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Funding Information: This work was supported by the National Natural Science Foundation of China (Grant No. U1803231). Publisher Copyright: © 2023, Higher Education Press. How sex-related root traits and soil microbes and their interactions respond to drought remains unclear. Here, we investigated how fine root traits and the composition of rhizosphere microbial communities in Populus euphratica females and males respond to drought in concert in 17-year-old plantations. Females increased specific root length (SRL) in response to drought. However, males showed no changes in their roots but significant increases in arbuscular mycorrhizal hyphal biomass and population of Gram-negative bacteria in the rhizosphere. Also, fungal symbiotroph communities associated with root systems in males differed from those in females under drought. We further demonstrated that the Gram-positive to Gram-negative bacteria ratios positively correlated with the SRL, while fungi to bacteria ratios were negatively correlated. Meanwhile, the relative abundance of symbiotrophs was negatively correlated with the SRL, while saprotroph abundance was positively correlated. Nevertheless, the relative abundance of symbiotrophs was positively correlated with the root carbon content (RCC). These findings indicate that microbial responses to drought depend highly upon the sex of the plant and microbial group and are related to root trait adjustments to drought. This discovery also highlights the role of plant-microbial interactions in the ecosystems of P. euphratica forest plantations.

Hundreds of land plant lineages have independently evolved separate sexes in either gametophytes (dioicy) or sporophytes (dioecy), but 43% of all dioecious angiosperms are found in just 34 entirely dioecious clades, suggesting that their mode of sex determination evolved a long time ago. Here, we review recent insights on the molecular mechanisms that underlie the evolutionary change from individuals that each produce male and female gametes to individuals specializing in the production of just one type of gamete. The canonical model of sex chromosome evolution in plants predicts that two sex-determining genes will become linked in a sex-determining region (SDR), followed by expanding recombination suppression, chromosome differentiation and, ultimately, degeneration. Experimental work, however, is showing that single genes function as master regulators in model systems, such as the liverwort Marchantia and the angiosperms Diospyros and Populus. In Populus, this type of regulatory function has been demonstrated by genome editing. In other systems, including Actinidia, Asparagus and Vitis, two coinherited factors appear to independently regulate female and male function, yet sex chromosome differentiation has remained low. We discuss the best-understood systems and evolutionary pathways to dioecy, and present a meta-analysis of the sizes and ages of SDRs. We propose that limited sexual conflict explains why most SDRs are small and sex chromosomes remain homomorphic. It appears that models of increasing recombination suppression with age do not apply because selection favours mechanisms in which sex determination depends on minimal differences, keeping it surgically precise.

Although hundreds of plant lineages have independently evolved dioecy (that is, separation of the sexes), the underlying genetic basis remains largely elusive1. Here we show that diverse poplar species carry partial duplicates of the ARABIDOPSIS RESPONSE REGULATOR 17 (ARR17) orthologue in the male-specific region of the Y chromosome. These duplicates give rise to small RNAs apparently causing male-specific DNA methylation and silencing of the ARR17 gene. CRISPR-Cas9-induced mutations demonstrate that ARR17 functions as a sex switch, triggering female development when on and male development when off. Despite repeated turnover events, including a transition from the XY system to a ZW system, the sex-specific regulation of ARR17 is conserved across the poplar genus and probably beyond. Our data reveal how a single-gene-based mechanism of dioecy can enable highly dynamic sex-linked regions and contribute to maintaining recombination and integrity of sex chromosomes.

Plant genetic sex determinants that mediate the transition to dioecy are predicted to be diverse, as this type of mating system independently evolved multiple times in angiosperms. Wild Vitis species are dioecious with individuals producing morphologically distinct female or male flowers; whereas, modern domesticated Vitis vinifera cultivars form hermaphrodite flowers capable of self-pollination. Here, we identify the VviPLATZ1 transcription factor as a key candidate female flower morphology factor that localizes to the Vitis SEX-DETERMINING REGION. The expression pattern of this gene correlates with the formation reflex stamens, a prominent morphological phenotype of female flowers. After generating CRISPR/Cas9 gene-edited alleles in a hermaphrodite genotype, phenotype analysis shows that individual homozygous lines produce flowers with reflex stamens. Taken together, our results demonstrate that loss of VviPLATZ1 function is a major factor that controls female flower morphology in Vitis., Unlike wild Vitis species, which produce either female or male flowers, modern grapevine cultivars form hermaphrodite flowers for self-pollination. Here, the authors report that the VviPLATZ1 (plant AT-rich sequence-and zinc-binding protein1) transcription factor functions in controlling female flower morphology determination.

Dioecy, the separation of sex at the individual level, evolved in angiosperms from hermaphroditic ancestors. One of the postulated evolutionary pathways is by way of the gynodioecious stage, i.e., the coexistence of female and hermaphroditic individuals. In a morphologically gynodioecious species, Daphne jezoensis., seed fertility in hermaphrodites is limited compared to that of females, indicating that its sexual system is close to dioecy. The aim of this study was to clarify whether the gynodioecy in this species is a stable sexual system or a transitional state leading to dioecy. We evaluated variations in the degree of pollen limitation, sexual and mating systems, floral morphology, sex ratio, and the genetic structure of 14 populations across the species’ distribution range in Japan. Sexual systems were similar among populations throughout the distribution range although the southernmost populations had unique genetic structures to some extent. Hermaphrodites were commonly self-compatible, but their fruiting capacity was low in every population. This suggests that gynodioecy composed of females and hermaphrodites having low seed fertility is a consistent sexual system in D. jezoensis. Low seed fertility in hermaphrodites throughout the distribution range suggests that this sexual system is not necessarily a transitional state from gynodioecy to dioecy if occasional seed production in hermaphrodites has any advantage. One possible ecological advantage for the maintenance of hermaphrodites is that seed production by autonomous self-pollination may be beneficial in the colonization process after long-distance seed dispersal.

In gymnosperms dioecy occurs in 64.6%, and sex chromosomes have been identified in 0.6% of the total species. All the Cycadidae, Gnetidae, Ginkgo and some species in Pinidae are dioecious. Heteromorphic sex pairs have been studied in six species where the heteromorphic sex may either be male giving rise to XX/XY system or female resulting in ZZ/WZ system. The chromosome determining the male sex may also be formed by the fusion of two telocentrics resulting in X1X2X1X2/X1X2Y system as in some Podocarpus species. In other cases where chromosomal hetromorphism is undetectable, the molecular markers and functional genomics have been applied to facilitate early sex detection while DNA methylation has also been indicated to be associated with sex determination. The multiple origins of dioecy is evident by its presence in evolutionarily distinct taxa, which provides an opportunity to explore diverse types and their mechanisms of sex determination in this group of plants.

PCD role in unisexual flowers. The developmental processes underlying the transition from hermaphroditism to unisexuality are key to understanding variation and evolution of floral structure and function. A detailed examination of the cytological and histological patterns involved in pollen and ovule development of staminate and pistillate flowers in the dioecious Opuntia robusta was undertaken, and the potential involvement of programmed cell death in the abortion of the sex whorls was explored. Flowers initiated development as hermaphrodites and became functionally unisexual by anthesis. Female individuals have pistillate flowers with a conspicuous stigma, functional ovary, collapsed stamens and no pollen grains. Male individuals have staminate flowers, with large yellow anthers, abundant pollen grains, underdeveloped stigma, style and an ovary that rarely produced ovules. In pistillate flowers, anther abortion resulted from the premature degradation of the tapetum by PCD, followed by irregular deposition of callose wall around the microsporocytes, and finally by microspore degradation. In staminate flowers, the stigma could support pollen germination; however, the ovaries were reduced, with evidence of placental arrest and ovule abortion through PCD, when ovules were present. We demonstrate that PCD is recruited in both pistillate and staminate flower development; however, it occurs at different times of floral development. This study contributes to the understanding of the nature of the O. robusta breeding system and identifies developmental landmarks that contribute to sexual determination in Cactaceae.

