Geographic parthenogenesis and the common tea‐tree stick insect of New Zealand

Worldwide, parthenogenetic reproduction has evolved many times in the stick insects (Phasmatidae). Many parthenogenetic stick insects show the distribution pattern known as geographic parthenogenesis, in that they occupy habitats that are at higher altitude or latitude compared with their sexual relatives. Although it is often assumed that, in the short term, parthenogenetic populations will have a reproductive advantage over sexual populations; this is not necessarily the case. We present data on the distribution and evolutionary relationships of sexual and asexual populations of the New Zealand stick insect, Clitarchus hookeri. Males are common in the northern half of the species’ range but rare or absent elsewhere, and we found that most C. hookeri from putative‐parthenogenetic populations share a common ancestor. Female stick insects from bisexual populations of Clitarchus hookeri are capable of parthenogenetic reproduction, but those insects from putative‐parthenogenetic populations produced few offspring via sexual reproduction when males were available. We found similar fertility (hatching success) in mated and virgin females. Mated females produce equal numbers of male and female offspring, with most hatching about 9–16 weeks after laying. In contrast, most eggs from unmated females took longer to hatch (21–23 weeks), and most offspring were female. It appears that all C. hookeri females are capable of parthenogenetic reproduction, and thus could benefit from the numerical advantage this yields. Nevertheless, our phylogeographic evidence shows that the majority of all‐female populations over a wide geographic area originate from a single loss of sexual reproduction.     

Parthenogenetic populations of the freshwater snail Campeloma limum occupy habitats with fewer environmental stressors than their sexual counterparts

1. Sexual organisms should have half the reproductive rate of their parthenogenetic counterparts (i.e. twofold cost of sex), so the plethora of sexual species relative to parthenogenetic species remains an evolutionary paradox. The rarity of parthenogenesis may in part be due to the accumulation of deleterious mutations. Indeed, parthenogenetic populations of the freshwater snail Campeloma limum have a greater mutation load relative to sexual populations of C. limum, although this does not directly affect their reproductive fitness. We hypothesise that although parthenogenesis has no direct effect on fitness in C. limum, mutation accumulation and environmental stress act synergistically to limit the distribution of parthenogenetic populations. 2. We evaluated this hypothesis, predicting that parthenogenetic populations of C. limum would inhabit sites with fewer environmental stressors than their sexual counterparts. We collected water quality, population density and individual size data at multiple time points from eight parthenogenetic and five sexual populations in the south‐eastern United States (Georgia and South Carolina). 3. Consistent with our hypothesis, sexual populations of C. limum inhabited poorer‐quality areas (sites with significantly lower dissolved oxygen and significantly more faecal coliform bacteria) than parthenogenetic populations. Despite these stressors, sexual populations still exhibited significantly higher population density than parthenogenetic populations. 4. Our findings support the hypothesis that mutation‐laden parthenogenetic C. limum populations occupy habitats with fewer environmental stressors relative to their sexual counterparts. Moreover, sexual C. limum populations inhabit lower‐quality habitats where they can presumably evade the twofold cost of sex in the absence of competition from their parthenogenetic counterparts.     

Sex at the margins: parthenogenesis vs. facultative and obligate sex in a Neotropical ant

Geographic parthenogenesis is a distribution pattern, in which parthenogenetic populations tend to live in marginal habitats, at higher latitudes and altitudes and island‐like habitats compared with the sexual forms. The facultatively parthenogenetic ant Platythyrea punctata is thought to exhibit this general pattern throughout its wide range in Central America and the Caribbean Islands. Workers of P. punctata from the Caribbean produce diploid female offspring from unfertilized eggs by thelytokous parthenogenesis, and mated females and males are rare. In contrast, workers in one colony from Costa Rica were incapable of thelytoky; instead mated workers produced all female offspring. Because sample sizes were very low in former studies, we here use microsatellite markers and explicit tests of thelytoky to examine the population genetic structure of ancestral and derived populations of P. punctata throughout the Caribbean and Central America. Populations from the Caribbean islands were fully capable of parthenogenesis, and population genetic signatures indicate that this is the predominant mode of reproduction, although males are occasionally produced. In contrast, the northernmost population on the mainland (Texas) showed signatures of sexual reproduction, and individuals were incapable of reproduction by thelytoky. Contrary to expectations from a geographic parthenogenesis distribution pattern, most parts of the mainland populations were found to be facultatively thelytokous, with population genetic signatures of both sexual and parthenogenetic reproduction.     

