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Premise of research. Vascular plants depend on sexual recombination for generating new genetic variability to meet environmental needs. Nevertheless, members of the early-divergent fern genus Botry...

For most angiosperms, flowers are critical to reproduction because they increase rates of outcrossing. Flowers are highly variable in numerous traits, including size, shape, and color. Most of this...

Premise of research. The wide diversity of floral traits seen among plants is shaped by neutral and selective evolutionary processes. In outcrossing species, sexual selection from competing pollen ...

Premise of research. Tacca is a monocot genus characterized by an unusual reproductive display, with plants possessing umbelliform inflorescences subtended by highly conspicuous leafy bracts and filiform bracteoles with colors ranging from dark maroon to white. Paradoxically, previous studies indicated very low levels of outcrossing in some showy Tacca species, implicating self-pollination as the primary reproductive strategy.Methodology. We describe visual observations and collections made of floral visitors to flowering individuals of Tacca cristata Jack in Singapore.Pivotal results. Female ceratopogonid midges were observed to visit the flowers at anthesis and were collected with pollen specifically placed on their dorsal thoracic region. A significant proportion of flowering plants was attacked by florivores.Conclusions. These findings suggest that at least some showy species of Tacca may be pollinated to some extent through insect visitation. The reproductive biology of Tacca, including inflorescence...

Premise of research. Early-divergent angiosperms typically possess hermaphroditic flowers but often lack any biochemically mediated self-incompatibility mechanism. Although outcrossing is generally promoted in these taxa by protogyny, which is effective for precluding autogamy (self-fertilization within a flower), other mechanisms are necessary to prevent geitonogamy (self-pollination between different flowers within an individual). In this article we investigate the occurrence and efficacy of inter- and intraindividual floral synchrony for preventing geitonogamy in the early-divergent angiosperm Desmos chinensis (Annonaceae) within the context of the overall floral biology and pollination ecology of the species.Methodology. We assess the floral phenology (at the flower, individual plant, and population levels), floral scent chemistry, pollination ecology, and breeding system of D. chinensis.Pivotal results. Desmos chinensis is pollinated by small Amystrops beetles (Nitidulidae) that are attracted by flor...

Premise of research. It is known that outcrossing rates and mating structure are important factors determining the genetic structure of populations. Moreover, the composition and abundance of pollinator communities visiting flowers determine the distances over which the pollen is likely to be dispersed, which in turn affects the genetic structure of plant populations. For invasive plants, high outcrossing rates produce genetically diverse propagules for the colonization of uninvaded areas and facilitate the establishment of novel gene combinations that may affect their invasive ability. Here, we examine how differences in pollinator communities visiting flowers of the invasive shrub Lonicera maackii (Caprifoliaceae), growing along the edge and in the interior of a woodlot, affect their outcrossing rates and mating structure.Methodology. Five microsatellite marker loci were used to determine the outcrossing rates, levels of biparental inbreeding, and effective number of pollen donors siring the seed crop o...

Premise of research. Plant sexuality evolves in response to genetic and ecological pressures that shape mating systems and floral traits. Comparing such systems and traits within phylogenies can show evolutionary trends in reproduction and floral morphology without the problems posed by the nonindependence of species. We compared sexual systems in seven species of the monophyletic genus Ariocarpus to elucidate major differences in floral morphology and potential relationships between biogeographic rarity and reproductive traits. We hypothesized that floral morphology would be linked to outcrossing rates and that deviations from strict outcrossing and reduced fertility would be present in rare species.Methodology. In one population per species, floral biology was detailed to characterize breeding systems, and mating systems were determined through pollination treatments. Discriminant analysis was used to identify floral traits that separate flowers into species. Phylogenetically independent contrasts were ...