BackgroundPersimmon (Diospyros kakiThunb.) has various labile sex types, and studying its sex differentiation can improve breeding efficiency. However, studies on sexual regulation patterns in persimmon have focused mainly on monoecy and dioecy, whereas little research has been published on andromonoecy. In order to reveal the sex differentiation regulation mechanism of andromonoecious persimmon, we performed histological and cytological observations, evaluatedOGIandMeGIexpression and conducted phytohormones assays and mRNA and small RNA transcriptome analyses of the male and hermaphroditic floral buds of the andromonoecious persimmon ‘Longyanyeshi 1’.ResultsStages 2 and 4 were identified as the critical morphological periods for sex differentiation of ‘Longyanyeshi 1’ by histological and cytological observation. At both stages,OGIwas differentially expressed in male and hermaphroditic buds, butMeGIwas not. This was different from their expressions in dioecious and monoecious persimmons. Meantime, the results of phytohormones assays showed that high IAA, ABA, GA3, and JA levels at stage 2 may have promoted male floral bud differentiation. However, high JA levels at stage 4 and high ZT levels at stages 2 and 4 may have promoted hermaphroditic floral bud differentiation. In these phytohormone biosynthesis and signaling pathways, 52 and 54 differential expression genes (includingAux/IAA,ARFs,DELLA,AHP,A-ARR,B-ARR,CYP735A,CRE1,PP2C,JAZ,MYC2,COI1,CTR1,SIMKK,ACO,andMPK6) were identified, respectively. During the development of male floral buds, five metacaspases genes may have been involved in pistil abortion. In addition,MYB,FAR1,bHLH, WRKY,andMADStranscription factors might play important roles in persimmon floral bud sex differentiation. Noteworthy, miR169v_1, miR169e_3, miR319_1, and miR319 were predicted to contribute to phytohormone biosynthesis and signaling pathways and floral organogenesis and may also regulate floral bud sex differentiation.ConclusionThe present study revealed the differences in morphology and phytohormones content between male and hermaphroditic floral buds of ‘Longyanyeshi 1’ during the process of sex differentiation, and identified a subset of candidate genes and miRNAs putatively associated with its sex differentiation. These findings can provide a foundation for molecular regulatory mechanism researching on andromonoecious persimmon.

The high occurrence of dioecy on oceanic islands has attracted much attention from plant evolutionary biologists. Tournefortia argentea (Boraginaceae), an Indo-Pacific strand plant, has an unusual sexual system which remains poorly understood. Here, we investigated the sexual system of T. argentea based on morphological and micromorphological studies, as well as manipulated pollination treatments. Two morphs of plants were found in the natural populations on Xisha Islands: hermaphrodite and female plants. Each sexual morph exhibited different floral traits, showing a modest form of sexual dimorphism. Flowers of hermaphrodites display significantly larger size in corolla and anther than those of females. Fruit sets under natural condition and hand pollination in hermaphroditic individuals were both far lower than that of female ones, which shows characteristics of both leaky dioecy and gynodioecy. We investigated how these differences in floral traits between females and hermaphrodites affect the choice of pollinators. Although females and hermaphroditic flowers in T. argentea attracted the same assemblage of pollinator species, including Apidae, Syrphidae and Utetheisa lotrix (Arctiidae), the reduction in allocation to pollinator-attracting structures (corolla) and rewarding products caused a significant reduction in Apidae visitation to female flowers. Accordingly, T. argentea is another example of a gynodioecious species of flowering plants in Boraginaceae with clear sexual dimorphism occurring in oceanic islands. The sexual system in T. argentea may imply a transitional stage in the evolution to dioecy and may have been favored during colonization to newly reclaimed habitats, where mating partners may be limited.

Dioecy and the dynamics of its evolution are intensely investigated aspects of plant reproduction. Seabuckthorn (Hippophae rhamnoides ssp. turkestanica) is an alpine shrub growing wild in certain parts of western Himalaya. The previous studies have reported heteromorphic sex chromosomes in the species and yet marker-based studies indicate high similarity between the male and female genomes. Lack of information on sexual system in the species has further complicated the situation. A systematic study was thus undertaken to understand the sexual system in seabuckthorn and to discern the extent of similarity/dissimilarity between the male and female genomes by generating a large number of markers using amplified fragment length polymorphism and representational difference analysis. Floral biology and regular monitoring of species revealed the presence of polygamomonoecious (PGM) plants in most populations at a low percentage (~2–4%). PGM plants showed low pollen production and overall low fertility, suggesting a monoecy–paradioecy pathway at function. The results of the marker study demonstrated that there are limited differences between male and female genomes and these differences were not uniform across the populations in the Leh-Ladakh region, especially when the geographical distance increases. Results also suggest that a dynamic partitioning of genomes is operational between the two genders of seabuckthorn and differences are not homogenized across the populations. Both reproductive biology-based and DNA marker-based studies indicate that genders have separated recently. The present study proposes seabuckthorn as a promising model system to study evolution of dioecy and sex determination.

This study focuses on variation in reproductive systems in Acer (maple) species, and how Acer might be a useful genus for understanding floral morphology and the evolution of these highly variable systems. The goals were (1) to conduct a survey of reproductive characters in the genus Acer with an emphasis on floral morphometrics of A. ginnala Maxim., A. tataricum L., and the hybrid A. ginnala × tataricum and (2) present a compilation of the reproductive biology of Acer based on the literature and online information. Morphometric analyses showed that the hybrid A. ginnala × tataricum exhibits intermediacy in vegetative morphology and some floral traits compared to the parents. Literature and an online survey revealed some information on the reproductive biology of 40 Acer species, plus the hybrid, representing only 32.3% of the taxonomic diversity. The reproductive modes of Acer include monoecy, dioecy, trioecy, andromonoecy, androdioecy, and andropolygamy, with a trend towards unisexuality. Temporal patterns of maturation of reproductive parts revealed duodichogamy, heterodichogamy, simultaneous, and dichogamy. The pollination syndrome is entomophilous, anemophilous, and ambophilous. We conclude that the evolutionary and geographic success of Acer lie in the versatility of sexual systems, diverse modes of maturation of reproductive organs, reproductive success, and the ability to colonise wider areas.

In sexually plastic Acer pensylvanicum, determination of sex can occur extremely late, within three weeks of spring flowering. Physical damage causing complete vascular tissue severance results in increased female expression. Species with environmental sex determination are rare amongst angiosperms but widely distributed across taxa. The timing of floral development in species that change sex based on environmental cues is unexplored. We investigated the timing of differentiation of sexual organs in buds of Acer pensylvanicum, an understory tree in eastern North America with environmental sex determination. We collected branches from individuals at three collection times in the early spring of 2016 and kept them in a warm greenhouse until anthesis. All individuals exhibited complete or partial female inflorescences in the greenhouse in one or more collection. However, none of these same individuals produced only female flowers in the field. Unlike many other woody species that differentiate bud sexual primordia 9–12 months prior to flowering, A. pensylvanicum may differentiate the sexual organs in its flower buds as late as three weeks prior to anthesis. In a separate series of branch collections in 2017, we found that the stress response to cutting leads to increased female sex expression in branches, while earlier warm temperatures (e.g., those caused by growing in a protected greenhouse environment) or increased carbohydrate availability does not. Given the labile sex determination system of A. pensylvanicum, the ability to delay differentiation of buds into male or female until shortly before spring flowering would allow individual trees to respond to sex-determining damage cues as late as mid-spring. This supports the hypothesis that A. pensylvanicum may not exhibit the lag-time characteristic of temperate spring and early-summer flowering woody species and may change sex expression in response to stress.

