Self-compatible Hermaphrodites Research Articles

Population genetic structure in a self-compatible hermaphroditic snail is driven by drift independently of its contemporary mating system.

Genetic drift, gene flow, and natural selection commonly influence population genetic diversity. In populations of self-compatible hermaphrodites, the mating system (e.g., self-fertilization) further reduces individual heterozygosity. Furthermore, selfing, as a form of inbreeding, significantly impacts genetic drift by reducing effective population size (N e). This can potentially accelerate genetic drift, particularly in small populations where self-fertilization is likely during founder events. To investigate the roles of genetic drift and contemporary mating system in populations of the freshwater snail Lymnaea stagnalis, we examined their effective population sizes (N e) and Tajima's D values, which reflect genetic drift over extended time periods, as well as estimates of within-population selfing rates and pairwise relatedness reflecting contemporary mating system. We used 4054 SNP markers obtained using restriction site associated DNA (RAD) sequencing from individuals in five snail populations originating from geographically closely located ponds. We found strong population genetic structure and differences in genetic diversity among populations. Covariation between genetic diversity and N e estimates and Tajima's D values suggested drift being an important determinant of genetic diversity and structure in these populations. However, this effect was independent of the contemporary mating system, as indicated by the similarity of selfing rates and relatedness estimates among populations. Thus, founder events (possibly including historical inbreeding) and/or drift due to small sizes of L. stagnalis populations are likely to explain their genetic structure and limit within-population genetic diversity.

Propensity for selfing varies within a population of hermaphroditic snails: coexistence of selfers, outcrossers and mixed-mating individuals.

To understand mating-system evolution in self-compatible hermaphrodites, variation in selfing rates is highly relevant. Empirical studies are rarely designed to capture variation between individuals, instead often comparing species and populations. Yet, evolution primarily occurs within populations, rendering among-individual variation essential. Observed individual selfing rates depend on the environment (e.g. differences in mate availability) and individuals' propensity for selfing. We quantified individual variation in selfing propensity in the snail Radix balthica by conducting laboratory mating trials that manipulated mate availability (low versus moderate) and estimating selfing rates from progeny arrays. We also measured female lifetime fitness. We found substantial among-individual variation in selfing propensity, including pure selfers (32%), pure outcrossers (31%) and mixed-mating individuals that selfed and outcrossed (37%). Experimental levels of mate availability did not significantly affect selfing rates. Selfers had reduced female liftetime fitness. Our results show that the propensity for selfing can differ considerably among individuals, with similar proportions of selfers, outcrossers and mixed maters. As mate availability did not affect selfing, our 'moderate' experimental level of mate availability might still have been too low to prompt selfers to outcross. This and the observed fitness differences also cautiously suggest that investigating the heritability of selfing propensities might be worthwhile in this population.

Sexual systems of Tropical Dry Forest trees in restored habitats

Background: Obligate outcrossing species (such as dioecious species) are more vulnerable to habitat fragmentation than self-compatible hermaphrodite trees.
 Questions: What is the effect of restoration intervention in the populations of dioecious, monoecious, and hermaphrodite tree species in permanent agricultural landscapes?
 Study site and dates: 8-year-old restoration settings in the seasonally dry tropical forest in Sierra de Huautla Biosphere Reserve, Morelos, Mexico.
 Methods: We measure richness and density of dioecious, monoecious, and hermaphrodite tree species with Diameter at Breast Height ≥ 2.5 cm in perturbed, 8-year-old restoration settings (plantings and natural succession) and conserved habitats.
 Results: Overall habitats, 2,042 trees from 46 species in 18 families were identified; the highest percentage of species and trees were hermaphrodites (63 % of species and 78 % of trees), the lowest percentage of species were monoecious (9 %) and the lowest percentage of trees (10 %) were dioecious. In the plantings were registered the largest number of species (37 species) and in the conserved habitat the largest number of trees (852 trees). Density and richness of dioecious, monoecious, and hermaphrodite species did not differ by habitat, contrary to our predictions. For dioecious trees, the restored and conserved habitats showed similar composition heterogeneity. Composition of hermaphrodites in agricultural landscapes differed the most with the conserved habitat.
 Conclusions: Dioecious and monoecious species are favored in agricultural landscapes due to their use value. Selection of species for restoration plantings should consider sexual systems of trees to recover not only forest structure but also ecosystem function.

