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Phoradendron hexastichum is a bird-dispersed mistletoe that infects the dioecious tree Cecropia schreberiana. Because both species share frugivore seed dispersers, we hypothesized that female Cecropia would have a greater probability and intensity of mistletoe infection than males due to more frequent visitation by shared frugivores. Over 50% of female Cecropia were infected, in contrast with 25% of males. On average, female trees had twice as many mistletoes as male trees. Infection probability and intensity increased with basal area in females but not in males, suggesting that lifetime reinfection was also female biased. We found mistletoe frugivores visiting uninfected fruiting females twice as often as males. Although mistletoes were mostly consumed by the mistletoe specialist Euphonia musica, we did not record Euphonia visiting uninfected Cecropia trees. Uninfected Cecropia trees were frequently visited by generalist frugivores (such as Spindalis portoricensis) that used both mistletoes and Cecropia fruits. The Cecropia–frugivores– Phoradendron network of interactions seems to have led to the spatial linkage of the two plant species through directional dispersal, to plant–plant facilitation through shared frugivores, and to bird–bird facilitation in which generalist frugivores start new foci of infection that specialist frugivores can use.

Most flowering plants rely on animal pollinators to transfer male gametes between individuals, and thus a significant problem for gender dimorphic plants is that pollinators often avoid female flowers. Here we show for the first time that one important reason pollinators shun female flowers is because they do not smell like males. We compared emission rates and floral scent composition in a gynodioecious wild strawberry (Fragaria virginiana) where females receive half as many visits by generalist pollinators as conspecific hermaphrodites. We used floral extracts to determine the source of sexually dimorphic odor and pollinator responses. Specifically, we used extracts of whole flowers and specific floral parts in choice tests to determine that pollinators preferred the scent of hermaphrodite flowers over those of females and that this discrimination was due primarily to the scent of hermaphrodite anthers. These data conclusively show that scent can be a major driver of pollinator behavior in gender dimorphic plants. Our results also indicate that scent is an important modulator of pollinator behavior even in a small flowered, weakly scented species visited by generalist pollinators, and not just peculiar to intensely scented, deceptive, or specialized pollination systems.

We explore the effects of gender dimorphism on resistance (damage avoidance) and tolerance (growth and fitness compensation) to a flower bud clipping weevil in a gynodioecious wild strawberry. Using both natural populations and a common garden, we document pervasive hermaphrodite-biased damage and identify several floral traits associated with resistance. Lower flower number, lower pollen production per flower, and earlier flowering date were associated with higher resistance. Because the sex morphs do not differ in flower number or flowering time, the presence of pollen is the main factor determining sex-differential resistance. We confirmed that simulated clipping was a good surrogate for weevil clipping, and we used simulated clipping to assess sex-morph- and sex-function-specific tolerance. We found that females were less tolerant than hermaphrodites in terms of seed and fruit production, but the sex morphs were similarly unable to compensate fully in flower production. Hermaphrodites showed some ability to compensate in pollen per flower, but this was not large enough to offset flower losses, leading to net undercompensation for pollen per plant. We evaluated potential mechanisms for tolerance and found that the number of reserve buds was the most consistent predictor of tolerance. Finally, we found that herbivory shifted sex expression of hermaphrodites toward greater femaleness via both plastic changes in allocation and direct effects of loss of male-functioning flowers. Our work indicates that the effects of herbivory on sexual system evolution can include sexual phenotype-biased herbivore damage and sex-dependent tolerance, as well as herbivory-induced changes in hermaphrodite sex allocation. In addition, this study suggests that pollen production is subject to selection via herbivores that opposes selection mediated by pollinators.

A richly sporophyte-producing population of the dioecious perennial clonal moss Hylocomium splendens was followed for five years in order to investigate the longterm costs of sporophyte production. Female mature segments were divided into two subpopulations: sporophyte producing and reference without sporophytes. Ultimate (long-term) population growth rates of l 5 1.091 and 1.258, respectively, were found for the two subpopulations by building separate deterministic transition matrix models. By bootstrapping each subpopulation, the difference in l between subpopulations was found to be significant (P , 0.001). Life-table response experiment analysis attributed the lower population growth rate of the SP subpopulation to inferior ramification rates, lower survival, and inferior size development of mature segments. Considerable variation in H. splendens sporophyte production among years indicates large among-year variation in investment in sexual reproduction, probably related to resource status. Establishment of H. splendens from spores in natural habitats has never been observed. Nevertheless, sexual spores may play a role in establishment of H. splendens populations on new substrates. Key words: bootstrapping; boreal forest (southeastern Norway); bryophyte; clonal plant; elasticity; Hylocomium splendens; life-table response experiment; matrix models; models, transition matrix; population growth rate; sexual reproduction; somatic costs.