An introduced species’ propagule pressure strongly influences the genetic diversity and evolutionary potential of its descendants and even the likelihood of biological invasion. We examined population genetic consequences arising from introduction of the invasive annual grass Bromus tectorum into the central United States (U.S.). The origin and frequency of introductions were investigated by assembling allozyme diversity data from 60 widely spaced populations. At least five introduction events contributed to the grass’s genetic diversity in the central U.S. Populations in this region have fewer alleles (30 vs. 43) and polymorphic loci (5 vs. 13) than native populations, evidence of a genetic bottleneck. However, the populations are, on average, more genetically diverse and less structured than native populations. Assembly within central U.S. populations of previously allopatric genotypes may have allowed the formation, via outcrossing, of a rare multilocus genotype. Genetic admixtures may have occ...

Angadenia berteroi is a tropical perennial subshrub with large yellow flowers that sets very few fruits in its native pine rockland habitat. To learn more about the breeding system of this rare species, we collected seeds from two populations and grew plants for greenhouse study. The flowers open before sunrise. They have a complex floral arrangement (the anthers form a conical structure surrounding the stigma, which has a secondary pollen presentation) that promotes outcrossing from visits by long-tongued pollinators. Hand-pollinations show that A. berteroi is mostly self-incompatible, with greatest fruit set, fruit length, and seedling emergence resulting from crosses between unrelated individuals. The low fruit set observed in natural populations may be due to low visitation by pollinators, matings between closely related individuals, or both.

Mixed mating (producing a mix of selfed and outcrossed seed) is common in flowering plants and is ecologically important because it potentially offers reproductive assurance against pollination failure. However, selfed seed offers no reproductive assurance unless it overcomes inbreeding depression. In trees and other long-lived plants, genetic evidence suggests that selfed seed seldom matures to adulthood, presumably because of severe inbreeding depression. We measured inbreeding depression over 11 yr in two populations each of two New Zealand trees, Fuchsia excorticata and Sophora microphylla, from pollination to germination and performance in both pots and the field. The accumulated inbreeding depression was very high in both cases (F. excorticata: and 0.84; S. microphylla: and 0.99) and largely late acting, especially in F. excorticata. Fewer than 10% (and

Consistent differences in the physiological performance of wild populations of closely related plant taxa may be the result of environmentally induced phenotypic plasticity or adaptive evolution (or a combination of the two). Here we report the results of a field study of physiological and fitness-related traits in geographically proximate sister taxa in the annual wildflower genus Clarkia (Onagraceae) and interpret the differences between them in light of their ecological and reproductive differences. Within two pairs of taxa, the predominantly autogamous (self-fertilizing) taxon flowers and completes its life cycle before its pollinator-dependent (predominantly outcrossing) counterpart growing in sympatry or at similar elevations in the southern Sierra Nevada. Selfers generally exhibited higher rates of photosynthesis and transpiration than their outcrossing sister taxa, and, except for the earliest-flowering (autogamous) taxon, both photosynthetic and transpiration rates tended to decline as the season...

Theory suggests that mating‐system parameters in plants may be strongly influenced by pollinator type. We examined the outcrossing rate, biparental inbreeding, and inbreeding depression in two closely related species of Aquilegia, A. formosa and A. pubescens, that differ in their major floral visitors (hummingbird and hawkmoth, respectively). Population‐level estimates of the outcrossing rate were generally high, ranging from 0.42 to 0.89 and averaging 0.79 for A. formosa and 0.69 for A. pubescens; however, the two rates did not differ significantly. The similarities in the outcrossing rates of the two species suggest that both hummingbird and hawkmoth pollination systems provide pollen movement patterns that promote a high degree of outcrossing. However, we also found that a substantial amount of outcrossing (13%–46%) was due to mating between relatives (biparental inbreeding). Using Ritland’s genetic marker–based approach to estimate inbreeding depression, we found complete selection against inbred prog...

In many insect‐pollinated angiosperms, stamens are grown with anthers cohered, which might affect pollination processes, but the ecological significance of this fusion of anthers remains unexplored. We studied the breeding system and pollination process of didynamous Campsis grandiflora (Bignoniaceae), with anther fusion in pairs. Pollen removal and deposition were studied in the field for naturally fused versus experimentally separated anthers. Campsis grandiflora flowers were protandrous, with a low pollen:ovule ratio, were mostly outcrossed, and were self‐compatible, with pollinators needed. The main pollinators were pollen‐collecting halictid bees and nectar‐feeding vespid wasps. Halictid bees scooped pollen on long anthers for a significantly longer time period and removed twice as much pollen from fused anthers than from separated anthers. Vespid wasps contacted fused short anthers more frequently and more precisely and removed nearly twice as much pollen than when contacting separated short anthers...