Effects of clonality on the genetic variability of rare,insular species: the case of Ruta microcarpa from the Canary Islands

Many plant species combine sexual and clonal reproduction. Clonal propagation has ecological costs mainly related to inbreeding depression and pollen discounting; at the same time, species able to reproduce clonally have ecological and evolutionary advantages being able to persist when conditions are not favorable for sexual reproduction. The presence of clonality has profound consequences on the genetic structure of populations, especially when it represents the predominant reproductive strategy in a population. Theoretical studies suggest that high rate of clonal propagation should increase the effective number of alleles and heterozygosity in a population, while an opposite effect is expected on genetic differentiation among populations and on genotypic diversity. In this study, we ask how clonal propagation affects the genetic diversity of rare insular species, which are often characterized by low levels of genetic diversity, hence at risk of extinction. We used eight polymorphic microsatellite markers to study the genetic structure of the critically endangered insular endemic Ruta microcarpa. We found that clonality appears to positively affect the genetic diversity of R. microcarpa by increasing allelic diversity, polymorphism, and heterozygosity. Moreover, clonal propagation seems to be a more successful reproductive strategy in small, isolated population subjected to environmental stress. Our results suggest that clonal propagation may benefit rare species. However, the advantage of clonal growth may be only short‐lived for prolonged clonal growth could ultimately lead to monoclonal populations. Some degree of sexual reproduction may be needed in a predominantly clonal species to ensure long‐term viability.     

Functional rare males in diploid parthenogenetic Artemia

Functional males that are produced occasionally in some asexual taxa – called ‘rare males’ – raise considerable evolutionary interest, as they might be involved in the origin of new parthenogenetic lineages. Diploid parthenogenetic Artemia produce rare males, which may retain the ability to mate with females of related sexual lineages. Here, we (i) describe the frequency of male progeny in populations of diploid parthenogenetic Artemia, (ii) characterize rare males morphologically, (iii) assess their reproductive role, using cross‐mating experiments with sexual females of related species from Central Asia and characterize the F1 hybrid offspring viability and (iv) confirm genetically both the identity and functionality of rare males using DNA barcoding and microsatellite loci. Our result suggests that these males may have an evolutionary role through genetic exchange with related sexual species and that diploid parthenogenetic Artemia is a good model system to investigate the evolutionary transitions between sexual species and parthenogenetic strains.     

Could sex be maintained through harmful males?

All else being equal, parthenogenetic females should produce as many surviving daughters as sexual couples produce daughters plus sons. Hence the resources spent on producing sons are a cost of sex and parthenogenetic females economize on sons. It has recently been shown that a small competitive advantage of sexual individuals can recoup this large reproductive disadvantage, while the adaptation behind the competitive advantage might differ from case to case. One hypothesis that has not yet been considered as a potential competitive advantage is that males could differentially harm parthenogenetic females, for example, through harassment, toxic seminal fluids, or infanticide. Harmful male functions result from the selection for males that maximise their fitness at the expense of females in the context of sexual conflict. Unless parthenogenetic lineages can maintain their resistance against harmful male functions, a competitive advantage for sex should be a by-product benefit of sexual conflict.
Mutations that make males harm parthenogenetic females worse than sexual ones, however, can be seen as evolutionary spite. The spiteful trait is not the production of costly sons, but the production of males that discriminate against parthenogenetic females. Spiteful behaviour can be positively selected, if it acts against negatively related victims. Sexual and parthenogenetic individuals within a population should usually be negatively related, because the genomes of the sexual individuals are bound together by recombination while those of parthenogenetic individuals will be identical except for divergence through mutation.
Some unusual cases of parthenogenesis are discussed in the light of this new hypothesis and an experimental approach for testing it is suggested.     

