Is sexual selection operating in the androdioecious clam shrimp,Eulimnadia texana (Crustacea: Conchostraca)?

Experiments were performed to document the existence of intersexual or intrasexual selection in the clam shrimp,Eulimnadia texana. Individuals within this species are either males or hermaphrodites. Hermaphrodites can self their own eggs or outcross with a male, but they cannot outcross with other hermaphrodites. Theoretical considerations suggest that both intrasexual and intersexual selection could be occurring on the part of the hermaphrodites and the males. When males were given a choice between two non-gravid hermaphrodites of different sizes, they did not exhibit a mating preference based upon size. When two males of different sizes were isolated with a single nongravid receptive hermaphrodite, the hermaphrodite showed no preference between the two males. There was evidence, however, of male-male competition for receptive hermaphrodites and of mate guarding on the part of the males. During aggressive encounters between twp males, the larger of the two had a significant advantage over the smaller, and larger males were always the victors hermaphrodite takeovers occurred as a result of male-male conflict. Hermaphrodites appear to control the mating process both by struglling with males when they are not receptive to them and by selfing in the presence of males. This suggests that hermaphrodites withhold receptivity cues from males, or produce non-receptivity cues, when they are going to self. Though hermaphrodites do not appear to select males based upon size, they make a selection between selfing and outcrossing by controlling the use of receptivity signals.     

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.     

LIFETIME INBREEDING DEPRESSION,PURGING, AND MATING SYSTEM EVOLUTION IN A SIMULTANEOUS HERMAPHRODITE TAPEWORM

Classical theory on mating system evolution suggests that simultaneous hermaphrodites should either outcross if they have high inbreeding depression (ID) or self‐fertilize if they have low ID. However, a mixture of selfing and outcrossing persists in many species. Previous studies with the tapeworm Schistocephalus solidus have found worms to self‐fertilize some of their eggs despite ID. The probability for selfing to spread depends on the relative fitness of selfers, as well as the genetic basis for ID and whether it can be effectively purged. We bred S. solidus through two consecutive generations of selfing and recorded several fitness correlates over the whole life cycle. After one round of selfing, ID was pronounced, particularly in early‐life traits, and the conservatively estimated lifetime fitness of selfed progeny was only 9% that of the outcrossed controls. After a second generation of selfing, ID remained high but was significantly reduced in several traits, which is consistent with the purging of deleterious recessive alleles (the estimated load of lethal equivalents dropped by 48%). Severe ID, even if it can be rapidly purged, likely prevents transitions toward pure selfing in this parasite, although we also cannot exclude the possibility that low‐level selfing has undetected benefits.     

BEST OF BOTH WORLDS? ASSOCIATION BETWEEN OUTCROSSING AND PARASITE LOADS IN A SELFING FISH

Mixed-mating strategies (i.e., intermediate levels of self-fertilization and outcrossing in hermaphrodites) are relatively common in plants and animals, but why self-fertilization (selfing) rates vary so much in nature has proved difficult to explain. We tested the hypothesis that parasites help maintain mixed-mating using a partially selfing fish (Kryptolebias marmoratus) as a model. We show that outcrossed progeny in the wild are genetically more diverse and less susceptible to multiple parasite infections than their selfed counterparts. Given that outcrossing in K. marmoratus can only be attained by male-hermaphrodite matings, our data provide an explanation for the coexistence of males and hermaphrodites in androdioecious species where hermaphrodites are unable to outcross among themselves. Moreover, our study provides evidence that parasites contribute to maintaining mixed-mating in a natural animal population.     

EVOLUTION OF GYNODIOECY AND MAINTENANCE OF FEMALES: THE ROLE OF INBREEDING DEPRESSION,OUTCROSSING RATES,AND RESOURCE ALLOCATION IN SCHIEDEA ADAMANTIS (CARYOPHYLLACEAE)