Recently, the biotic context for sexual and mating system evolution in plants has received special attention; however, the significance of interactions with antagonists has only begun to be revealed. We investigated the effect of florivory on reproduction and inbreeding depression by simulating damage on selfed and outcrossed progeny of hermaphrodites of Fragaria virginiana and recording the response of reproduction, as well as measuring tolerance to florivory. While both florivory and inbreeding affected reproduction, their effects were independent with respect to sexual traits but not an asexual trait; inbreeding depression was florivory and family dependent, specifically, for plantlet production. Plants were intolerant of florivory in terms of flowers, moderately tolerant in terms of fruit, and most tolerant in terms of plantlets. However, only under severe damage was intolerance statistically significant. Inbreeding did little to change these patterns. Our findings indicate that florivory does not con...

Self-fertilization can provide reproductive assurance during periods of low or unreliable pollinator visitation. Therefore, periods of low pollen receipt may favor evolutionary shifts from cross-fertilization to self-fertilization. Although reproductive assurance is hypothesized to be important in mating system evolution, it has been quantified in relatively few species. Leptosiphon jepsonii shows variation in the mode of selfing, with transient self-incompatibility conferring delayed selfing seen in some individuals and early self-compatibility present in others. Autofertility, reproductive assurance, and pollen limitation were quantified in three populations of L. jepsonii that vary in the timing of self-compatibility and rate of self-fertilization. Plants in all three populations were capable of high seed set through autonomous self-fertilization, and selfing provided significant reproductive assurance in two populations, including one with the highest frequency of delayed selfing. Confidence intervals revealed no difference in reproductive assurance among populations. We conclude that both early and delayed self-compatibility can confer reproductive assurance and alleviate pollen limitation in this species. Results from this study are integrated with previous studies on the same three populations. We synthesize data on inbreeding depression, outcrossing rates, and floral biology for a comprehensive evaluation of the factors affecting the evolution of mixed mating in L. jepsonii.

As seed and fruit size can influence the success of later life history stages, it is important to understand their sources of variation. In this study, we examined the effects of pollen load and donor diversity on variation in seed and fruit mass of outcrossing senita cacti (Pachycereus schottii) in the Sonoran Desert. By massing all individual seeds per fruit from pollen supplementation and donor diversity experiments, we were able to examine their effects on seed and fruit mass, compared with intra‐ and interplant variation. Seed and fruit mass showed up to 13‐ and 15‐fold variations, respectively. Pollen load did statistically increase seed and fruit mass, but it explained

Plant volatiles mediate numerous interactions between plants and insects, yet few studies have examined variation in volatile production within plant populations or the genetic and environmental causes of this variation. Here we document the effects of inbreeding and maternal family on volatile production by horsenettle Solanum carolinense L. (Solanaceae). We collected volatiles from ramets (clones) of each of 12 genets (genotypes) of horsenettle grown in four agricultural fields with natural levels of herbivory. The 12 genets included self‐ and cross‐pollinated progeny from six maternal plants. We found that inbreeding reduced total volatile production relative to that of outcrossed plants. We also found a breeding‐by‐family interaction for the total amount and blend of volatiles, indicating genetic variation among families for inbreeding depression. Analysis of outcrossed plants alone (a random sample from the population) revealed a genet effect on the total amount and blend of volatiles released, indic...