Coexistence of Hermaphrodites and Males in Androgenetic Clam Corbicula fluminea Müller in Shirakawa River, Kyoto, Japan

Sex, maturity, and the incidence of males in a population of hermaphroditic Corbicula fluminea Muller from Shirakawa River, Kyoto, Japan, are reported. Two shell color types (yellow and green) of this species are recognized. High frequencies of females (63.3% yellow and 58.5% green) are represented among individuals in this population. Histological examination of gonads and measurements of shell length suggest this species first matures as a female before commencing spermatogenesis (i.e., it is a protogynous simultaneous hermaphrodite). Low frequencies of males (mostly less than 5%), the testes of which are filled with spermatozoa, are reported; no histological evidence for any change from males to hermaphrodites, or hermaphrodites to males, is found. Requirements for maintaining males, and for their successful reproduction in self-compatible hermaphrodite populations, are proposed as follows: (1) the protogynous nature of the hermaphrodite, (2) low self-fertilization rates of hermaphrodites, (3) more spermatozoa and greater survival of males than hermaphrodites, and (4) high population densities, ensuring males and hermaphrodites are in close proximity.

Mate availability determines use of alternative reproductive phenotypes in hermaphrodites

Abstract In many species, individuals can employ alternative reproductive phenotypes, with profound consequences for individual fitness and population dynamics. This is particularly relevant for self-compatible hermaphrodites, which have exceptionally many reproductive options. Here we investigated the occurrence of reproductive phenotypes in the simultaneously hermaphroditic freshwater snail Radix balthica under experimentally simulated conditions of low versus moderate population density. We captured all mating behavior on camera and measured individual female lifetime reproductive success. We found every possible reproductive phenotype: (1) both male and female (i.e., truly hermaphroditic) reproduction, (2) purely female and (3) purely male reproduction, (4) male reproduction combined with self-fertilization and (5) female mating activity, (6) pure self-fertilization without mating and (7–8) two types of reproductive failure. Variation in alternative reproductive phenotypes was explained by mate availability (10.8%) and individual condition, approximated by a snail’s mean daily growth rate (17.5%). Increased mate availability resulted in a lower diversity of reproductive phenotypes, in particular increasing the frequency of true hermaphrodites. However, it lowered phenotype-specific fecundities and hence reduced the population growth rate. Snails in better condition were more likely to reproduce as true hermaphrodites or pure females, whereas low-condition snails tended to suffer reproductive failure. Overall, we show substantial variation in alternative reproductive phenotypes in a hermaphrodite, which is possibly in part maintained by fluctuations in population density and thus mate availability, and by variation in individual condition. We also provide evidence of an almost 2-fold increase in clutch size that can be ascribed specifically to mating as a female.

The mutational decay of male-male and hermaphrodite-hermaphrodite competitive fitness in the androdioecious nematode C. elegans.

Androdioecious Caenorhabditis have a high frequency of self-compatible hermaphrodites and a low frequency of males. The effects of mutations on male fitness are of interest for two reasons. First, when males are rare, selection on male-specific mutations is less efficient than in hermaphrodites. Second, males may present a larger mutational target than hermaphrodites because of the different ways in which fitness accrues in the two sexes. We report the first estimates of male-specific mutational effects in an androdioecious organism. The rate of male-specific inviable or sterile mutations is ⩽5 × 10-4/generation, below the rate at which males would be lost solely due to those kinds of mutations. The rate of mutational decay of male competitive fitness is ~ 0.17%/generation; that of hermaphrodite competitive fitness is ~ 0.11%/generation. The point estimate of ~ 1.5X faster rate of mutational decay of male fitness is nearly identical to the same ratio in Drosophila. Estimates of mutational variance (VM) for male mating success and competitive fitness are not significantly different from zero, whereas VM for hermaphrodite competitive fitness is similar to that of non-competitive fitness. Two independent estimates of the average selection coefficient against mutations affecting hermaphrodite competitive fitness agree to within two-fold, 0.33-0.5%.

Reduced mate availability leads to evolution of self-fertilization and purging of inbreeding depression in a hermaphrodite.