Levels of inbreeding depression, outcrossing rates, and phenotypic patterns of resource allocation were studied to examine their relative importance in the maintenance of high numbers of females in gynodioecious Schiedea adamantis (Caryophyllaceae), an endemic Hawaiian shrub found in a single population on Diamond Head Crater, Oahu. In studies of inbreeding depression in two greenhouse environments, families of hermaphrodites exhibited significant inbreeding depression (δ = 0.60), based on a multiplicative fitness function using seeds per capsule, germination, survival, and the inflorescence biomass of progeny. Differences between inbred and outcrossed progeny were smallest at the early stage of seeds per capsule and more pronounced at the later stages of survival and inflorescence production. These results are consistent with inbreeding depression caused by many mutations of small effect. Using allozyme analyses, the inbreeding coefficient of adult plants in the field was not significantly different from zero, implying that δ in nature may be equal to one. The single locus estimate of the outcrossing rate for hermaphrodites was 0.50 based on progeny that survived to flowering; corrected for the disproportionate loss before flowering of progeny from selfing, the adjusted outcrossing rate at the zygote stage was 0.32, suggesting that considerable selfing occurs in hermaphrodites. Females were totally outcrossed. When females and hermaphrodites were compared for reproductive output in the field, females produced over twice as many seeds per plant as hermaphrodites, primarily because females had far more capsules per inflorescence than hermaphrodites. Females had greater mass per seed than hermaphrodites in the field, either because of greater provisioning or reduced inbreeding depression. There was no significant differential mortality with respect to sex over a seven year period. The higher number of seeds per plant of females, combined with substantial inbreeding depression and relatively high selfing rates for hermaphrodites, are probably responsible for the maintenance of females in this population. The predicted frequency of females based on data for seed production, the adjusted selfing rate, and inbreeding depression is 42%, remarkably close to the observed frequency of 39%. High levels of inbreeding depression suggest that considerable quantitative genetic variation is present for traits affecting fitness in this population, despite low allozyme variability and a presumed founder effect.     

Patterns of self compatibility, inbreeding depression, outcrossing, and sex allocation in a marine bryozoan suggest the predominating influence of sperm competition

Sex allocation by simultaneous hermaphrodites is theoretically influenced by selfing rate, which is in turn influenced by the benefits of enhanced genomic transmission and reproductive assurance relative to the cost of inbreeding depression. The experimental investigation of these influences in seed plants has a rich pedigree, yet although such an approach is equally relevant to colonial invertebrates, which globally dominate subtidal communities on firm substrata, such studies have been scarce. We reared self‐compatible genets of the marine bryozoan Celleporella hyalina s.l. in the presence and absence of allosperm, and used molecular genetic markers for paternity analysis of progeny to test theoretical predictions that: (1) genets from focal populations with high selfing rates show less inbreeding depression than from focal populations with low selfing rates; (2) genets whose selfed progeny show inbreeding depression prefer outcross sperm (allosperm); and (3) genets bias sex allocation toward female function when reared in reproductive isolation. Offspring survivorship and paternity analysis were used to estimate levels of inbreeding depression and preference for outcrossing or selfing. Sex allocation was assessed by counting male and female zooids. As predicted, inbreeding depression was severe in selfed progeny of genets derived from the populations with low self‐compatibility rates, but, with one exception, was not detected in selfed progeny of genets derived from the populations with higher self‐compatibility rates. Also, as predicted, genets whose selfed progeny showed inbreeding depression preferred outcrossing, and a genet whose selfed progeny did not show inbreeding depression preferred selfing. Contrary to prediction, sex allocation in the majority of genets was not influenced by reproductive isolation. Lack of economy of male function may reflect the over‐riding influence of allosperm‐competition in typically dense breeding populations offering good opportunity for outcrossing. We suggest that hermaphroditism may be a plesiomorphic character of the crown group Bryozoa, prevented by phylogenetic constraint from being replaced by gonochorism and therefore not necessarily adaptive in all extant clades. © 2009 The Linnean Society of London, Biological Journal of the Linnean Society, 2009, 98 , 519–531.     

MUTATIONAL MELTDOWN IN SELFING ARABIDOPSIS LYRATA

The majority of plant species and many animals are hermaphrodites, with individuals expressing both female and male function. Although hermaphrodites can potentially reproduce by self‐fertilization, they have a high prevalence of outcrossing. The genetic advantages of outcrossing are described by two hypotheses: avoidance of inbreeding depression because selfing leads to immediate expression of recessive deleterious mutations, and release from drift load because self‐fertilization leads to long‐term accumulation of deleterious mutations due to genetic drift and, eventually, to extinction. I tested both hypotheses by experimentally crossing Arabidopsis lyrata plants (self‐pollinated, cross‐pollinated within the population, or cross‐pollinated between populations) and measuring offspring performance over 3 years. There were 18 source populations, each of which was either predominantly outcrossing, mixed mating, or predominantly selfing. Contrary to predictions, outcrossing populations had low inbreeding depression, which equaled that of selfing populations, challenging the central role of inbreeding depression in mating system shifts. However, plants from selfing populations showed the greatest increase in fitness when crossed with plants from other populations, reflecting higher drift load. The results support the hypothesis that extinction by mutational meltdown is why selfing hermaphroditic taxa are rare, despite their frequent appearance over evolutionary time.     