Sphedamnocarpus andersonii, a new species from the Ihorombe Region of Madagascar, is described and illustrated. The twiggy, white-flowered shrubs are characterized by linear leaves, unique in the genus.

High Mn poisoned male and female Populus cathayana. The toxicity could be alleviated by exogenous ABA application. Intriguingly, ABA granted higher resistance to males than to females under high Mn stress because ABA could induce more blocking of Mn translocation to leaf in males than in females. Abscisic acid (ABA) is involved in plants' adaptive responses to various environmental stresses. However, little is known about the sex-related detoxification of ABA in plants under excess manganese (Mn) conditions. To reveal potentially different ABA detoxification mechanisms between Populus cathayana males and females against excess Mn exposure, photosynthesis performance, Mn2+ concentrations and morphologic changes were investigated. High Mn stress led to a more severe chloroplast destruction and, thus, greater reduction in the photosynthesis of P. cathayana females when compared to males. Under high Mn conditions, Mn reallocated mainly to leaves in females, while in males, it was distributed equally to roots and leaves. With the application of ABA, photosynthesis was restored more in males and more integrated grana in males than in females. It should be noted that Mn concentrations in males were lower in leaves and higher in roots and stems than those in females when treated with the combination of Mn and ABA. Conclusively, due to the reduction of root-shoot Mn transportation induced by ABA in P. cathayana males, males experienced less physiological injuries than do females, which suggest that males possess greater ABA-inducible resistance to Mn stress than do females.

Gynodioecy is the coexistence of hermaphrodites and females in a population. It is supposed to be an intermediate stage in the evolutionary pathway from hermaphroditism to dioecy in angiosperm. Hermaphrodites gain fitness through both seed and pollen production whereas females gain fitness only through seed production. As females spread in a gynodioecious population, sexual selection prompts hermaphrodites to invest in male function and male-biased hermaphrodites prevail. In the gynodioecious shrub Daphne jezoensis (Thymelaeaceae), female frequency is stably around 50% in most populations, and fruit-set rate of hermaphrodites is commonly low. Therefore, D. jezoensis is likely at a later stage in the evolutionary pathway. Female function of hermaphrodites (fruit-set rate, selfing rate, seed size, and germination rate) was assessed in three populations under natural conditions. In order to evaluate the potential seed fertility and inbreeding depression by selfing in hermaphrodites, hand pollination treatments were also performed. Over a 2-year period under natural conditions, 18-29% of hermaphrodites and 69-81% of females set fruit. Across all three populations, the mean fruit-set rate ranged 9.5-49.2% in females and only 3.9-10.2% in hermaphrodites. Even with artificial outcross-pollination, 59-91% of hermaphrodites failed to set any fruit. When self-pollination was performed in hermaphrodites, both of fruit-set and germination rates were decreased, indicating early-acting inbreeding depression. In addition, more than half of the hermaphrodite seeds were produced by selfing under natural pollination, but pollinator service was still required. Totally, hermaphrodites performed poorly as seed producers because of the intrinsically-low fruiting ability and a combination of autogamous selfing and strong inbreeding depression, indicating the absence of reproductive assurance. These results indicate that the mating system of D. jezoensis is functionally close to dioecy.

Most flowering plants are simultaneous hermaphrodites. Within species and even within local populations, sex allocation is usually highly plastic. Here, we link pollen sufficiency to the size of pollen-exchanging groups (i.e., pollen neighborhoods) and to pollen transfer efficiency, using an individual-based game-theoretic framework to determine the stable distribution of sex allocation that does not require the unrealistic assumption of infinitely large, panmictic populations. In the absence of selfing, we obtain the novel result that pollen limitation destabilizes hermaphroditism and favors separate sexes, whereas hermaphroditism remains stable without pollen limitation. With mixed mating, hermaphroditism is stable except when the fitness value of selfed offspring is less than half that of outcrossed offspring (i.e., strong inbreeding depression). In that case, the size of pollen neighborhoods, pollen transfer efficiencies, and the relative fitness of selfed offspring determine whether separate sexes or hermaphroditism is the stable outcome. The model thus predicts that separate sexes can derive from either of two ancestral states: obligate outcrossing under pollen limitation, or mixed mating (competing self-fertilization) under severe inbreeding depression. It also predicts conditions under which variance in sex-allocation among hermaphrodites within pollen exchanging groups along a gradient of pollen limitation can range from high (dioecy) to near zero (equal proportions of male and female investment).

Morus alba (white mulberry) is a cultivated species of mulberry that exists as both dioecious and monoecious plants in nature. Although it has been cultivated for years as an essential crop for sericulture, and although various cultivars are readily available, its sex determination mechanisms are not yet clear. In the present study, we aimed to elucidate sex determination mechanisms of mulberry using both genetic and genomic approaches. Female and male cultivars of dioecious M. alba were crossed, and the segregation of sex was analyzed in the F1 plants. In the F1 plants, female and male plants were segregated at an almost equal ratio. By comparing the restriction site associated DNA sequencing analysis data between female- and male-bulked DNA of these F1 plants, the male-bulk-specific RAD tags were predominantly identified. Thus, we presumed that the male M. alba is heterogametic whereas the female M. alba is homogametic. Based on the corresponding sequences of male-bulk-specific RAD tags, putative male-specific markers were developed, and these markers exhibited linkage with the male sex in individual F1 plants. However, genotypes of these markers did not always show an association with sex in the cultivars of M. alba, M. bombycis, and M. lhou. Around half of the analyzed female cultivars showed identical marker genotypes to male, whereas all the analyzed male and monoecious cultivars showed a consistent genotype. According to these results, the DNA markers developed in this study were found to be relevant to male flower determination, and are thus expected to be helpful for elucidating the mechanisms underlying the evolutionary and sex determination systems of mulberry plants.

Dioecious plant species having both male and female plants have been investigated regarding sex-related characteristics such as sex ratio, sex-differential resource requirements, and spatial segregation of the sexes. Habitat loss and fragmentation are major threats to the survival of plant populations, but dioecious species are particularly more prone to such habitat degradation than non-dioecious species because of their dimorphic sexual system. We examined the sex-related demographics of two Ilex cornuta populations being different regarding land use history. During 2016–2017, we examined I. cornuta trees with a basal diameter ≥ 1.5 cm in the Yongsu-ri population (YS population) and the Gotjawal Provincial Park population (GP population). Plant sex (male, female, or unsexed) was identified. The tree size (basal diameter and height of the main stem), clonal production (the ramet numbers per genet), and vitality for each clone were measured. The associations between population, sex, tree size, clonal production, and vitality were examined using ANOVAs and contingency table analyses. Finally, point pattern analyses using O-ring statistics were conducted to assess spatial patterns. Upon excluding unsexed trees, the YS population with 74 trees was significantly male-biased (0.66), while the GP population with only 26 trees had a 1:1 sex ratio. In both populations, males and females did not differ in tree size. Although the mean number of ramets differed significantly between populations, females tended to produce more ramets than males. The proportion of weak trees was significantly higher in the YS than in the GP population. Neither population showed evidence of spatial segregation of the sexes. The two populations of dioecious I. cornuta are characterized by the small number of trees and relatively high frequencies of non-reproductive trees. Both indicate that these populations are quite susceptible to environmental and genetic stochasticity. On the other hand, the differences between populations in sex ratio, clonal production, and vitality suggest that conservation efforts for I. cornuta need to be population-specific. In order to help recover and enable this vulnerable species to persist, it is necessary to find ways to enhance their sexual reproduction and simultaneously reduce habitat disturbances due to anthropogenic activities.