Baker’s law posits that self‐compatible (SC) plants will be more successful than self‐incompatible (SI) plants in long‐distance colonization because a single propagule can establish a viable population. Oceanic islands represent ideal systems to test Baker’s law because insular lineages have, without question, originated from long‐distance dispersal. The dilemma of Baker’s law is that one propagule of an SC plant would establish a population with low genetic diversity, which could limit subsequent evolution. By contrast, a single propagule from an SI ancestor, having originated from an outcrossing source population, would provide more diversity but could not undergo sexual reproduction. We examined this issue by studying the breeding system of members of the flowering plant genus Tolpis (Asteraceae), a small (nine to 13 species), monophyletic lineage in the Canary Islands archipelago. A combination of floral morphology, pollen‐ovule ratio, autogamous seed set, and genetic data indicates that only one ende...

Cleistogamous species constitute strong evidence for the stability of mixed selfing and outcrossing but have been largely overlooked as theoretical and empirical systems for studying the forces thought to influence matingsystem evolution. Inbreeding depression is expected to play a major role in the maintenance of outcrossing, yet reliable estimates of inbreeding depression for cleistogamous species are virtually nonexistent. We estimated both population- and family-level inbreeding depression for all life cycle stages for a population of the cleistogamous perennial Viola septemloba. Our estimate of cumulative inbreeding depression of 43% is substantially greater than two previous estimates for cleistogamous species, but it is not sufficient by itself to explain the maintenance of potentially outcrossed chasmogamous flowers.

Breeding systems exert profound effects on the amount and distribution of genetic diversity within and among populations. Knowledge of breeding systems is also important for understanding dynamics between coevolving organisms, e.g., pathogen‐host interactions. Here we study the breeding system of the obligate anther smut Microbotryum violaceum on Silene dioica. Microbotryum violaceum is capable of both inbreeding and outcrossing, but several recent studies on other host races have indicated that automixis via intrapromycelial mating is the predominant breeding system. Compared with conjugations between cells from different meioses, automixis results in slower loss of heterozygosity and faster production of infectious hypha. However, high rates of intrapromycelial matings have been suggested to invoke a fitness cost due to production of fewer infectious dikaryons. Working with single strains under standardized laboratory conditions, we studied traits that could influence the distribution of genetic variabi...

Evolutionary transitions from outcrossing to selfing are expected to cause a reduction in the effective population size and a corresponding increase in fixation rates of slightly deleterious mutations and decrease in fixation of advantageous mutations. Despite these predictions, evidence from genomic data does not suggest a significant reduction in the efficacy of selection associated with high levels of self‐fertilization. Here, we discuss opportunities for selfing populations to avoid an irreversible decline in fitness toward extinction and the implications for genome evolution. Most directly, large population sizes and the purging of deleterious recessive mutations can reduce genetic loads and slow the effects of genetic drift. Theory suggests that recombination rates may also evolve in response to the evolution of mating system, which can offset the harmful effects of inbreeding. Cytological data supporting the evolution of higher recombination rates in selfing species should be supplemented with gene...

Wind pollination is predominantly a derived condition in angiosperms and is thought to evolve in response to ecological conditions that render animal pollination less advantageous. However, the specific ecological and evolutionary mechanisms responsible for transitions from animal to wind pollination are poorly understood in comparison with other major reproductive transitions in angiosperms, including the evolution of selfing from outcrossing and dioecy from hermaphroditism. To investigate correlations between wind pollination and a range of characters including habitat type, sexual system, floral display size, floral showiness, and ovule number, we used a large-scale molecular phylogeny of the angiosperms and maximum likelihood methods to infer historical patterns of evolution. This approach enabled us to detect correlated evolution and the order of trait acquisition between pollination mode and each of nine characters. Log likelihood ratio tests supported a model of correlated evolution for wind pollination and habitat type, floral sexuality, sexual system, flower size, flower showiness, presence versus absence of nectar, and ovule number. In contrast, wind pollination and geographical distribution and number of flowers per inflorescence evolve independently. We found that in wind-pollinated taxa, nectar is lost more often and ovule number is reduced to one. We also found that wind pollination evolves more frequently in lineages already possessing unisexual flowers and/or unisexual plants. An understanding of the ecological and life-history context in which wind pollination originates is fundamental to further investigation of the microevolutionary forces causing transitions from animal to wind pollination.