SEX EXPRESSION,BREEDING SYSTEM,AND POLLEN BIOLOGY OF RICINOCARPOS PINIFOLIUS: A CASE OF ANDRODIOECY?

A population of 54 Ricinocarpos pinifolius (Euphorbiaceae) plants contained male plants, which produced only staminate flowers, and hermaphrodites, which produced staminate and pistillate flowers. The fraction of pistillate flowers ranged continuously from 0 to 0.68. Insect pollination was effective and fruit set virtually complete except for losses to herbivores. Self pollen, outcross pollen from male plants, and outcross pollen from hermaphrodites were all equivalent in viability, germination, tube growth, ovule penetration, and fruit setting ability. Inbreeding depression was manifested as late abortion of some selfed seeds. Geitonogamous selfing is largely prevented by temporal separation of male and female functions within plants. This temporal separation, combined with population-wide synchrony of flowering, may create unusual conditions allowing male plants at low frequency to match hermaphrodites in reproductive success.     

Nonrandom mating in Clarkia gracilis (Onagraceae): a case of cryptic self-incompatibility

In plants capable of both self-fertilization and outcrossing, the selfing rate depends on the proportion of self pollen in pollen loads and on the relative postpollination success of self pollen in siring offspring. While the composition of pollen loads is subject to unpredictable variation, paternity success of self vs. outcross pollen following pollen deposition may be controlled by maternal plants. This study examined postpollination paternity success in Clarkia gracilis ssp. sonomensis, in which deposition of self pollen is common. Pure loads of self and outcross pollen produced similar numbers of mature seeds, but equal mixtures of self and outcross pollen yielded more than three times as many outcrossed offspring as selfed offspring. The finding that the paternity success of self pollen depends on whether it is in competition with outcross pollen helps to explain an earlier finding that the selfing rate in experimental populations was highest when pollinator activity was lowest. Cryptic self-incompatibility allows paternity by self pollen when outcross pollen is unavailable.     

ECOLOGICAL GENETICS OF GYNODIOECY IN SILENE VULGARIS: RELATIVE FITNESS OF FEMALES AND HERMAPHRODITES DURING THE COLONIZATION PROCESS

Recent theoretical models have addressed the influence of metapopulation dynamics on the fitness of females and hermaphrodites in gynodioecious plants. In particular, selection is thought to favor hermaphrodites during population establishment because that sex should be less prone to pollen limitation, especially if self-fertilization is possible. However, inbreeding depression could limit this advantage. In this experimental study of Silene vulgaris, a weedy gynodioecious plant, the fitness of females and hermaphrodites was estimated from seed production in both mixed-sex populations and for individuals isolated from these populations by 20, 40, 80, or 160 m. In mixed populations females display statistically significant greater per capita seed production owing to higher capsule production and higher rates of seed germination. The fitness of both sexes declines with increasing isolation, but at different rates, such that in the 160-m treatment hermaphrodites are by far the more fit sex. Allozyme studies suggest that this differential decline is because the selfing rate in hermaphrodites increases as a function of isolation, at least partially compensating for a decline in the availability of outcross pollen. Overall, the negative effects of pollen limitation on females far outweighs the negative effects of inbreeding depression following selfing in hermaphrodites. Thus, extinction/recolonization dynamics would appear to favor hermaphrodites as long as seed dispersal events exceed some critical distance.     

FEMALE FREQUENCIES IN GYNODIOECIOUS POPULATIONS CORRELATED WITH SELFING RATES IN HERMAPHRODITES

Gynodioecious populations consist of separate hermaphroditic and female individuals. Females are at a selective disadvantage because they contribute genes to the next generation only through ovules, while hermaphrodites contribute genes through ovules and pollen. For females to be maintained in populations they must have some compensating selective advantage. The outcrossing hypothesis postulates that females are maintained because their progeny result from obligate outcrossing, whereas some of the progeny of hermaphrodites result from self-fertilization and are less fit because of inbreeding depression. If correct, the frequency of females should be positively correlated with selfing rates of hermaphrodites in populations. We found a strong positive correlation between female frequency and selfing rates of hermaphrodites (r = 0.91, P < 0.01) in eight gynodioecious populations of Hawaiian species of Bidens. Our results confirm that the obligate outcrossing of females is a major factor maintaining females in gynodioecious populations. However, the observed selfing rates are insufficient by themselves to account for the frequency of females in these populations.     