Evolutionary biologists have repeatedly argued that environmental factors have influenced the evolution of dioecy (males and females) from hermaphroditism in plants. While many researchers have focused on the role of pollinators and abiotic factors, far fewer have considered the enemy component of the environment. In this paper, I explore how herbivory may impact the evolution of dioecy from hermaphroditism through gynodioecy. This synthesis shows that the widespread occurrence of male-biased enemy attack in sexually dimorphic species is likely to have significant consequences for both the first and second steps in the evolution of dioecy through gynodioecy. I identify several ways that herbivory can influence sexual-system evolution, review the handful of studies that have explored them, and highlight exciting new avenues for research. From this review and synthesis, I conclude that consideration of enemies will provide a fresh dimension to our understanding of plant sexual systems.

Heterodichogamy, that is, dimorphic sex expression of protogynous (stigmas are receptive before pollen is shed) and protandrous (pollen is shed before stigmas are receptive) mating types provides a possible evolutionary pathway to dioecy (dimorphism of males and females) in plant species. I investigated whether the gender of protogynous or protandrous morphs is specialized in male or female function in a population of a heterodichogamous maple, Acer japonicum (Aceraceae), by examining the phenology of sex expression and phenotypic gender in two morphs. Most of the trees in the study (>90%) were bisexual and from either the protogynous (PG) or the protandrous (PA) type. The remaining trees (

The evolution and maintenance of combined vs. separate sexes in flowering plants is influenced by both ecological and genetic factors; variation in resources, partic- ularly moisture availability, is thought to play a role in selection for gender dimorphism in some groups. We investigated the density, distribution, biomass allocation, and physi- ology of sympatric monomorphic (cosexual) and dimorphic (female and male) populations of Wurmbea dioica in relation to soil moisture on the Darling Escarpment in southwestern Australia. Populations with monomorphic vs. dimorphic sexual systems segregated into wet vs. dry microsites, respectively, and biomass allocation patterns and physiological traits reflected differences in water availability, despite similarities in total ramet biomass between the sexual systems. Unisexuals flowered earlier at lower density, and they allocated sig- nificantly more biomass below ground to roots and corms than did cosexuals, which al- located more biomass above ground to leaves, stems, and flowers. Females, males, and cosexuals produced similar numbers of flowers per ramet, but unisexuals produced more ramets than cosexuals, increasing the total number of flowers per genet. Contrary to ex- pectation, cosexuals had significantly higher (more positive) leaf carbon isotope ratios and lower leaf nitrogen content than unisexuals, suggesting that cosexuals are more water-use efficient and have lower rates of photosynthesis per unit leaf mass despite their occurrence in wetter microsites. Cosexuals appear to adjust their stomatal behavior to minimize water loss through transpiration while maintaining high investment in leaves and reproductive structures. Unisexuals apparently maximize the acquisition and storage of both water and nitrogen through increased allocation to roots and corms and enhance the uptake of CO2 by keeping stomata more open. These findings indicate that the two sexual systems have different morphological and physiological features associated with local-scale variation in water availability.

Bias in the adult sex ratio of a population, if not due to a biased primary sex ratio, may be due to differential mortality of males and females, which can occur during any life history stage. Yet, although adult sex-ratio bias is common in dioecious plant species, little is known about the demography of prereproductive individuals. Using a RAPD-PCR (randomly amplified polymorphic DNA) marker that co-segregates with female phenotype, I studied juvenile demography in the saltmarsh grass Distichlis spicata, which exhibits within-population sex-ratio variation (spatial segregation of the sexes) and has genetic sex determination. First, I surveyed populations of D. spicata to ascertain whether any environmental factor co-varies with local sex ratio, and I found that habitats with female majorities are located at significantly lower elevations in the marsh than are habitats with male majorities. In randomly collected seeds, the sex ratio was significantly biased toward females, but male and female seeds did not differ in mass. Using a subsample of these seeds, I conducted greenhouse and field experiments to examine whether differences in seed germination or seedling survival between the sexes explain sex-ratio variation along the micro-elevation gradient in the marsh. In greenhouse treatments designed to mimic field habitats with female majorities, more male seeds germinated than did female seeds. In habitats with female majorities in the field, significantly more female seedlings survived an extreme high tide than did male seedlings (25.17% of females vs. 13.49% of males). These results suggest that, in habitats with female majorities, gender-specific bias in seedling survivorship, rather than seed germination, is a causal factor in the underlying sex-ratio bias. In habitats with male majorities, seed germination and seedling survival did not differ between the sexes. This study is the first to document environment-dependent differential seed germination and seedling mortality between males and females in a dioecious plant species.