The evolutionary transition from diploidy to polyploidy is prevalent in flowering plants and may result in correlated changes in mating system (outcrossing rate). Most theory predicts a shift toward self‐fertilization (decrease in outcrossing) in polyploids, but empirical evidence for this pattern and its underlying mechanisms is inconclusive or restricted to a few cases. In an analysis of variation in outcrossing rates among diploid‐polyploid species pairs from the literature, polyploids had lower outcrossing rates (higher selfing; \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \Declar...

We investigated genetic variation within and among 11 populations of Iris hexagona at its southern limits in the Florida peninsula by using 19 microsatellite loci. All of the populations contain varying numbers of identical multilocus genotypes, indicative of clonal reproduction. Two population samples consist largely of one clonal lineage and two clonal lineages: the first from the Caloosahatchee drainage west of Lake Okeechobee and the other from the Big Cypress Swamp. The populations are predominantly outcrossing, with high levels of heterozygosity, and show a highly significant pattern of isolation by distance that fits a modified stepping‐stone model. This pattern breaks down at the local level, however, where metapopulation dynamics or asymmetrical gene flow may exert a stronger effect on patterns of genetic diversity. Though the majority of genetic variation is within populations, 20% occurs between populations. Genetic distance resolves five clusters: four in the Caloosahatchee Valley and one in t...

An understanding of how selfing occurs and whether selfing incurs costs relative to outcrossing is necessary for an evolutionary analysis of self‐fertilization. We used glasshouse and field experiments to address these questions in the perennial herb Bulbine bulbosa. Flowers did not autonomously self‐pollinate, and geitonogamy was unlikely because plants opened only one short‐lived flower per day on average. Instead, selfing occurred via facilitated autogamy. We manipulated selfing rate and showed that seed set was negatively related to levels of selfing because many selfed zygotes aborted. Selfed seedlings were smaller and fewer survived compared with crossed seedlings, resulting in high cumulative inbreeding depression (0.85). Naturally pollinated plants had high ovule fertilization similar to that of experimentally cross‐pollinated and self‐pollinated plants. However, naturally pollinated plants produced fewer fruits and seeds than did cross‐pollinated plants but similar numbers as self‐pollinated plan...

Flower longevity often exhibits a plastic response to pollination and may vary adaptively to maximize pollen removal and deposition and minimize flower maintenance costs. Using Chamerion angustifolium (fireweed), we experimentally tested two predictions that follow from this hypothesis: (1) flower closure should be triggered early, before fertilization, if it is to reduce costs and adjust to maximize pollen receipt; and (2) flower closure should respond differently to different kinds (outcross, self) and amounts (none, low, high) of pollen. In C. angustifolium, flower closure was initiated 4 h after pollination, well before any ovules were fertilized (25 h). The addition of pollen to stigmas significantly shortened the time to closure compared with unpollinated controls. Flower closure times were reduced with increasing pollen loads, up to 700 pollen grains, as might be expected if closure was adaptive. Flower closure was slower in self‐pollinations than outcross pollinations at 24 and 48 h after pollinat...

Flowers with highly specialized pollination systems sometimes have the ability to self-pollinate, contradicting our notion that pollination specialization reflects selective pressures to ensure high maternal outcrossing rates. We survey the literature (80 species representing 38 families) for the simultaneous study of pollination and breeding systems. We demonstrate that pollination specialization is often associated with a variety of floral traits that facilitate delayed autonomous selfing at the end of the flower’s life span. While the potential autonomous selfing rate can be high, the actual autonomous selfing rate is often much lower, indicating that species in our survey are facultative selfers. Autonomous selfing was more commonly associated with protandry than with protogyny and was found in both herkogamous and nonherkogamous species. We conclude that pollination specialization can evolve independently of the ability to autonomously self-pollinate and that the presence of floral traits that promote pollination specialization and autonomous selfing in the same flower is not paradoxical. Pollination specialization can be reconciled with autonomous self-pollination when selective forces other than high maternal outcrossing rates are considered.