Selfing versus outcrossing propensity of the fungal pathogen Microbotryumviolaceum across Silenelatifolia host plants

In the fungal pathogen Microbotryumviolaceum mating (i.e. conjugation between cells of opposite mating type) is indispensable for infection of its host plant Silenelatifolia. Since outcrossing opportunities are potentially rare, selfing may be appropriate to ensure reproduction. On the other hand, outcrossing may create genetic variability necessary in the coevolutionary arms race with its host. We investigated the propensity of M. violaceum to outcross vs. self in different host environments. We used haploid sporidia from each of three strains from five fungal populations for pairwise mixtures of opposite mating type, representing either selfing or outcrossing combinations. Mixtures were exposed to leaf extract from seven S. latifolia plants. The proportion of conjugated sporidia quantified mating propensity. The identity of both fungal strains and host influenced conjugation. First, individual strains differed in conjugation frequency by up to 30%, and strains differed in their performance across the different hosts. Second, selfing combinations produced, on average, more conjugations than did outcrossing combinations. Selfing appears to be the predominant mode of reproduction in this fungus, and selfing preference may have evolved as a mechanism of reproductive assurance. Third, individual strains varied considerably in conjugation frequency in selfing and outcrossing combinations across different hosts. This indicates that conjugation between outcrossing partners could be favoured at least in some hosts. Since the dikaryon resulting from conjugation is the infectious unit, conjugation frequency may correspond with infection probability. This assumption was supported by an inoculation experiment, where high infectious sporidial dosage resulted in higher infections success than did low dosage. We therefore predict that sexual recombination can provide this pathogen with novel genotypes able to infect local resistant hosts.     

Inbreeding and outbreeding effects on pollen fitness and zygote survival in Silene nutans (Caryophyllaceae)

In plants, selfing and outcrossing may be affected by maternal mate choice and competition among pollen and zygotes. To evaluate this in Silene nutans, we pollinated plants with mixtures of (1) self‐ and outcross pollen and (2) pollen from within a population and from another population. Pollen fitness and zygote survival was estimated from the zygote survival and paternity of seeds. Self pollen had a lower fitness than outcross pollen, and selfed zygotes were less likely, or as likely, to develop into seeds. Hybrid zygotes survived as frequently or more than local zygotes, and pollen from one of the populations fertilized most ovules in both populations. Our results thus indicate strong maternal discrimination against selfing, whereas the success of outbreeding seems mostly affected by divergent pollen performance. The implications for the evolution of maternal mate choice are discussed.     

The mating system of Hydrophyllum appendiculatum,a protandrous species

Summary The mating system of Hydrophyllum appendiculatum (Hydrophyllaceae), a protandrous, self-compatible, monocarpic plant was examined using progeny arrays assayed at three polymorphic allozyme loci. We were particularly interested in the effect of ecological factors on spatial and temporal variation in outcrossing rates. Multilocus estimates of outcrossing rates in three populations ranged from 0.62–0.81 indicating that the majority of seeds are produced via outcrossing. The population estimates did not differ significantly from each other indicating that there is little or no spatial variation in the mating system of this species. The estimates were, however, significantly less than unity, which demonstrates that although mainly outcrossing, a significant fraction of seeds are produced by selfing. Estimates suggested that biparental inbreeding occurred, although it was statistically significant in only one population. Individuals of H. appendiculatum may remain in anthesis 3–4 weeks and produce up to 30 inflorescences. As a result, the possibility exists for the mating system to vary through the flowering season. Although the mean outcrossing rate was highest in the middle of the flowering phenology than at the beginning or end, there was no evidence for statistically significant temporal heterogeneity. We were also interested in determining if the size of the floral display (number of inflorescences produced by a plant) influenced the outcrossing rate. The results obtained by two different statistical analyses were contradictory; there was a significant positive correlation between size of floral display and outcrossing rate, but the outcrossing rates of large plants (\s> 8 inflorescences) did not differ significantly from small plants (> 8 inflorescences).     