Gender in flowering plants is governed by a complex interplay of genetic and environmental factors. The perennial aquatic herb Sagittaria latifolia displays remarkable variation within and among populations in gender expression, from monoecious populations composed of plants with differing numbers of female and male flowers to completely dioecious populations with separate female and male plants. Here we examine the role of environmental factors in modifying gender expression in these two sexual systems by investigating the relation between ramet size and flower number in three sex phenotypes (hermaphrodite, female, and male). In particular, we assess the extent to which hermaphroditic plants have the option of altering the number of female and male flowers to accommodate their specific environmental circumstances. We sampled 12 populations from diverse wetland habitats in southern Ontario, Canada, estimated their phenotypic gender, and examined the relations between ramet size and a range of vegetative and floral traits. On average, plants from dioecious and monoecious populations did not differ in leaf length, a correlate of ramet size. However, in monoecious populations with significant numbers of both male and hermaphroditic ramets, hermaphrodites were larger and produced more flowers than males. This contrasting pattern was also observed when plants were grown under glasshouse conditions. In monoecious populations, variation in ramet size did not affect the production of male flowers, whereas female flower production varied positively with plant size. These relations enabled statistical prediction of the dependence of gender on plant size and the frequency distribution of gender within monoecious populations. These relations also imply that the male phenotype can be determined environmentally, whereas the female phenotype cannot. The size dependence of floral sex ratios in monoecious populations provides novel insights into the likely evolutionary pathway by which dioecy has evolved from monoecy in Sagittaria.

We studied components of reproductive success in two dioecious fig species, Ficus exasperata and F. hispida, at a deciduous and an evergreen site in south India, over a two-year period. Unlike monoecious figs, which produce a mixture of wasps, seeds, and pollen in every fruit, dioecious fig species have hermaphrodite (functionally male) trees, which produce wasps and pollen only, and female trees, which produce seeds only. Di- oecious fig species are free of the seed-wasp trade-off involved within each monoecious fig fruit; wasp and seed production in dioecious species reflect selective pressures acting on each sex to maximize offspring production. We studied specific predictions of the reproductive behavior of dioecious fig species, predictions that were made based on studies of monoecious figs. Pollinator (and associated parasitic wasp) arrivals at trees bearing receptive inflores- cences were closely linked to the seasonal variation in receptive fig numbers. More wasps arrived at female trees of F. exasperata than at "male" trees, apparently because of syn- chronized wasp release by male trees during the time of synchronous female tree receptivity. Wasp numbers arriving at male and female trees of F. hispida did not differ significantly, apparently because inflorescence production on male and female trees was more evenly spread over time. Female inflorescences had more pollinators than male inflorescences in F. hispida; female and male inflorescences had approximately equal numbers of pollinators in F. exasperata. Hand-pollination experiments introducing one, three, or eight pollinators into figs of F. hispida showed that foundress number was a significant predictor of resultant wasp or seed number. In both fig species, female inflorescences initiated more flowers than male inflorescences, and produced more seeds than male inflorescences produced wasps. Female fruits were not larger than male fruits at maturity in either fig species. Only in F. exasperata were male fruits significantly smaller in the dry than in the wet site. Both species had a relatively high average level of pollination (>70%). The mean percentage of fruit matured was 56% in F. hispida, and a surprisingly high 78% in F. exasperata. Both fig species exhibited strong sex differences in foundress numbers, numbers of flowers initiated, and numbers of wasps or seeds matured, suggesting strong selection on components of reproductive success within each sex after dioecy evolved. Our quantitative analyses elucidate how some of the many factors in plant-pollinator relationships affect plant reproductive success in the fig-wasp mutualism.