Several different self‐incompatibility (SI) syndromes have been previously reported from Polemoniaceae, including differences in genetic control, timing of the response, and the location of pollen tube inhibition. In order to examine the evolutionary linkage between genetic control and site of pollen tube inhibition, crossing experiments and microscopic analyses were performed on an additional species of Polemoniaceae, Ipomopsis tenuifolia. Thirteen individuals from a natural population were used for the parental crosses; three F1 full‐sibling arrays were grown to maturity. Strong self‐incompatibility was found in the parental population on the basis of significant differences in the percentages of seed set in self‐ versus outcross matings. In the F1 arrays, none of the outcrosses or backcrosses was completely incompatible, leading to a large number of mating types. On the basis of the number of mating types in the F1 arrays, self‐recognition in this species must be based on full allelic matching of at le...

Inter–simple sequence repeat (ISSR) molecular markers were used to characterize inter‐ and intrapopulation genetic diversity in populations of two contrasting species of Viola, Viola striata of the eastern United States and Canada and Viola grahamii of central Mexico. Both species show a mixed breeding system with the production of both chasmogamous and cleistogamous flowers, although the temporal pattern of flowering varies between the two species. In both species the majority of the seeds produced in a given season are the result of cleistogamous flowers; additionally, V. grahamii reproduces asexually by means of runners. Our results indicate similar high levels of genetic diversity, low levels of population subdivision, and weak interpopulation differentiation in both species. In particular V. grahamii showed a slightly higher level of genetic diversity than V. striata, which does not reproduce vegetatively. Modest chasmogamous reproduction is proposed to maintain levels of outcrossing sufficient to am...

A flexible mating system may enable self‐compatible plants to prevent self‐fertilization under certain circumstances. Campanulastrum americanum is a self‐compatible, protandrous herb. Although within‐plant pollen transfer is likely and self pollen can produce a full set of seeds, selfing in natural populations is rare. We investigated explanations for the high outcrossing rate. One possibility was that self pollen contacts the stigma rarely. We surveyed floral displays in nature to gauge the potential for within‐plant pollen transfer. Floral displays in nature are large enough to enable frequent geitonogamous pollinator visits and subsequent self‐pollination. Alternatively, the high outcrossing rate could result from cryptic self‐incompatibility, a mechanism that favors outcross pollen over self pollen. To determine whether outcross pollen has a seed‐siring advantage over self pollen, we pollinated maternal plants with equal mixtures of self and outcross pollen. Genotyping the offspring revealed that outc...

An isolated 180‐ha stand of Pinus caribaea var. hondurensis (Caribbean pine) exists at El Pinal, Guatemala, in seasonal marshlands 18 km northeast of the old Mayan capital city of Tikal. One hypothesis suggests that during the zenith of their culture between 600 and 900 AD, the Mayans planted the pine stand as a source of resinous lighter wood (ocote) to start cooking fires. We examine the alternate hypothesis that El Pinal is of natural origin. Allozyme analyses were used to compare genetic diversity levels at El Pinal with seven neighboring natural stands of Caribbean pine in Belize and Guatemala. Results indicated that the El Pinal population had a high proportion of polymorphic loci (88%), had four unique alleles, exhibited an outcrossing rate of 91%, and differed little from larger natural populations of Caribbean pine located 65–100 km away. Only 2.3% of the total genetic diversity (GST) was found among the eight populations, which suggests high historic levels of gene flow. A climatic model (FloraM...

The timing of self‐compatibility was found to vary within and among populations of Leptosiphon jepsonii, a narrowly distributed California annual species. In each of 17 populations, some individuals exhibited transient self‐incompatibility, a phenomenon that confers delayed selfing, while others were fully self‐compatible upon flower opening, allowing for concurrent self‐ and cross‐fertilization. The frequency of initially self‐compatible individuals varied significantly among populations, ranging from 3.2% to 71.0%. A comparison of outcrossing rate estimates for three populations was consistent with the hypothesis that early self‐compatibility promotes higher selfing rates. Population means for each of three floral morphological traits were significantly correlated with a population index of self‐incompatibility that reflects the frequency of fully self‐compatible and transiently self‐incompatible individuals. A high frequency of self‐compatibility was associated with shorter corolla tubes, smaller corol...