Rates of deleterious mutation and the evolution of sex in Caenorhabditis

A variety of models propose that the accumulation of deleterious mutations plays an important role in the evolution of breeding systems. These models make predictions regarding the relative rates of protein evolution and deleterious mutation in taxa with contrasting modes of reproduction. Here we compare available coding sequences from one obligately outcrossing and two primarily selfing species of Caenorhabditis to explore the potential for mutational models to explain the evolution of breeding system in this clade. If deleterious mutations interact synergistically, the mutational deterministic hypothesis predicts that a high genomic deleterious mutation rate (U) will offset the reproductive disadvantage of outcrossing relative to asexual or selfing reproduction. Therefore, C. elegans and C. briggsae (both largely selfing) should both exhibit lower rates of deleterious mutation than the obligately outcrossing relative C. remanei. Using a comparative approach, we estimate U to be equivalent (and < 1) among all three related species. Stochastic mutational models, Muller's ratchet and Hill-Robertson interference, are expected to cause reductions in the effective population size in species that rarely outcross, thereby allowing deleterious mutations to accumulate at an elevated rate. We find only limited support for more rapid molecular evolution in selfing lineages. Overall, our analyses indicate that the evolution of breeding system in this group is unlikely to be explained solely by available mutational models.     

The evolution from females to hermaphrodites results in a sexual conflict over mating in androdioecious nematode worms and clam shrimp

The nematode worm Caenorhabditis elegans and the clam shrimp Eulimnadia texana are two well‐studied androdioecious species consisting mostly of self‐fertilizing hermaphrodites and few males. To understand how androdioecy can evolve, a simple two‐step mathematical model of the evolutionary pathway from a male–female species to a selfing‐hermaphrodite species is constructed. First, the frequency of mutant females capable of facultative self‐fertilization increases if the benefits of reproductive assurance exceed the cost. Second, hermaphrodites become obligate self‐fertilizers if the fitness of selfed offspring exceeds one‐half the fitness of outcrossed offspring. Genetic considerations specific to C. elegans and E. texana show that males may endure as descendants of the ancestral male–female species. These models combined with an extensive literature review suggest a sexual conflict over mating in these androdioecious species: selection favours hermaphrodites that self and males that outcross. The strength of selection on hermaphrodites and males differs, however. Males that fail to outcross suffer a genetic death. Hermaphrodites may never encounter a rare male, and those that do and outcross only bear less fecund offspring. This asymmetric sexual conflict results in an evolutionary stand‐off: rare, but persistent males occasionally fertilize common, but reluctant hermaphrodites. A consequence of this stand‐off may be an increase in the longevity of the androdioecious mating system.     

The effect of protandry on siring success in Chamerion angustifolium (Onagraceae) with different inflorescence sizes

Abstract Protandry, a form of temporal separation of gender within hermaphroditic flowers, may reduce the magnitude of pollen lost to selfing (pollen discounting) and also serve to enhance pollen export and outcross siring success. Because pollen discounting is strongest when selfing occurs between flowers on the same plant, the advantage of protandry may be greatest in plants with large floral displays. We tested this hypothesis with enclosed, artificial populations of Chamerion angustifolium (Onagraceae) by experimentally manipulating protandry (producing uniformly adichogamous or mixed protandrous and adichogamous populations) and inflorescence size (two-, six-, or 10-flowered inflorescences) and measuring pollinator visitation, seed set, female outcrossing rate, and outcross siring success. Bees spent more time foraging on and visited more flowers of larger inflorescences than small. Female outcrossing rates did not vary among inflorescence size treatments. However, seed set per fruit decreased with increasing inflorescence size, likely as a result of increased abortion of selfed embryos, perhaps obscuring the magnitude of geitonogamous selfing. Protandrous plants had a marginally higher female outcrossing rate than adichogamous plants, but similar seed set. More importantly, protandrous plants had, on average, a twofold siring advantage relative to adichogamous plants. However, this siring advantage did not increase linearly with inflorescence size, suggesting that protandry acts to enhance siring success, but not exclusively by reducing between-flower interference.     