The main goal of this study was to evaluate sex expression at geographic and individual levels in Ochradenus baccatus, an Israeli desert shrub. Although this species was formerly thought to be strictly dioecious, in a survey of 24 Ochradenus populations along a latitudinal rainfall gradient, we found sex expression to be extremely variable. While females reproduced only by seeds, males ranged across a continuum from "pure males" that produced only pollen to "inconstant males," which produced seeds in addition to pollen. For sexual dimorphism to evolve from cosexuality, it is believed that the sex morphs must be compensated for their loss of one sex function (e.g., pollen production) by an increase in the remaining sex function (i.e., seed production). In our study we found that females produced 9-20 times as many seeds as males, and we showed in a controlled laboratory experiment that their offspring had higher germination and seedling growth rates. Pure males invested more biomass in stamens than did inconstant males. Previously published models concerned with the evolution of sexual dimorphisms have predicted that: (1) the frequency of inconstant males should be inversely related to the frequency of females, and (2) if the degree of male inconstancy is influenced by environ- mental factors, then seed production in males should decline along with habitat quality. We found support for both of these predictions. Although the sex ratio did not differ significantly from 1:1 in 19 of the 24 populations surveyed, the frequency of male incon- stancy did vary geographically. The fraction of males that produced fruit in a population declined on a north-south gradient that reflected a three-fold decline in rainfall. Among populations, the frequency of females was negatively correlated with the frequency of inconstant males. We postulate that the advantages of conditional fruit production in males (i.e., ability to produce seeds during periods of unpredictable pollinator service) will result in gynodioecy and male inconstancy remaining a stable feature of populations of 0. bac- catus.

A growing body of research documents the importance of plant genetic effects on arthropod community structure. However, the mechanisms underlying these effects are often unclear. Additionally, plant genetic effects have largely been quantified in common gardens, thus inflating the estimates of their importance by minimizing levels of natural variation. Using Valeriana edulis, a dioecious plant with genetically based sex determination, we conducted surveys and experiments on wild-grown individuals to document field patterns of arthropod association between the sexes and the mechanisms underlying these plant genetic effects. Three years of surveys revealed strong and consistent sex-biased arthropod association in wild-grown plants: female plants supported 4-fold, 1.5-fold, and 4-fold higher densities of aphids, aphid predators, and aphid-tending ants, respectively, compared to males. There was mixed evidence that the female bias for aphids was due to higher plant quality, while we found no difference between plant sexes in aphid preference or the top-down effects of predators and tending ants. Female bias for ants was due to both the greater attractiveness of female plants (direct effect mediated by floral nectar) and an independent, weaker effect of higher aphid abundance on females (density-mediated indirect effect). Conversely, the female bias for predators was driven solely by the greater attractiveness of female plants. We did not find interaction modification, i.e., ant-aphid and predator-aphid interactions were equivalent between plant sexes. Plant sex explained 0.24%, 2.28%, and 4.42% of the variance in aphids, predators, and ants, respectively, values comparable to but slightly weaker than those previously reported from common-garden studies. In contrast to the prediction of diminished plant genetic effects with increasing trophic level, we show how weak indirect effects on predators and parasitoids (via herbivores) can be complemented by strong direct effects via common plant traits (floral resources). In summary, we document direct and indirect effects of genetically based sex on a multi-trophic arthropod community that were expressed in wild- grown plants across multiple years.

The angiosperm flora of the Hawaiian Islands, with its high incidence of dioecy, has been central in development of hypotheses about the evolution of dioecy. Based on a recent taxonomic treatment of the Hawaiian angiosperm flora, we analyzed biogeo- graphic patterns of dioecy as well as the association of dioecy with ecological traits po- tentially important in the evolution of dioecy for native current species and genera. We also analyzed patterns for presumed colonists of the flora to control better for phylogenetic patterns in these traits. Dioecy is associated with woodiness, especially trees; however, this pattern does not hold for gynodioecy. Within woody species, dimorphism (dioecy, gyno- dioecy, subdioecy, polygamodioecy) is associated with mesic habitats and occurs more frequently in species with lowland and lowland-montane distributions. In contrast, in the endemic Hawaiian Alsinoideae (Schiedea and Alsinidendron) and in Bidens (two groups with autochthonous evolution of dimorphism), dimorphism is associated with more xeric habitats, and for the former group, with wind pollination. For the entire angiosperm flora, dimorphic species are associated with flowers that are small and green. Woody dioecious species and genera are disproportionately associated with wind pollination; hermaphroditic species are disproportionately bird pollinated primarily because of one species-rich lineage in the Campanulaceae. There was no association of dimorphism and pollinator type for colonists. Because of a few species-rich lineages, dimorphism is associated with dry fruits at the specific level, but at the generic level and for colonists, dimorphism is associated with fleshy fruits. Dioecious and gynodioecious species are found more often on older islands, a result of speciation of dimorphic colonists as well as autochthonous evolution of dimorphism. Single- island endemism is not associated with dioecy in the flora as a whole or with dimorphism within lineages evolving dimorphism autochthonously; thus Baker's law is not supported within the Hawaiian Islands. Because the Hawaiian angiosperm flora originated from a minimum of only 291 colonists, many of the associations of dimorphism with ecological traits occur because of the influence of only a few species-rich lineages. Better knowledge of phylogenetic patterns and further ecological studies, particularly within those groups evolving dioecy autochthonously, are needed to determine causality.