Asparagus officinalis (garden asparagus) is a dioecious perennial crop, and the dioecy (i.e., sex) of A. officinalis can affect its productivity. In A. officinalis, flower anthers in female plants fail to accumulate callose around microsporocytes, fail to complete meiosis, and degenerate due to cell death. Although 13 genes have been implicated in the anther development of male and female flowers, it is unclear how these genes regulate the cell death in female flower anthers. The aim of this study was to narrow down factors involved in this process. TUNEL staining and Feulgen staining of female flower microsporocytes suggest that female microsporocytes enter a previously undetected meiosis-like process, and that the cell death occurs independently of this meiosis-like process, excluding the possibility that the cell death is caused by the cessation of meiosis. RNA sequencing with individual floral organs (tepals, pistils and stamens) revealed that several genes possibly regulating the cell death, such as metacaspase genes and a Bax inhibitor-1 gene, are differentially regulated between female and male flower anthers, and that genes involved in callose accumulation are up-regulated only in male flower anthers. These genes are likely involved in regulating the cell death in female flower anthers in A. officinalis.

Lindera glauca (Lauraceae) is an economically important East Asian forest tree characterized by a dioecy in China and apomixis in Japan. However, patterns of population genetic diversity and structure of this species remain unknown for this species due to a lack of efficient molecular markers. In this study, we employed Illumina sequencing to analyze the transcriptomes of the female and male flower buds of L. glauca. We retrieved 59,753 and 75,075 unigenes for the female and male buds, respectively. Based on sequence similarity, 44,379 (74.27 %) unigenes for the female and 45,414 (60.49 %) unigenes for the male were matched to public databases. We identified 11,127 putative differentially expressed genes between the female and male buds and 20,048 expressed sequence tag-simple sequence repeats (EST-SSRs). From 3147 primer pairs designed successfully, 120 were selected for validation of polymorphism, and 13 could reliably amplify polymorphic bands and exhibited moderate levels of genetic diversity (e.g., N A = 4.42; H E = 0.56) when surveyed across 96 individuals of altogether six L. glauca populations from China and Japan. One of the three population genetic clusters identified in China was fixed in Japan, suggesting a historical population bottleneck following island immigration. The present study has generated a wealth of transcriptome data for future functional genomic research focused on the variable reproductive system of L. glauca (dioecy, apomixis) as well as EST-SSR markers for population genetics studies and its intriguing evolutionary shift from dioecy to apomixis in the wake of island colonization.

The Chenopodiaceae Tribe Anserineae Dumort was proposed to include the genus Spinacia and the genus Blitum. In addition to the recent domestication of Spinacia, the tribe demonstrates extensive evolution within its floral development. We test whether the development of dioecy, monoecy, and protogyny is reflected differentially among floral developmental versus non-floral developmental genes, and whether recent domestication leaves traces in the phylogenetic relationship within the genus Spinacia. The phylogenetic predictions consistently support the sister relationship of Spinacia sp. to a Blitum clade consisting of Blitum bonus-henricus, Blitum virgatum, and Blitum nuttallianum. Relative rates tests indicate a generally faster rate of nucleotide substitutions within Spinacia. Tests of selection indicate that there is generally purifying selection acting on the sequences. In addition, insertion/deletion (indel) events occur more prominently within the Spinacia clade and occur in both coding and intron regions. The phylogenetic relationships within this tribe calls into question the hypothesis that dioecy in Spinacia evolved from a monoecious grade. The evidence for purifying selection in Spinacia suggests that the increased nucleotide substitution rates are not driving protein evolution, in contrast to evidence of protein sequence and structure evolution driven by indels. There is no footprint of domestication on sequence evolution, and we cannot detect phylogenetic signals that would support separation of the Spinacia accessions into three distinct taxa.

Sexual allocation pattern is considered highly relevant for the maintenance of females in the process of evolution from hermaphrodites to dioecy. Theoretical and empirical studies predict that gynodioecious plants should invest more resources in male function under harsher environments and/or when female frequency is higher; and that there are trade-offs between male and female function. We studied sexual allocation pattern in the gynodioecious species Cyananthus macrocalyx in two populations in SW China. The results showed that although the total flower mass in hermaphrodites was significantly higher than that in females, females and hermaphrodites allocated similar biomass to female function (pistil biomass). As a consequence, females allocated relatively more resource to female function, while hermaphrodites allocated relatively more resource to pollinator attractiveness. There was no difference in total flower mass, pistil mass, and pollen production in hermaphrodites between the two populations. These results suggest that the females compensate for the disadvantage of lacking male function by allocating relatively more resource to female function, while hermaphrodites gain fitness through both male and female functions. This study supports the idea that if females are to be maintained in a gynodioecious population, they must allocate more resource to female functions than is necessary for hermaphrodites. However, our results were in contrast to those of previous studies, in that trade-offs between male and female functions and male-biased allocation under high female frequency were not observed. This study adds to the body of research on plant sexual allocation, and for that it is, at least at some aspects, contrary to previous statements, it could be helpful for advancing the theoretical predictions in the future.

Dioecious plant populations are composed of two distinct sexual morphs representing the female and male counterparts. Dioecy in restinga areas has been strongly associated with fleshy fruits, woodiness, generalist entomophily, and resource-poor environments. The ability of vegetative reproduction is another ecological trait commonly associated with dioecy, but not shown for the woody dioecious species in restingas. This study aimed to investigate whether the distribution of sexual morphs in populations is due to occurrence of vegetative propagation as a reproductive strategy among dioecious woody species on sandy coastal plains. We also intended to evaluate possible effects of sex ratio on sexual reproduction (female reproductive success). The study was conducted in the sandy coastal plains (restinga) of Marica, Rio de Janeiro State. We studied six woody dioecious species. During the reproductive period, all individuals of each species and your nearest neighbor were marked using GPS, along an area with approximately 5.1 ha. The sex of the individuals was recorded. This study provides evidence of the occurrence of vegetative propagation in the woody dioecious species studied and indicates that the ability to reproduce in this manner varies between sexual morphs, generating male-biased populations in most cases. Our data suggest that asexual reproduction by vegetative propagation of the male morphs can have an effect on sexual reproduction, as it increases the amount of staminate flowers, and therefore the supply of the pollen grains. Our results provide insights into vegetative reproduction as a strategy to increase male plant fitness and an alternative to populate environments under stress.

The co-existence of males, females and hermaphrodites, a rare mating system known as trioecy, has been considered as an evolutionarily transient state. In nematodes, androdioecy (males/hermaphrodites) as found in Caenorhabditis elegans, is thought to have evolved from dioecy (males/females) through a trioecious intermediate. Thus, trioecious species are good models to understand the steps and requirements for the evolution of new mating systems. Here we describe two new species of nematodes with trioecy, Auanema rhodensis and A. freiburgensis. Along with molecular barcodes, we provide a detailed analysis of the morphology of these species, and document it with drawings and light and SEM micrographs. Based on morphological data, these free-living nematodes were assigned to a new genus, Auanema, together with three other species described previously. Auanema species display convergent evolution in some features with parasitic nematodes with complex life cycles, such as the production of few males after outcrossing and the obligatory development of dauers into self-propagating adults.