Consequences of inbreeding for offspring fitness and gender in Silene vulgaris,a gynodioecious plant

Abstract In gynodioecious plants, hermaphrodite and female plants co‐occur in the same population. In these systems gender typically depends on whether a maternally inherited cytoplasmic male sterility factor (CMS) is counteracted by nuclear restorer alleles. These restorer alleles are often genetically dominant. Although plants of the female morph are obligatorily outcrossing, hermaphrodites may self. This selfing increases homozygosity and may thus have two effects: (1) it may decrease fitness (i.e. result in inbreeding depression) and (ii) it may increase homozygosity of the nuclear restorer alleles and therefore increase the production of females. This, in turn, enhances outcrossing in the following generation. In order to test the latter hypothesis, experimental crosses were conducted using individuals derived from four natural populations of Silene vulgaris, a gynodioecious plant. Treatments included self‐fertilization of hermaphrodites, outcrossing of hermaphrodites and females using pollen derived from the same source population as the pollen recipients, and outcrossing hermaphrodites and females using pollen derived from different source populations. Offspring were scored for seed germination, survivorship to flowering and gender. The products of self‐fertilization had reduced survivorship at both life stages when compared with the offspring of outcrossed hermaphrodites or females. In one population the fitness of offspring produced by within‐population outcrossing of females was significantly less than the fitness of offspring produced by crossing females with hermaphrodites from other populations. Self‐fertilization of hermaphrodites produced a smaller proportion of hermaphroditic offspring than did outcrossing hermaphrodites. Outcrossing females within populations produced a smaller proportion of hermaphrodite offspring than did crossing females with hermaphrodites from other populations. These results are consistent with a cytonuclear system of sex determination with dominant nuclear restorers, and are discussed with regard to how the mating system and the genetics of sex determination interact to influence the evolution of inbreeding depression.     

POLLEN DISCOUNTING AND THE SPREAD OF A SELFING VARIANT IN TRISTYLOUS EICHHORNIA PANICULATA: EVIDENCE FROM EXPERIMENTAL POPULATIONS

Floral traits that increase self-fertilization are expected to spread unless countered by the effects of inbreeding depression, pollen discounting (reduced outcross pollen success by individuals with increased rates of self-fertilization), or both. Few studies have attempted to measure pollen discounting because to do so requires estimating the male outcrossing success of plants that differ in selfing rate. In natural populations of tristylous Eichhornia paniculata, selfing variants of the mid-styled morph are usually absent from populations containing all three style morphs but often predominate in nontrimorphic populations. We used experimental garden populations of genetically marked plants to investigate whether the effects of population morph structure on relative gamete transmission by unmodified (M) and selfing variants (M‘) of the mid-styled morph could explain their observed distribution. Transmission through ovules and self and outcross pollen by plants of the M and M’ morphs were compared under trimorphic, dimorphic (S morph absent), and monomorphic (L and S morphs absent) population structures. Neither population structure nor floral morphology affected female reproductive success, but both had strong effects on the relative transmission of male gametes. The frequency of self-fertilization in the M' morph was consistently higher than that of the M morph under all morph structures, and the frequency of self-fertilization by both morphs increased as morph diversity of experimental populations declined. In trimorphic populations, total transmission by the M and M' morphs did not differ. The small, nonsignificant increase in selfing by the M' relative to the M morph was balanced by decreased outcross siring success, particularly on the S morph. In populations lacking the S morph, male gamete transmission by the M' morph was approximately 1.5 times greater than that by the M morph because of both increased selfing and increased success through outcross pollen donation. Therefore, gamete transmission strongly favored the M' morph only in the absence of the S morph, a result consistent with the distribution of the M' morph in nature. This study indicates that floral traits that alter the selfing rate can have large and context-dependent influences on outcross pollen donation.     

To self or not to self? Absence of mate choice despite costly outcrossing in the fungus Podospora anserina

Fungi have a large potential for flexibility in their mode of sexual reproduction, resulting in mating systems ranging from haploid selfing to outcrossing. However, we know little about which mating strategies are used in nature, and why, even in well-studied model organisms. Here, we explored the fitness consequences of alternative mating strategies in the ascomycete fungus Podospora anserina. We measured and compared fitness proxies of nine genotypes in either diploid selfing or outcrossing events, over two generations, and with or without environmental stress. We showed that fitness was consistently lower in outcrossing events, irrespective of the environment. The cost of outcrossing was partly attributed to non-self recognition genes with pleiotropic effects on fertility. We then predicted that when presented with options to either self or outcross, individuals would perform mate choice in favour of the reproductive strategy that yields higher fitness. Contrary to our prediction, individuals did not seem to avoid outcrossing when a choice was offered, in spite of the fitness cost incurred. Our results suggest that, although functionally diploid, P. anserina does not benefit from outcrossing in most cases. We outline different explanations for the apparent lack of mate choice in face of high fitness costs associated with outcrossing, including a new perspective on the pleiotropic effect of non-self recognition genes.