Because of its presumed high levels of dioecy (separate male and female plants), study of the native Hawaiian angiosperm flora has been important in development of many hypotheses about conditions favoring the evolution of dioecy. The importance of ecological correlates with dioecy has proven difficult to assess, however, because of lack of data on the origins of dioecy in the Hawaiian Islands. Clearly, these correlations are of greater interest in taxa where dioecy evolved in the Hawaiian Islands (autochthonous evolution of dioecy) than in taxa that are the result of dioecious colonists with subsequent speciation in the Hawaiian Islands. Because the Hawaiian flora is small and extremely isolated, colonists can be identified and their breeding systems hypothesized, thus allowing inferences on the origins of dioecy. Using current taxonomic information, the incidence of dioecy in native Hawaiian angiosperm species is 14.7%, lower than previous estimates, but still the highest of any known flora worldwide. Ten percent of the colonists were sexually dimorphic (dioecious, gynodioecious, polygamodioecious, and subdioecious), and over half (55.2%) of current dimorphic species are in lineages arising from dimorphic colonists, showing that dimorphism is high in part because colonists were dimorphic. Autochthonous evolution of dimorphism occurred in at least 12 lineages (e.g., hermaphroditic colonists of Bidens (Asteraceae), the Hawaiian Alsinoideae (Caryophyllaceae), and Hedyotis (Rubiaceae) led to species-rich lineages that include many dimorphic species). One-third (31.8%) of current dimorphic species are in lineages arising from monomorphic colonists. Dioecy in the Hawaiian Islands is a result of both dimorphic colonists as well as evolution of dioecy in Hawaiian lineages from hermaphroditic colonists. The high incidence of dimorphism is not because dimorphic colonists evolved more species per colonist than monomorphic colonists. Detailed studies of individual lineages are critical to elucidate causal factors in the evolution of dioecy.

The ecological basis for breeding-system evolution was explored in Ecbal- Hum elaterium (L.) A. Rich. (Cucurbitaceae). Two subspecies are parapatric on the Iberian Peninsula: dioicum (dioecious) is found in southern, more arid regions and elaterium (mon- oecious) is found in northern, wetter regions. Six experimental gardens were set up in three sites that span the climatic gradient of the species' natural range in Spain in order to address two issues: (1) gender specialization in reproductive characters and life history, and (2) ecological specialization of the two subspecies across environments. Overall, the dioecious subspecies outperformed the monoecious subspecies. Monoecious plants, on average, were shorter-lived in all environments. Females of the dioecious sub- species produced similar numbers of female flowers, but more fruits and seeds, than did monoecious plants (cosexuals). The fruits of females contained more, but smaller, seeds than the fruits of cosexuals. Males always produced more male flowers than did cosexuals, disproportionately so with increased plant size (canopy volume); no such disproportionate increase in seed production was found in females. A geographic analysis of gender (proportion of female flowers) in the monoecious plants revealed a striking pattern: dimorphism with increased gender differentiation in drier sites, in contrast to monomorphism with respect to gender in wetter sites. Differences between the subspecies and between habitat types can be related directly to breeding-system differences. The higher overall performance of dioecious plants may be due to enhanced heterozygosity resulting from outcrossing enforced by dioecy. On the other hand, the gender differentiation of the monoecious plants in the drier sites may reflect gender allocation "decisions" enforced by limited resource availability.