Allozyme variation (17 loci coding 11 enzymes) was investigated in 14 populations of the fern Athyrium filix-femina var. asplenioides arrayed at differing elevations and latitudes in the southern Appalachians. Allozyme fingerprints showed that asplenioides individuals comprise meandering, overlapping clones usually � 3 m in extent, occasionally forming larger clones of up to 17 m. Levels of genetic variability in populations (means: A ¼ 2:01, P ¼ 32:8, H ¼ 0:115) were near the averages for both ferns and seed plants. General conformance to Hardy-Weinberg expectations indicated a predominantly outcrossing mating system. Hierarchical F statistic analysis and occasional deficits and excesses of heterozygotes indicated population substructure. Similar allele frequencies across all populations resulted in low to moderate FST values (mean FST ¼ 0:069; range ¼ 0:013 � 0:112) and high values of genetic similarity (mean S ¼ 0:944; mean I ¼ 0:992). Hierarchical analysis indicated that neither regional proximity (FXY ¼� 0:009) nor elevation (FXY ¼� 0:007) contributed substantially to divergence among populations (FXY ¼ 0:056), a result corroborated by UPGMA analysis that clustered together populations from different regions and of different elevational class. Southern Appalachian asplenioides differed from more eastern asplenioides populations of the piedmont and coastal plain in having higher frequencies of Pgm-2 c and Tpi-2 B , alleles characteristic of the more northern variety angustum. Nonetheless, genetic distinctness of the two varieties was maintained. We hypothesize that higher frequencies of angustum alleles in the southern Appalachian asplenioides populations are the result of introgression from angustum that persisted at high elevations as both taxa migrated northward following the retreat of the Wisconsinan glacier.

The anther smut fungus Microbotryum violaceum has a mixed mating system, being able to either self or outcross. The infection success may be influenced by selfing or outcrossing. It will affect the amount and distribution of genetic variation within natural populations and, in particular, in this system, may explain the maintenance of common highly deleterious alleles linked to the mating‐type locus. Using artificial inoculations with two different mixtures of strains that have different identifiable genotypes on the host plant Silene latifolia, we estimate infection success and selfing propensity of this fungus. Fungal spores were placed in flowers, mimicking natural spore deposition by pollinators. Successful infection was found in 31.5% of inoculated plants, and infection success depended on the number of inoculations and host plant sex, females having a higher per‐contact risk but males a higher global probability of infection. Many host individuals were infected by more than one fungal genotype, and ...

The showy corolla of Mimulus guttatus is generally considered an adaptation to attract pollinators. We use phenotypic manipulations to demonstrate that corolla morphology plays a critical mechanical role in both outcrossing and self‐fertilization. In the absence of pollinators, the lower portion of the corolla facilitates autogamy by retaining pollen released from the anthers. A substantial portion of self‐fertilization seems to occur late in the floral life span as the stigma contacts the corolla. When pollinators are present, the corolla facilitates outcrossing before, during, and after insect visitation. A large fraction of cross‐pollen is actually captured by the corolla and not by the stigma. This “indirect” pathway for pollen reception suggests that a large fraction of cross‐pollination in M. guttatus actually occurs long after a pollinator departs from a flower.

Experimental self- and cross-pollinations revealed that Leptosiphon jepsonii (formerly Linanthus jepsonii), an annual species, possesses a floral age-dependent form of self-incompatibility (SI) that confers delayed selfing. In field and growth room studies of two populations, self-pollination produced few to no pollen tubes when flowers were first open, but growth of outcross pollen tubes indicated that stigmas were receptive during this phase. By the second day of a flower’s duration, however, self pollen tubes grew readily, and pollen tube numbers in self- and outcross-pollinated flowers were not significantly different. Experimental crosspollinations among full siblings indicated the presence of two mating types, consistent with allelic segregation at a single SI locus. On the basis of these results and the site of pollen rejection, we hypothesize that the floral age-dependent pollen-pistil phenomenon is a modified form of the single-locus sporophytic SI found in a closely related species, Leptosiphon parviflorus. To explore the potential effect of transient SI on the timing and rate of self-fertilization, we used allozyme genetic markers to determine the paternity of seeds produced by mixed self and outcross pollen loads. Consistent with the pollen tube data, nearly all progeny from mixed pollinations of day 1 flowers were outcrossed, and the proportion of selfed progeny increased significantly with floral age. Transient SI may be an evolutionarily stable strategy or a transitional step in the evolution from SI to complete selfing.