Objective Mercurialis annua is a wind-pollinated annual plant that has long been used as a model for the study of ploidy and sexual-systems evolution. However, no molecular markers are yet available for genetic studies of its diploid populations. Here, we develop and characterize a set of eight polymorphic microsatellite markers for diploid dioecious M. annua. Results Following an SSR-enrichment protocol, 13 microsatellite markers were proposed, eight of which yielded successful amplification and polymorphism. We screened the eight microsatellite loci in 100 individuals. The number of alleles per marker ranged from 6 to 12, and observed heterozygosity ranged from 0.57 to 0.76. To estimate potential allele scoring errors, these individuals’ offspring were genotyped for the same loci, and error rates were estimated from parentage analyses. Error rates ranged from 0 to 6.8%. Cross-amplification tests were performed for congeneric M. huetti and M. canariensis, with successful amplification for seven and six of the eight loci, respectively. The novel microsatellite markers proposed here will be crucial for a multitude of genetic studies of M. annua and further establish its importance as a model species for addressing ecological and population genetic questions.

Gene flow in plant populations is heavily affected by species sexual systems. In order to study the effect of sexual systems on genetic structure, we examined plastid and nuclear DNA of 12 dioecious (males and females) and 18 trioecious (males, females and hermaphrodites) populations of Salix myrsinifolia—a boreal shrub with slow range expansion. Populations were located along latitudinal gradients across submarginal and marginal parts of the range. Individuals of each sex morph were all hexaploid. We identified 10 chloroplast DNA haplotypes and scored 205 polymorphic bands with amplified fragment length polymorphism. We found dioecious populations that differed from trioecious populations via the presence of four unique haplotypes and significant difference in Nei’s gene diversity index (0.119 vs. 0.116) and down-weighed marker value (1.17 vs. 1.02). The latter parameter, together with haplotype and nucleotide diversity, significantly decreased with latitude similar to the expansion front. Also, we found that 89% of hermaphrodite individuals belong to one distinct in tree parsimony network haplotype. This frequency significantly decreased with latitude towards the expansion front. We suspect that the presence of hermaphrodites in trioecious populations may represent a trade-off between the possibility of producing progeny by single hermaphrodites and genetic variability loss through autogamy. S. myrsinifolia benefits from trioecious sexual systems under colonization events. This phenomenon is no longer a gain closer to the core of the species range.

Flowering plants are known to exhibit vast diversity of sexual systems encompassing bisexual, monoecious and dioecious conditions. Dioecy offers opportunities to explore separately the male and female programmes giving an insight to the evolutionary, developmental and molecular processes leading to separate mechanisms for sex expression. Mechanisms controlling sex can either be genetic or epigenetic (physiological and environmental). Plant hormones too influence sex expression. An active Y sex determination system and an X to autosomes ratio systems are common amongst the flowering plants. Advances in our understanding of sex determination has been addressed both by conventional as well as molecular approaches. Using conventional techniques mainly cytogenetics, sex chromosomes in some dioecious plants have been identified and characterized. Surprisingly, the presence of well defined sex chromosomes was found in only few species. Some sex linked genes have also been identified and characterized using molecular approaches but none of these genes have a direct link to sex determination. Molecular markers have been employed to resolve the enigma associated with dioecism to a certain extent. Its application in plant breeding is immensely beneficial. Positively, it would be beneficial for validation of sex prior their sex expression at larger perspectives. The present review therefore emphasizes the mode of sex determination among dioecious plants vis-a-vis summarizes the works related to gender specific markers generated using male and female plants from agriculturally important dioecious crops.

The mutualistic interaction between Ficus spp. and their pollinating fig wasps (Agaonidae) centres on the plants’ unique inflorescences—their figs. Each Ficus species is pollinated by foundresses of host-specific fig wasps which enter figs to lay eggs in the female flowers. Most foundresses are trapped in the first figs they enter, but in some species wingless foundresses can re-emerge and subsequently enter and oviposit into further figs. We investigated whether number of potential oviposition sites, age of the fig and age of the wasp influence the likelihood of re-emergence of lone foundresses of the Asian fig wasp Kradibia (=Liporrhopalum) tentacularis from previously un-entered figs of Ficus montana. Likelihood of re-emergence was not influenced by wasp age or flower numbers (resource abundance), but was more frequent from older figs that had waited longer to be pollinated. Laying eggs in several figs offers clear advantages, but foundresses often failed to re-emerge despite being unable to lay all their eggs. Resource quality not quantity appears to be the main influence on the fig wasp’s oviposition decisions. The physical difficulty that the wasps experience when trying to re-emerge may prevent it, even when re-emergence would be advantageous for both the insect and its host plant, but older fig wasps were not detectably ‘weaker’ than younger individuals.

The link between life history traits and mating systems in diploid organisms has been extensively addressed in the literature, whereas the degree of selfing and/or inbreeding in natural populations of haploid-diploid organisms, in which haploid gametophytes alternate with diploid sporophytes, has been rarely measured. Dioecy has often been used as a proxy for the mating system in these organisms. Yet, dioecy does not prevent the fusion of gametes from male and female gametophytes originating from the same sporophyte. This is likely a common occurrence when spores from the same parent are dispersed in clumps and recruit together. This pattern of clumped spore dispersal has been hypothesized to explain significant heterozygote deficiency in the dioecious haploid-diploid seaweed Chondrus crispus. Fronds and cystocarps (structures in which zygotes are mitotically amplified) were sampled in two 25 m(2) plots located within a high and a low intertidal zone and genotyped at 5 polymorphic microsatellite loci in order to explore the mating system directly using paternity analyses. Multiple males sired cystocarps on each female, but only one of the 423 paternal genotypes corresponded to a field-sampled gametophyte. Nevertheless, larger kinship coefficients were detected between males siring cystocarps on the same female in comparison with males in the entire population, confirming restricted spermatial and clumped spore dispersal. Such dispersal mechanisms may be a mode of reproductive assurance due to nonmotile gametes associated with putatively reduced effects of inbreeding depression because of the free-living haploid stage in C. crispus.

Spatial segregation of the sexes (SSS) has been reported in (30 dioecious species from 24 plant families. Such convergent evolution can arise via shared constraints and/or shared selective pressures but the extent to which SSS is adaptive is unresolved. In particular, because mating in plants occurs primarily among near neighbors, any fitness benefits of SSS should usually be offset by the separation of mating partners during reproduction. It has been argued that the problem of mating in plants with SSS should be alleviated in plants with more reliable modes of pollination, such as wind pollination. Accordingly, we evaluated the association between SSS and pollination mode using phylogenetically controlled comparative analyses. Moreover, because outcrossing plants produce many more pollen grains than ovules, females should compete less intensely for mating partners than males. We therefore predicted that any fitness costs arising from segregation should be greater for males than females. We evaluated this prediction using spatially explicit stochastic simulations. The comparative analyses indicated that there is indeed a positive association between wind pollination and SSS, but also that SSS might be associated with clonal growth. Because methods used for detecting SSS can be biased by clonality, this raises the possibility that some cases of SSS are 'false positives'. The simulation results supported the prediction that male fitness is more strongly affected by segregation than female fitness. Contrary to arguments based on anecdotal evidence, increasing the effectiveness of pollen dispersal had little effect on the fitness of segregated males. Instead, our results indicated that the evolutionary maintenance of SSS requires substantial performance advantages to segregated males. However, there is little empirical

Coccinia grandis, a dioecious cucurbitaceous vegetable crop possess different morphological forms such as male, female and morphologically perfect plants. Mitotic analysis revealed the presence of 2n =24 chromosomes in the male and female plants. However, the plant with morphologically perfect flowers, revealed 2n = 28 chromosomes. Though all the morphotypes had a predominance of nearly median (nm) chromosomes, the plants with morphologically perfect flowers showed more nearly median (nm) and median (m) chromosomes than both the male and female plants. The unique karyotype with predominance of ‘m’ and ‘nm’ chromosomes of these plants is in consistency with its morphology indicating the ancestral bisexual condition of the species. The findings of the present study therefore indicate that dioecy in C.grandis with well defined sex chromosomes has evolved from bisexual ancestors relatively recently and has involved chromosomal changes such as translocations or fusions. Formation of anther and production of malformed pollen grains in morphologically perfect plants also suggests that the genes required for stamen development may be located on the X chromosomes or autosomes, however, genes related to pollen maturation should be strictly under the control the Y chromosome.