Basic models of mating-system evolution predict that hermaphroditic organisms should mostly either cross-fertilize, or self-fertilize, due to self-reinforcing coevolution of inbreeding depression and outcrossing rates. However transitions between mating systems occur. A plausible scenario for such transitions assumes that a decrease in pollinator or mate availability temporarily constrains outcrossing populations to self-fertilize as a reproductive assurance strategy. This should trigger a purge of inbreeding depression, which in turn encourages individuals to self-fertilize more often and finally to reduce male allocation. We tested the predictions of this scenario using the freshwater snail Physa acuta, a self-compatible hermaphrodite that preferentially outcrosses and exhibits high inbreeding depression in natural populations. From an outbred population, we built two types of experimental evolution lines, controls (outcrossing every generation) and constrained lines (in which mates were often unavailable, forcing individuals to self-fertilize). After ca. 20 generations, individuals from constrained lines initiated self-fertilization earlier in life and had purged most of their inbreeding depression compared to controls. However, their male allocation remained unchanged. Our study suggests that the mating system can rapidly evolve as a response to reduced mating opportunities, supporting the reproductive assurance scenario of transitions from outcrossing to selfing.

Pollinator dependency, pollen limitation and pollinator visitation rates to six vegetable crops in southern India

We investigated levels of pollinator dependency and pollinator visitation rates to flowers of six vegetable crops: brinjal (aubergine), tomato, chilli pepper (Solanaceae), okra (Malvaceae), bitter and snake gourds (Cucurbitaceae) in six small family farms in the Coimbatore region of southern India. We tested the null hypothesis that fruit set in these crops would be independent of pollinators. We assessed fruit set through self and cross pollination by pollen augmentation, by pollinator exclusion and open pollination. We evaluated pollen limitation by comparing percentage fruit set by hand outcrossed pollen with open pollination; pollinator dependency by differences in percentage fruit set by open pollination and autogamous pollination; and visitation rates to flowers by pollinating insects. Tomato, chilli and okra produced self-compatible hermaphrodite flowers, with higher levels of autogamous fruit set (32-76%) and significantly lower levels of pollinator dependency (0-37%), whereas andro-monoecious brinjal and monoecious gourds had significantly lower levels of fruit set through autogamy, and higher levels of pollinator dependency. Pollen limitation was not evident in any crop. Diverse pollinating insects visited the flowers, and the frequency of visits by different pollinator taxa differed with crop type. Native vegetation and uncultivated land may enhance pollinator diversity in small farms.

Reproductive assurance drives transitions to self-fertilization in experimental Caenorhabditis elegans.

BackgroundEvolutionary transitions from outcrossing between individuals to selfing are partly responsible for the great diversity of animal and plant reproduction systems. The hypothesis of ‘reproductive assurance’ suggests that transitions to selfing occur because selfers that are able to reproduce on their own ensure the persistence of populations in environments where mates or pollination agents are unavailable. Here we test this hypothesis by performing experimental evolution in Caenorhabditis elegans.ResultsWe show that self-compatible hermaphrodites provide reproductive assurance to a male-female population facing a novel environment where outcrossing is limiting. Invasions of hermaphrodites in male-female populations, and subsequent experimental evolution in the novel environment, led to successful transitions to selfing and adaptation. Adaptation was not due to the loss of males during transitions, as shown by evolution experiments in exclusively hermaphroditic populations and in male-hermaphrodite populations. Instead, adaptation was due to the displacement of females by hermaphrodites. Genotyping of single-nucleotide polymorphisms further indicated that the observed evolution of selfing rates was not due to selection of standing genetic diversity. Finally, numerical modelling and evolution experiments in male-female populations demonstrate that the improvement of male fitness components may diminish the opportunity for reproductive assurance.ConclusionsOur findings support the hypothesis that reproductive assurance can drive the transition from outcrossing to selfing, and further suggest that the success of transitions to selfing hinges on adaptation of obligate outcrossing populations to the environment where outcrossing was once a limiting factor.Electronic supplementary materialThe online version of this article (doi:10.1186/s12915-014-0093-1) contains supplementary material, which is available to authorized users.