The effects of geitonogamy on the seed set of a protogynous hermaphroditic tree, Magnolia obovata Thunb., were investigated by electrophoretic allozyme analysis and pollination experiments. The multilocus outcrossing rates (tm) estimated in 1995 and 1997 by using four polymorphic loci were 0.39 and 0.42, respectively, indicating that more than half of the seeds are produced by geitonogamous selfing. Manual pollination demonstrated that the follicle set per fruit (frequency of ripe carpels) and seed set per fruit from self‐pollination were 2.6 and 3.8 times less than those from cross‐pollination, respectively, and the frequency of aborted seeds from self‐pollination was significantly higher than that from cross‐pollination. There was no significant difference between self‐ and cross‐pollination in the fruit set and the frequency of remnant ovules (ovules unfertilized or aborted before endosperm development). These results indicate that inbreeding depression is a primary factor reducing the seed set of self...

Previous studies have shown that the nectar-feeding bat, Leptonycteris curasoae, is the major pollinator of Pachycereus pringlei (cardon), a columnar cactus whose populations are either gynodioecious or trioecious in the Sonoran Desert. On the basis of evidence of pollinator limitation in females and low flower visitation rates, a hypothesis has been proposed to explain geographic variation in the breeding system of P. pringlei. The pollinator abundance hypothesis predicts high frequencies of males and females in areas where L. curasoae is abundant and high frequency of hermaphrodites in areas where this bat is uncommon. We test this hypothesis by describing the following: first, the relationship between sex frequencies and bat abundance as revealed by nocturnal pollen deposition on stigmas of female plants and, second, the extent of pollinator limitation for fruit set in females and hermaphrodites in four populations of P. pringlei from coastal Sonora, Mexico. We also describe geographic variation in the relative fitness of females compared with hermaphrodites and outcrossing rates and inbreeding depression of self-compatible hermaphrodites in these populations. Some of these parameters should vary with bat abundance according to this hypothesis. Sex-frequency distributions showed a clear latitudinal pattern, with gynodioecious populations in the southern range and trioecious populations in the northern range. Large differences in nocturnal pollen deposition in females and abundance of L. curasoae were detected among populations but not as predicted by the pollinator abundance hypothesis. Fruit set was greater in females than in hermaphrodites in different pollination treatments, and no evidence of pollinator limitation was detected in either sex class. In general, the relative fitness of females was greater than hermaphrodites in most populations. Outcrossing rates in hermaphrodites were close to 1.0 in each population. Field and experimental data indicated no evidence of inbreeding depression. Our results do not support the hypothesis that the sex-frequency distribution of P. pringlei is closely correlated with the abundance of L. curasoae but do demonstrate that females outperform hermaphrodites in gynodioecious and trioecious populations. Studies of the distribution of mtDNA variants are needed to understand geographic variation in the breeding system of P. pringlei.

The evolution of inbreeding is common throughout the angiosperms, although little is known about the developmental and genetic processes involved. Lycopersicon pimpinellifolium (currant tomato) is a self‐compatible species with variation in outcrossing rate correlated with floral morphology. Mature flowers from selfing and outcrossing populations differ greatly in characters affecting breeding behavior (petal, anther, and style lengths); other flower parts (sepals, ovaries) show minimal differences. A quantitative trait locus (QTL) mapping of the floral characters associated with breeding‐system differences in this species showed QTLs of major effect (>25%) on phenotypic variance for anther and style length. Here, we examined the developmental basis for the difference in mature flower forms associated with the different breeding systems. The two floral forms at initial stages of growth showed no significant differences in external morphology. Anthers of selfing flowers, which were smaller at maturity and ...