The evolution of reproductive systems has intrigued evolutionary biologists for well over a century. Recent empirical and theoretical work has elucidated the evolution of dioecy (separate males and females) from hermaphroditism in many plant species. The reverse transition, evolving hermaphroditism from dioecy, has occurred many times in animals, and yet is poorly studied relative to its reverse analog in plants. Crustaceans in the sub-order Spinicaudata have evolved hermaphroditism from dioecy three separate times, in some cases forming all-hermaphroditic species and in others forming androdioecious (males + hermaphrodites) species. Herein we report evidence of hermaphroditism in a fourth spinicaudatan genus: the newly described Calalimnadia. We present sex ratio and anatomical evidence that Calalimnadia mahei comprises selfing hermaphrodites, with no males being found in over 10,000 offspring reared. We combine these reproductive results with those of other Spinicaudata to estimate the evolution of hermaphroditism in this crustacean sub-order. We use these genetic data combined with anatomical evidence to suggest that C. mahei represents a fourth, independent derivation of hermaphroditism from dioecy in these reproductively labile crustaceans.

The evolutionary pathway between hermaphroditism and dioecy draws widespread interests, and androdioecy is rarely achieved as an intermediate state between the two breeding systems. Flower bud differentiations in the pistils of hermaphrodites and the pistillodes of males in androdioecious Osmanthus fragrans L. were investigated by paraffin sectioning to elucidate the evolution to androdioecy. Results showed that the regularity and rhythm in flower bud differentiation between males and hermaphrodites were almost consistent and included six main stages. However, the hermaphrodites always lagged behind the males at each stage. The apical floret in the same inflorescence developed earlier than did the lateral ones in both hermaphrodites and males. The most significant difference between males and hermaphrodites was observed at the carpel differentiation stage. Two carpel primordia appeared inside the stamens of both males and hermaphrodites at the initial stage. These two carpels gradually fused with each other in hermaphrodites and eventually developed into a normal pistil with a stigma, a style, and an ovary. However, a cavity grew conspicuously over time between two carpels as developed in males. The two carpels eventually developed into a pistillode with two independent bracteal tissues. However, from the whole development process, the male retained the developmental residue of the hermaphrodite. Thus, the pistillodes of males could be traced to the pistils of hermaphrodites. This finding shows that males may be derived from hermaphrodites in O. fragrans. On the basis of this finding and previous studies on Oleaceae, androdioecy could be regarded as a transition from hermaphroditism to dioecy in this family.

Two new species of Miconia from Bolivia are described: Miconia galeiformis and Miconia neei. Both species occur in Andean montane forests. Miconia galeiformis (sect. Chaenopleura) is distinctive within Miconia due to the presence of setose stems, petioles and abaxial leaf surface, and a large, globose stigma. Miconia neei bears unisexual flowers, and can be distinguished from similar species of Miconia sect. Cremanium in Bolivia based on the furfuraceous stems and young nodes, in addition to serrulate leaf margins. The documentation of dioecy in Miconia neei and the occasional occurrence of unisexual flowers in Miconia galeiformis adds to our burgeoning understanding of complex breeding systems within Miconia.

The host-specific relationship between fig trees (Ficus) and their pollinator wasps (Agaonidae) is a classic case of obligate mutualism. Pollinators reproduce within highly specialised inflorescences (figs) of fig trees that depend on the pollinator offspring for the dispersal of their pollen. About half of all fig trees are functionally dioecious, with separate male and female plants responsible for separate sexual functions. Pollen and the fig wasps that disperse it are produced within male figs, whereas female figs produce only seeds. Figs vary greatly in size between different species, with female flower numbers varying from tens to many thousands. Within species, the number of female flowers present in each fig is potentially a major determinant of the numbers of pollinator offspring and seeds produced. We recorded variation in female flower numbers within male and female figs of the dioecious Ficus montana growing under controlled conditions, and assessed the sources and consequences of inflorescence size variation for the reproductive success of the plants and their pollinator (Kradibia tentacularis). Female flower numbers varied greatly within and between plants, as did the reproductive success of the plants, and their pollinators. The numbers of pollinator offspring in male figs and seeds in female figs were positively correlated with female flower numbers, but the numbers of male flowers and a parasitoid of the pollinator were not. The significant variation in flower number among figs produced by different individuals growing under uniform conditions indicates that there is a genetic influence on inflorescence size and that this character may be subject to selection.

This study examined the pollination ecology of Myrsine parvifolia in an area of sandy coastal plain, namely restinga. Male and female plants were distributed in an aggregated form with sex ratio of 1.7♂:1♀. Flowers are open dish-shaped, greenish-white, odorless, and devoid of nectaries. The female flower has a well-exposed sessile stigma. The anthers of the male flower release pollen grains in a powdery cloud. The pollen grains are small, dry, prolate spheroidal, 3–5-colpate and have a scabrous exine. We concluded that M. parvifolia is wind pollinated by: the presence of pollen grains in the air; fruit production in inflorescences isolated from access by biotic pollinators, but not by wind; the female flowers were not visited by biotic agents. Our results reinforce the hypothesis that pollination by wind is the main pollination system in Myrsine. Pollen limitation has been recorded for M. parvifolia and also for other dioecious insect-pollinated species from restinga as Guapira opposita and Tapirira guianensis, but not for Coccoloba arborescens and Protium brasiliense. Natural and manual fruit set of all of them was low, suggesting that other factors, in addition to pollen limitation, are involved in the regulation of fruit production. Studies on floral biology may help to clarify the variable rates of pollen limitation found for the dioecious species from restinga. However, as they are woody and perennial, and therefore pass through several reproductive cycles throughout life, factors that influence female reproductive success may vary over the reproductive events, due to abiotic and biotic conditions.