Females of dioecious plant species are expected to show greater trade-offs with other life history traits than males because of their high investment in reproduction. We investigated life history traits and resource allocation patterns for both males and females of the dioecious plant Silene latifolia, to document allocation both prior to and after re- productive investment. In addition, the impact of fruit production on subsequent allocation to reproduction and growth was investigated by varying the number of flowers pollinated on females. The mean date of first reproduction was the same for males and females. Early- flowering plants of both sexes invested less in leaf biomass than plants that delayed flow- ering. The percentage of flowers pollinated had a significant effect on how resources were allocated, and on the amount and timing of further flower production. Sexual dimorphism was observed in resource allocation patterns after, but not prior to, investment in repro- duction. Overall, pollinated females were found to have more leaf biomass and invest nearly twice as much in reproduction as males, in spite of an apparent trade-off between leaf production and reproduction within both females and males. We conclude that sexual di- morphism in life history traits that affects resource acquisition causes the resource pool that is allocated to be variable, rather than static, thereby masking the underlying trade- offs.

In dioecious plants, intersex differences in the frequency and size dependence of flowering may often be important proximate determinants of a population's flowering sex ratio. Such differences have been little investigated among Southeast Asian rain forest trees, where dioecy is perhaps best represented in the world's flora. In this study we recorded flowering activity and sex of reproductive individuals in two separate flowering seasons for 2600 trees representing three species of Aporusa and two species of Baccaurea (all Euphorbiaceae) in a primary rain forest in peninsular Malaysia. We found neither sexually mixed trees nor sex switching of trees between years. Flowering sex ratios for the species of smallest stature, A. microstachya and B. parviJflora, consistently exhibited a significant degree of male bias, which was greater in years with lower overall levels of flowering in the population. Two-year cumulative sex ratios were significantly male biased in these two species and in a second relatively small-statured species of Aporusa. The size distributions for male trees broadly overlapped those of female trees in all species. In the smaller statured species, male trees displayed a significantly greater degree of relative size variation than did female trees, suggesting that male trees begin flowering at a smaller size, but also grow to a larger size, than do females. Also, males were significantly more likely to flower in both years than were females in the smaller species studied. Sexual dimorphism in the frequency and size dependence of flowering has previously been explained as a result of higher energy costs of reproduction in females than in males. We suggest that this physiological constraint is most likely to play a strong role in energy- limited environments. We therefore predict that, as found in this study, male-biased sex ratios and associated patterns of sexual dimorphism may generally be most pronounced among diminutive treelets of the rain forest understory.

Flowering sex ratios of dioecious plants are commonly male-biased but rarely female-biased. While greater costs of reproduction from females have been repeatedly demonstrated and explain male biases, male reproductive costs almost never exceed female costs, making the origins of female biases enigmatic. I investigated the seagrasses Phyllospadix scouleri and P. serrulatus (surfgrasses), which have some of the most extreme female-biased sex ratios documented (>90% female), to identify the mechanisms driving sex ratio bias. I developed sex-linked amplified fragment length polymorphism (AFLP) markers and applied them to three P. scouleri life stages at four sites to determine when during the life cycle sex ratio bias arises. Sex ratios were even among seedlings but became more female-biased at later life stages, indicating that sex ratios were driven by male-biased mortality. To identify when during the life cycle sex ratio bias developed, I examined sex differences in survival among seedlings and three aspects of reproductive costs that could potentially generate biased sex ratios under field conditions. No differences in seedling survival between the sexes were detected, and there was no evidence of substantial sex differences in costs of reproduction. I found no support for a trade-off between current and future reproduction or between reproductive investment and growth. Thus, costs of reproduction appear unlikely to drive sex ratio bias in surfgrass. Instead, small sex differences in growth and survival spread across the life cycle appear to be responsible for female-biased sex ratios and suggest that life history trade-offs other than reproductive costs drive sex ratio bias.