Evidence of selfing hermaphroditism in the clam shrimp Cyzicus gynecia (Branchiopoda: Spinicaudata)

The branchiopods display a broad range of reproductive modes, including dioecy, hermaphroditism and parthenogenesis. An order within Branchiopoda, Spinicaudata or the “clam shrimp” are also reported to have all three of these breeding systems; yet parthenogenesis has only been inferred on the basis of a lack of males in several clam shrimp species. Herein we report a detailed analysis of the breeding system of one of these supposed parthenogenetic clam shrimp: Cyzicus gynecia (Mattox, 1950). A RAPD genetic analysis across a three state geographic range (New Jersey, Massachusetts, and New York) showed high levels of genetic differentiation among populations indicative of reproduction without males. Additionally, functional male gametes were found to be produced in a small region of the gonad located just posterior to the head. Thus, we posit that the purported parthenogenetic females of C. gynecia are instead functioning hermaphrodites that produce a small amount of sperm anteriorly in an ovotestis that they then use to fertilize their own eggs. These findings suggest that there are, in fact, no parthenogenetic species within Spinicaudata, but rather all “female” species are most likely self-compatible hermaphrodites.

Mate guarding behavior in clam shrimp: the influence of mating system on intersexual conflict

Mate guarding is a male strategy to gain access to receptive females but often results in antagonistic interactions between the sexes because of different costs/ benefits of guarding. In addition to social, morphological, and physiological parameters, the type of mating system should also affect the strength of the conflict and thus the guarding duration. Specifically, when compared to females, self-compatible hermaphrodites might have reduced bene- fits of outcrossing. We investigated mate guarding in dioecious (co-presence of females and males) and andro- dioecious (co-presence of hermaphrodites and males) branchiopod crustaceans. Both sexes in androdioecious systems should shift their guarding times to lower values relative to dioecious systems because (1) androdioecious males are present in lower percentages than dioecious males and thus encounter rates with receptive mates are relatively greater for them; and (2) hermaphrodites should have low incentive to incur high costs of mate guarding, having the alternative of self-fertilization, and thus should be highly eager to resist. While females preferred short guarding times, when allowed to control the guarding duration (males tethered), dioecious males did not increase their guarding duration when females (treated with muscular relaxant) could not resist, in contrast to what has previously been found for androdioecious males. This indicates that hermaphrodites are more willing to resist mate guarding than females. The among-species comparisons supported our hypotheses: compromised guarding times were signif- icantly lower in androdioecious than in dioecious species. The introduction of ap arameter (mating system) not previously investigated in mate guarding models resulted in a powerful test of mate guarding theory, adding a valuable contribution to our understanding of intersexual conflict.

DENSITY-DEPENDENT SELF-FERTILIZATION AND MALE VERSUS HERMAPHRODITE SIRING SUCCESS IN AN ANDRODIOECIOUS PLANT

Models of mating-system evolution emphasize the importance of frequency-dependent interactions among mating partners. It is also known that outcross siring success and the selfing rate in self-compatible hermaphrodites can be density dependent. Here, we use array experiments to show that the mating system (i.e., the outcrossing rate) and the siring success of morphs with divergent sex allocation strategies are both density dependent and frequency dependent in androdioecious populations of the wind-pollinated, annual plant Mercurialis annua. In particular, the outcrossing rate is a decreasing function of the mean interplant distance, regulated by a negative exponential pollen fall-off curve. Our results indicate that pollen dispersed from a male inflorescence are over 60% more likely to sire outcrossed progeny than equivalent pollen dispersed from hermaphrodites, likely due to the fact that males, but not hermaphrodites, disperse their pollen from erect inflorescence stalks. Because of this difference, and because males of M. annua produce much more pollen than hermaphrodites, the presence of males in the experimental arrays reduced both the selfing rate and the outcross siring success of hermaphrodites. We use our results to infer a density threshold below which males are unable to persist with hermaphrodites but above which they can invade hermaphroditic populations. We discuss our findings in the context of a metapopulation model, in which males can only persist in well-established populations but are excluded from small, sparse populations, for example, in the early stages of colonization.