The papaya Y-linked region showed clear population structure, resulting in the detection of the ancestral male population that domesticated hermaphrodite papayas were selected from. The same populations were used to study nucleotide diversity and population structure in the X-linked region. Diversity is very low for all genes in the X-linked region in the wild dioecious population, with nucleotide diversity π syn = 0.00017, tenfold lower than the autosomal region (π syn = 0.0017) and 12-fold lower than the Y-linked region (π syn = 0.0021). Analysis of the X-linked sequences shows an undivided population, suggesting a geographically wide diversity-reducing event, whereas two subpopulations were observed in the autosomes separating gynodioecy and dioecy and three subpopulations in the Y-linked region separating three male populations. The extremely low diversity in the papaya X-linked region was probably caused by a recent, strong selective sweep before domestication, involving either the spread of a recessive mutation in an X-linked gene that is beneficial to males or a partially dominant mutation that benefitted females or both sexes. Nucleotide diversity in the domesticated X samples is about half that in the wild Xs, probably due to the bottleneck when hermaphrodites were selected during domestication. The extreme low nucleotide diversity in the papaya X-linked region is much greater than observed in humans, great apes, and the neo-X chromosome of Drosophila miranda, which show the expected pattern of Y-linked genes

Fine-scale spatial genetic structure (SGS) has profound ecological and genetic consequences for plant populations, and some studies indicate that clonal reproduction may significantly enhance SGS. Clonality is widespread among dioecious species, but little is known about the relationship between clonal reproduction and SGS in the frame of sexual dimorphism. We asked the following questions: (1) Is there a sexually dependent pattern of SGS in white poplar population? (2) What is the relationship between clonal reproduction and SGS? and (3) Does this relationship have a sex-specific component? Using 16 microsatellite markers, genetic structure including fine-scale SGS and clonality of females and males of white poplar were investigated. Significant SGS was noted for both sexes at the ramet and genet levels. At the genet level, males had 2.7-fold higher SGS than that of females. Clonality significantly contributed to SGS only in females. A sibship structure revealed with pedigree analysis and clustering-based methods among males was likely the major factor of the observed SGS. The sexes differed in their clonal growth strategies. Spatial positioning of ramets in female clones suggested foraging behavior and/or avoidance of competition, while for male clones it indicated more expansion and space colonization. The obtained results led us to conclude that sexual dimorphism in life history traits may affect the course and rate of demo-genetic processes acting in natural populations of dioecious species. To our knowledge, this is the first study demonstrating a sex-specific pattern of SGS in natural populations of dioecious species.

Simple sequence repeats (SSRs) have been widely used in the construction of linkage maps, quantitative trait loci (QTLs) mapping, and marker-assisted selection (MAS). The availability of the sequenced Actinidia chinensis (kiwifruit) genome allows for the inexpensive and efficient development of microsatellite markers. In this study, a total of 49,067 SSRs were identified and characterized in the genome sequences of kiwifruit. Dinucleotide repeats are the most abundant SSRs, with the AG/TC motif accounting for 44.2 % of all SSRs in the genome. Fifty-five newly derived SSRs, together with 46 previously available SSRs, were integrated into linkage maps of an interspecific kiwifruit population. In addition, eight sex-linked SSR markers (including one previously published SSR) were mapped in the sex-related region on the LG25, suggesting that recombination is partially suppressed to maintain dioecy in kiwifruit. The SSRs developed from this study are a valuable resource for kiwifruit genetics and will contribute to the use of MAS in early sex determination of dioecious plant breeding.

Hippophae salicifolia D. Don (Seabuckthorn) grows in stressful environment of high altitude under conditions of low temperature and low availability of water. We have studied gender based differences in physiochemical response to cold stress in male and female plants of Seabuckthorn. After 24 h of cold stress about 32 and 66 % higher electrolyte leakage (EL) was recorded in male and female plants respectively. Relative water content (RWC) at the end of 24 h stress was higher in male plants (~64 %) compared to female plants (~60 %). Proline content in leaf samples of cold stressed male and female plants also increased upon cold stress. After 24 h about 2.7 fold higher amount of proline was assessed in male and female in comparison to control plants. Similarly, about two fold increase in the specific activities of superoxide dismutase (SOD) and catalase was also observed upon cold stress in male and female plants. These findings have important inferences for community of molecular biologists exploring seabuckthorn genome for agronomically important genes.

Like other species of the genus Taxus, European yew trees contain taxanes, including paclitaxel (T) and its precursor 10-deacetylbaccatin III (10-DAB). Taxanes are one of the most effective anticancer drugs. This study was undertaken to describe the levels and patterns of taxane variation in the Sudetian region (SW Poland). Paclitaxel (T) and 10-deacetylbaccatin III (10-DAB) concentrations were analysed in five populations. Needles and twigs were analysed from 60 individuals (30 males and 30 females) in each population. In addition, morphometric measurements were taken in the populations to obtain light intensity coefficients (specific leaf area, SLA). High variability in the taxane contents at both intra and interpopulational levels was found. Nevertheless, females had a significantly higher taxane content compared to males. Because taxanes are carbon-based secondary metabolites, females have higher rate of gas exchange of females compared to males. This was probably an adaptation to greater reproductive effort incurred by females. In this regard, female individuals seem to be better for selecting elite cultivars with a higher taxane production. The relationship between light intensity and taxane content was not significant. Shading, important for optimizing crop production, should not reduce the concentration of taxanes.

The dioecious plant Silene latifolia depends on nocturnal insects for pollination. To increase the chance of cross-pollination, pollen grains seem to be released and stigmas seem to be receptive simultaneously at night. We divided the floral development of S. latifolia into 1-20 stages, and determined the timetables of male and female function. The corolla of both male and female flowers opens at sunset (1900 hours) and closes at sunrise (0900 hours). To investigate the period of the reproductive phase of male and female function, we measured the germination rate on a pollen medium and the pollen germination rate on stigma during the period when stamens and stigmas were viable in the timetable. Male flowers had early- and late-maturing stamens that had the highest pollen viability, germination rate and pollen tube growth at midnight (0000 hours) at 1 day after flowering (DAF) and 0000 hours at 2 DAF. In contrast, female flowers maintained a germination rate of nearly 100 % from 1800 hours at 1 DAF to 1200 hours at 3 DAF. These results suggested that S. latifolia transferred the matured pollen grains from male flowers to female flowers only at night.

Sex is an ecologically important form of genetic variation in dioecious plants, with males and females generally differing in constitutive resistance to herbivores. Yet little is known about sexual dimorphism with respect to induced or indirect defense, or whether sex-based differences are underlain by trade-offs among modes of defense. We compared male and female Valeriana edulis plants for constitutive and induced direct resistance to two herbivores, an early-season caterpillar and a late-season aphid, and for constitutive and induced indirect resistance in terms of abundance of natural enemies and aphid-tending ants. No sexual dimorphism was found in constitutive direct plant resistance, yet the sexes differed for constitutive indirect resistance, with 78 % more natural enemies and 117 % more ants present on females than males. Past feeding damage by caterpillars induced direct and indirect resistance in both males and females, increasing caterpillar development time by 26 % and the abundance of natural enemies by 147 %. Caterpillar feeding did not induce direct resistance with respect to caterpillar final mass or aphid performance. In all cases, there were no interactions between the effects of caterpillar damage and plant sex. In summary, plant sexual dimorphism and induced responses to herbivore damage independently influenced herbivore performance and the composition of arthropod communities at higher trophic levels.

Hedyotis caerulea possesses two distinct floral morphs that are generally found in equal numbers in naturally occurring populations. Flowers either possess a relatively long style and short anthers, called a “pin,” or a short style and long anthers, called a “thrum.” This placement of reproductive organs is considered herkogamous and distylous, as it encourages outcrossing by restricting pollination to individuals of the alternate morph. Numerous species have been described as distylous without quantitative data establishing stigma-anther reciprocity. Here we assess those assumptions in H. caerulea by measuring stigma height, anther height and a suite of additional floral traits across multiple localities. All populations surveyed were isoplethic, although variation among them was present in all floral traits measured as well as for pollen diameter, pollen count, flower dry weight, and seed set. Pins produced smaller pollen than thrums, but made more of them. Thus, the total volume of pollen was similar for pins and thrums, and seed set was similar, suggesting that each morph has equal male and female fitness with no movement towards dioecy. Given a significant degree of variation found in the morphometric analysis, and that two of the three measures used to assess reciprocity were not consistent with predictions of precise symmetry, extensive change is possible where selection is acting on these traits. Even so, the distylous mating system in H. caerulea appears to be stable.