A well-known, but largely untested, prediction in plant reproductive ecology is that dioecious taxa should produce larger, more, higher-quality, or better-defended seeds than cosexual taxa. Using a data set composed of 972 species in 104 families, representing the flora of the Tambopata Wildlife Reserve (Madre de Dios, Peru), we evaluated the first component of this prediction, examining ecological and evolutionary relationships between breeding system and mean seed size with two kinds of tests. First, we conducted cross-species analyses to determine whether species with different breeding systems differed significantly with respect to mean individual seed size. Second, we used a hypothesized phylogeny to identify pairs of the most closely related taxa or clades within the Tambopata community that differed with respect to breeding system. Comparing pair members allowed us to determine whether evolutionary divergence in breeding system (between taxa with unisexual vs. cosexual individuals) was consistently associated with evolutionary change in seed size. In both analyses, we controlled for potentially confounding effects of growth form by examining these relationships within woody and nonwoody taxa. Cross-species analyses revealed that dioecious species produced larger seeds than cosexual species among woody species, shrubs, lianas (each growth form analyzed separately), and all species pooled, but not among trees. Phylogenetically independent contrasts upheld the significant association between breeding system and seed size among woody taxa, lianas, and all taxa pooled, but not among shrubs. We discuss the implications of our findings for evolutionary hypotheses regarding associations between dioecy and seed size.

10 páginas, 1 tabla, 5 figuras., Intraspecific variation in resistance to herbivory among genders and life-history phases of primary producers can significantly alter the ecological and evolutionary consequences of plant–herbivore interactions. Seaweeds (macroalgae) with complex life histories have multiple distinct phases with associated variation in traits that can potentially lead to differences in resistance to consumers and provide a unique system in which to simultaneously test the effects of sex and life-history stage on herbivory. We tested the susceptibility to grazing of the three life-history stages and separate sexes of the chemically defended red alga Asparagopsis armata against the sea hare Aplysia parvula, and we related this to the plant quality traits of different stages and genders. Differences in nutrient content and halogenated secondary metabolites between life-history phases were highly sex dependent. Male gametophytes had a low concentration of secondary metabolites and the highest nutrient content. The highest secondary metabolite content was found within the female gametophyte, in the wall of the reproductive structures (cystocarps) that contain the microscopic carposporophyte phase. Feeding choices by A. parvula were consistent with differences in algal quality and defense and resulted in the haploid male gametophytes being the most preferred food type. The diploid carposporophyte found inside the chemically rich cystocarps was the least consumed life-history stage. Selective herbivory of male gametophytes by A. parvula is consistent with an observed shift in gametophyte sex ratio in the field from unity at the beginning of the reproductive season to female bias at the end. The variation in susceptibility to herbivory found between sex and life-history stages of A. armata represents the first example of sex-biased consumption in seaweeds and may contribute to the maintenance of complex life histories such as those found in red algae., This work was partially supported by an FPI Postgraduate Award to A. Verge´s from the Spanish Ministry of Science and Education (REN-2002- 04020-C02) and the Centre for Marine Bio-Innovation.

We document through correlative and experimental analyses sex—biased herbivory in the arroyo willow, Salix lasiolepis. Male willows typically support higher densities of five species of tenthredinid sawflies than do female willows. Significant intersexual variation sawfly densities exists in naturally occurring willow clones and in clones grown in a common garden. Arroyo willow is sexually dimorphic in leafing phenology and in aspects of leaf chemistry. Male willows have significantly longer vegetative shoots than do females at the time of sawfly oviposition (P = .01). Males also have significantly higher foliar concentrations of nitrogen, phosphorus, and potassium than do females. Female willows have a marginally significant (P = .093) higher concentration of total phenols. Of the dimorphic traits, only variation in leaf phenology is a consistent predictor of variation in sawfly densities among willow clones. See full-text article at JSTOR

The reproductive success of an individual plant is in part a function of its capacity to assimilate resources, and this in turn is partially a result of plant morphology. Simmondsia chinensis, jojoba, a dioecious desert shrub, is dimorphic in secondary sexual characters in some desert populations, where females on average have larger leaves and more open canopies than males. The function of this sexual dimorphism was examined by investigating the importance of shoot morphology to reproductive components of plant fitness. The reproductive behavior of individuals tended to be associated with different morphological attributes for both males and females. Males with more “male—type” morphology tended to have more inflorescences and a greater likelihood of flowering than did males structured more like females, though they also had smaller inflorescences. Females with more “female—type” construction produced more flower buds in one of the study years and also produced heavier seeds than females with more male—like structure. Females from four different morphological classes allocated 1.5–4.5 times as much to reproduction as did males, but showed wide variability in whether or not they set fruit. Therefore females either had a much higher or a much smaller reproductive allocation than most males. Fruit set in the population studied is not limited by pollen availability, but is presumably constrained by abiotic resource availability. The asymmetry between sexes in the morphology of reproductively more successful individuals points to a potential for a resource—based evolutionary origin of sexual dimorphism in jojoba, arising from different morphological optima for each sex.