Predominance of outcrossing in Lymnaea stagnalis despite low apparent fitness costs of self‐fertilization

We have quantified the natural mating system in eight populations of the simultaneously hermaphroditic aquatic snail Lymnaea stagnalis, and studied the ecological and genetic forces that may be directing mating system evolution in this species. We investigated whether the natural mating system can be explained by the availability of mates, by the differential survival of self- and cross-fertilized snails in nature, and by the effects of mating system on parental fecundity and early survival. The natural mating system of L. stagnalis was found to be predominantly cross-fertilizing. Density of snails in the populations had no relationship with the mating system, suggesting that outcrossing rates are not limited by mate availability at the population densities observed. Contrary to expectations for outcrossing species, we detected no evidence for inbreeding depression in survival in nature with inferential population genetic methods. Further, experimental manipulations of mating system in the laboratory revealed that self-fertilization had no effect on parental fecundity, and only minor effects on offspring survival. Predominance of cross-fertilization despite low apparent fitness costs of self-fertilization is at odds with the paradigm that high self-fertilization depression is necessary for maintenance of cross-fertilization in self-compatible hermaphrodites.

DNA Extraction from Resting Eggs of the Clam Shrimp Eulimnadia Texana (Branchiopoda: Spinicaudata: Limnadiidae)

Abstract Large branchiopod crustaceans inhabiting ephemeral ponds are well adapted to their highly unpredictable habitat with a life cycle that includes a short-lived adult stage and a long-lived, desiccation-resistant egg stage. One well studied large branchiopod is the clam shrimp Eulimnadia texana, an androdioecious species with populations comprised of males and self-compatible hermaphrodites. Likely due to the harsh environments in which the eggs are found, e.g., dry areas with high heat and high irradiation, extraction of DNA from individual eggs for genetic analyses can be problematic. Here we report a number of modifications that we have tested allowing for increased efficiency and success in extracting high quality DNA from the eggs, nauplii, and adults of E. texana that may prove useful for similar studies of other species of large branchiopods.

When males and hermaphrodites coexist: a review of androdioecy in animals

Androdioecy (populations consisting of males and hermaphrodites) is a rare mating system in plants and animals: up to 50 plants and only 36 animals have been described as being androdioecious, with most of the latter being crustaceans. To date, a thorough comparative analysis of androdioecy in animals has not been undertaken. Herein we present such an analysis. Androdioecy has only been extensively surveyed in 2 animal taxa: the nematode Caenorhabditis and the clam shrimp Eulimnadia. The other major taxon having androdioecious species is the Cirripedia (barnacles), but there are only limited studies on androdioecy in this group. In animals, androdioecy is found either in species that have morphologically and ecologically distinct sexes (that is, hermaphrodites and small, "complemental" males) that are derived from hermaphroditic ancestors (that is, the barnacles) or in species that have similarly-sized males and hermaphrodites that have been derived from dioecious ancestors (the remaining androdioecious species). We suggest that the barnacles have evolved a sexual specialization in the form of these complemental males that can more efficiently use the constrained habitats that these barnacles often experience. For the remaining species, we suggest that androdioecy has evolved as a response to reproductive assurance in species that experience episodic low densities. Additionally, we hypothesize that the development of mechanisms allowing reproductive assurance in species with a number of sexually differentiated traits is most likely to result in androdioecy rather than gynodioecy (mixtures of females and hermaphrodites), and that these species may be developmentally constrained to stay androdioecious rather than being capable of evolving into populations solely consisting of efficient, self-compatible hermaphrodites. We conclude by suggesting several areas in need of further study to understand more completely the evolution and distribution of this interesting mating system in animals.

Early blooming's challenges: extended flowering season, diverse pollinator assemblage and the reproductive success of Gynodioecious Daphne laureola.

The scarcity and unpredictability of active pollinators during late winter in temperate areas tends to favour extended flowering seasons and increased floral longevity in early blooming species, which are usually pollinated by diverse sets of insects. Daphne laureola is a gynodioecious woody perennial that flowers from January to April in southern Spain, a period characterized by cold temperatures, frequent rains and irregular snowfalls. Pollinators were excluded at four different times during the flowering season in order to determine the effect of decreased exposure to pollinators on fruit set in female and hermaphrodite individuals. The role of nocturnal and diurnal pollination on reproductive success in each gender was simultaneously evaluated by selective exclusion. A 50 % reduction in the flowering period decreased fruit set of females by 50 %, whereas the corresponding decrease in self-compatible hermaphrodites was only approx. 25 %. Day-active hymenopterans and lepidopterans were infrequent visitors, and nocturnal pollinators were inefficient, suggesting that pollen beetles, Meligethes elongatus, were the main pollinators of D. laureola in the study region. Beetles were less abundant in pollenless females, although discrimination did not apparently result in pollination limitation of female reproduction. A preference of beetles for sunny locations emphasized the relevance of abiotic conditions for pollination of this early blooming shrub.