Distyly has been interpreted as a mechanism that promotes cross-pollination among conspecific plants and as one of the routes leading to the evolution of dioecy. In one of the possible evolutionary pathways, pollinators may disrupt intermorph pollen flow, and, as a consequence, floral morphs may gradually specialize as either male or female (functional dioecy). Natural patterns of pollen deposition and fruit and seed production were estimated in Palicourea demissa (Rubiaceae) and used as parameters to assess functional gender differences between floral morphs. Pollen flow was asymmetrical in P. demissa. Long-styled flowers were more effective than short-styled flowers in pollen deposition towards compatible stigmas, whereas short-styled flowers were more effective in legitimate pollen receipt. Accordingly, short-styled plants produced more fruits and viable seeds than long-styled plants. The contributions of male and female function to the potential functional gender were equivalent in both morphs. However, the realized functional gender deviated significantly from the potential functional gender in both morphs, in which short-styled plants were more successful through their female function, but long-styled plants through their male function. If pollinators disrupt the complementarities of pollen transfer between the two morphs (asymmetric pollen flow), the expression of a more profitable gender is expected in each morph. Thus, our results support the hypothesis that dioecy may evolve in distylous populations through the gradual specialization of each morph as either male or female.

The evolution of dioecy from heterostyly has been well documented, but detailed studies on this transitional process are rare. Here we report the occurrence of cryptic dioecy in a perennial liana species with stigma-height dimorphism, Morinda parvifolia Bartl. ex DC. (Rubiaceae). Floral morphology, ancillary characters and cross compatibility of long-styled (L-morph) and short-styled (S-morph) were examined. L-morph and S-morph display obvious pistil dimorphisms, with the stigma of S-morph lacking papillae cells. Both floral morphs show similar pollen morphology, although pollen viability is higher in S-morph than in L-morph. S-morph flowers produce viable pollen grains but much reduced stigma and set no fruits, functioning as males; L-morphs, although with viable pollen grains and receptive stigmas, exhibit strong self- and intramorph incompatibility, with self- and intramorph pollen tubes arrested in the stigma lobes and the upper part of style, respectively, resulting in L-morphs functioning only as females. The species thus has physiological androdioecy but functional dioecy. This might be the first case showing the possibility that androdioecy could be a mid-stage in the pathway of dioecy evolving from stigma-height dimorphism.

This paper examines the history of sex ratio theory and the effects of multiple variables on individual and population sex ratios. It also provides examples where plants have been used to test major predictions of sex ratio theory. Then, using over 200 studies from the literature, dioecious plant species are categorized based on their life form, pollination agent, fruit dispersal agent, and sex ratio. A loglinear analysis is used to look at possible correlations between the sex ratio of a population and other life history characteristics. These data are used to examine the predictions made by De Jong et al. (Journal of Evolutionary Biology 15:7, 2002), that relative pollen and seed dispersal distances can be used to predict sex ratio bias. Despite the limited sample size, strong relationships are still observed. 93% of insect pollinated dioecious vines that have biotically dispersed fruit have male-biased sex ratios. Conversely, 61% of shrubs that are wind pollinated and have abiotic fruit dispersal have female-biased sex ratios.

The Random Amplified Polymorphic DNA (RAPD) technique was used to amplify DNA segments, with the objective of finding markers linked to sex determination in male and female plants of Piper betle L. Two bulks of DNA were made drawing one each from male and female, by pooling an equal volume of DNA samples from each group of individual contributing to the bulk analysis. Fifty different random decamer primers were screened with the two bulks to identify markers associated with sex expression of which only four primers were found to be associated with sex expression. These four primers were then tested with individual plant DNA samples where sex-associated RAPD markers were identified. A ~1,400 and ~850 bp fragment from the primer OPA04 and OPN 02 respectively was found to be present in all the male individuals and absent in all the female plants. In another primer, a ~980 bp amplification product from the primer OPC 06 was present only in the female individuals. A common primer OPA 08 showed both male and female specific markers of 650 and 1,200 bp respectively. Thus, the three male- specific RAPD markers OPA041400, OPA08650 and OPN02850 and two female-specific markers OPA081200 and OPC06980 can reliably differentiate the male and female plants of P. betle L. Ploidy comparison also showed the differences in male and female plants.

The breeding system, floral morphology, morph frequency, biomass allocation to floral whorls, pollination and reproductive efficiency were examined in four distylous and three monomorphic Melochia species. Monomorphic species were self-compatible and distylous species were self-incompatible. Flowers of homomorphic species were longer than those of distylous species. Herkogamy was significantly higher in pin than in thrum morphs of the two distylous species, and monomorphic species exhibited the lowest values of herkogamy. Pollen/ovule ratios were similar between monomorphic and dimorphic species, irrespective of the self-incompatibility level. Biomass allocation to flowers was biased toward non-sexual structures, attraction and support. Androecium biomass and androecium/gynoecium biomass ratio were larger in thrum than in pin flowers of distylous species indicating maleness in the thrum morph and femaleness in the pin morph. There was no clear difference between fruit set of monomorphic and dimorphic species; however, the greatest fruit set was found in the monomorphic species, M. pyramidata, which is a self-compatible species. Fruit set was significantly higher in pin than in thrum morphs in three out four distylous species and fecundity was only significantly higher in pin morphs of M. caracasana and M. parvifolia. Melochia species have generalist pollination systems. According to the taxonomic classes and number of pollinator species, M. caracasana and M. parvifolia have similar generalist pollination systems. Our comparative analyses of the characters between floral morphs of distylous species and the relationship with these characters in monomorphic species allow divergences and similarities to be established and different evolutionary trends to be postulated in the breeding systems of Melochia species. Specifically, M. parvifolia and M. caracasana are apparently biased toward monomorphy and dioecy, respectively.

Elucidating the mechanisms of species coexistence is a crucial goal in ecology. Theory suggests that, when resource abundance fluctuates, coexistence can be achieved if each species in a competing pair is better at exploiting resources at opposite extremes of a fluctuating resource spectrum. Nonetheless, the proximal mechanisms allowing coexistence remain largely unexplored. In a previous paper, we showed that the coexistence of two Atriplex species was facilitated by their varying demographic response (in survival, growth and recruitment) to fluctuation in water availability. Here we explore the effect of spatial distribution, and pollen and resource limitation on the reproductive success (production of viable seeds) of the same two species. An analysis of their spatial distribution showed that Atriplex acanthocarpa had a clumped distribution, which is thought to increase the effectiveness of pollination in wind-pollinated plants, while Atriplex canescens had a random distribution, a pattern expected to restrict wind-pollination success. A pollen and resource (water and nutrients) addition experiment implemented through a repeated-measures design demonstrated that seed viability of A. canescens was both pollen and resource limited, but that these effects were negligible in A. acanthocarpa. Under natural conditions, pollen limitation restricted seed number in A. canescens to only one-third of that recorded when manual pollination was performed. By decreasing its fecundity (and consequent potential seedling recruitment), pollen limitation reverses the competitive advantage of A. canescens over A. acanthocarpa when the limiting resource (water) is abundant and seedling recruitment takes place. To our knowledge, our study of this congeneric pair in the Chihuahuan Desert is the first to document a link between pollen limitation and species coexistence.