Does the Pollinator Abundance Hypothesis Explain Geographic Variation in the Breeding System ofPachycereus pringlei?

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.

Reproductive strategies in some arctic Saxifraga (Saxifragaceae), with emphasis on the narrow endemic S. svalbardensis and its parental species

Arctic saxifrages show conspicuous reproductive and chromosomal variation. We examined sexual and asexual traits in 43 phytotron-cultivated Svalbard populations of nine species, including the endemic, supposedly entirely asexual and aneupolyploid S. svalbardensis and its parental species, S. cernua and S. rivularis. All species were self-compatible hermaphrodites with low pollen/ovule ratios, including the strongly protandrous S. cernua, which previously has been reported as self-incompatible with an androdioecious mating system. Spontaneous selfing resulted in considerable seed set in several species and a few seeds in S. svalbardensis and S. cernua; hand-selfing and cross-pollination often increased seed set in the two latter species. Self-fertilized seeds of S. svalbardensis and S. cernua were viable and developed into normal, vigorous plants. Saxifraga rivularis and its close relative S. hyperborea were strongly autogamous. The bulbil-reproducing S. svalbardensis and S. cernua showed extreme variation in fertility, probably because of frequent aneuploidy. Many plants of S. cernua were fully fertile, suggesting that although natural seed set rarely has been observed, sexual reproduction is frequent enough to maintain its previously reported high levels of clonal diversity. Some plants of S. svalbardensis were also fairly fertile. This species may have considerable evolutionary potential; sexual events can lead to increasingly fertile genets with euploid chromosome numbers.

Reproductive strategies in some arctic Saxifraga (Saxifragaceae), with emphasis on the narrow endemic S. svalbardensis and its parental species

Arctic saxifrages show conspicuous reproductive and chromosomal variation. We examined sexual and asexual traits in 43 phytotron-cultivated Svalbard populations of nine species, including the endemic, supposedly entirely asexual and aneupolyploid S. svalbardensis and its parental species, S. cernua and S. rivularis. All species were self-compatible hermaphrodites with low pollen/ovule ratios, including the strongly protandrous S. cernua, which previously has been reported as self-incompatible with an androdioecious mating system. Spontaneous selfing resulted in considerable seed set in several species and a few seeds in S. svalbardensis and S. cernua; hand-selfing and cross-pollination often increased seed set in the two latter species. Self-fertilized seeds of S. svalbardensis and S. cernua were viable and developed into normal, vigorous plants. Saxifraga rivularis and its close relative S. hyperborea were strongly autogamous. The bulbil-reproducing S. svalbardensis and S. cernua showed extreme variation in fertility, probably because of frequent aneuploidy. Many plants of S. cernua were fully fertile, suggesting that although natural seed set rarely has been observed, sexual reproduction is frequent enough to maintain its previously reported high levels of clonal diversity. Some plants of S. svalbardensis were also fairly fertile. This species may have considerable evolutionary potential; sexual events can lead to increasingly fertile genets with euploid chromosome numbers.

Rates of inbreeding in the androdioecious clam shrimp Eulimnadia texana

Populations of the clam shrimp Eulimnadia texana exhibit androdioecy, which is a mixed mating system composed of males and self-compatible hermaphrodites. It has been suggested that such mating systems are evolutionarily unstable, and yet most populations of E. texana appear to exhibit both outcrossing and selfing (a mixed mating strategy). Genetic and sex-ratio features of seven populations of these clam shrimp confirm that the majority of these populations show a mixture of inbreeding and outcrossing modes of reproduction. Additionally, we suggest that the relationship of inbreeding rate with male frequency indicates that mating is not random, as was suggested in a previous model of the mating system of E. texana.