Embryo production in a parthenogenetic snail (Potamopyrgus antipodarum) is negatively affected by the presence of other parthenogenetic females

Abstract. Stimuli associated with copulatory behavior are often needed to maximize reproductive output in internally fertilized sexual taxa. Although non-pseudogamous parthenogenetic females have no need for sperm, parthenogens descended from sexual ancestors may still require copulatory stimuli to reach their full reproductive potential. Retention of physiological dependence on copulation in parthenogens could facilitate the maintenance of sexual reproduction in species where sexual and parthenogenetic individuals coexist if parthenogens do not receive enough copulatory stimuli to achieve maximal daughter production. A laboratory experiment was conducted to determine whether embryo production in parthenogenetic female snails ( Potamopyrgus antipodarum ) is dependent on male presence. Rather than male presence, this experiment showed that embryo production is affected by the number of coexisting parthenogens. Specifically, parthenogens housed with fewer other parthenogens produced significantly more embryos than parthenogens housed with a greater number of other parthenogens, regardless of male presence and total population size. This result indicates that copulatory dependence is not likely to contribute to the maintenance of sex in P. antipodarum . Instead, it demonstrates that females of P. antipodarum negatively affect each other's reproduction, and suggests that females of P. antipodarum may exert a larger competitive influence than males of P. antipodarum . Moreover, this finding raises the possibility that highly parthenogenetic and consequently female-dense populations of P. antipodarum may experience decreased reproductive output when population size is large and resources are limiting.     

Clonal diversity driven by parasitism in a freshwater snail

One explanation for the widespread abundance of sexual reproduction is the advantage that genetically diverse sexual lineages have under strong pressure from virulent coevolving parasites. Such parasites are believed to track common asexual host genotypes, resulting in negative frequency‐dependent selection that counterbalances the population growth‐rate advantage of asexuals in comparison with sexuals. In the face of genetically diverse asexual lineages, this advantage of sexual reproduction might be eroded, and instead sexual populations would be replaced by diverse assemblages of clonal lineages. We investigated whether parasite‐mediated selection promotes clonal diversity in 22 natural populations of the freshwater snail Melanoides tuberculata. We found that infection prevalence explains the observed variation in the clonal diversity of M. tuberculata populations, whereas no such relationship was found between infection prevalence and male frequency. Clonal diversity and male frequency were independent of snail population density. Incorporating ecological factors such as presence/absence of fish, habitat geography and habitat type did not improve the predictive power of regression models. Approximately 11% of the clonal snail genotypes were shared among 2–4 populations, creating a web of 17 interconnected populations. Taken together, our study suggests that parasite‐mediated selection coupled with host dispersal ecology promotes clonal diversity. This, in return, may erode the advantage of sexual reproduction in M. tuberculata populations.     

The Persistence of Facultative Parthenogenesis in Drosophila albomicans

Parthenogenesis has evolved independently in more than 10 Drosophila species. Most cases are tychoparthenogenesis, which is occasional or accidental parthenogenesis in normally bisexual species with a low hatching rate of eggs produced by virgin females; this form is presumed to be an early stage of parthenogenesis. To address how parthenogenesis and sexual reproduction coexist in Drosophila populations, we investigated several reproductive traits, including the fertility, parthenogenetic capability, diploidization mechanisms, and mating propensity of parthenogenetic D. albomicans. The fertility of mated parthenogenetic females was significantly higher than that of virgin females. The mated females could still produce parthenogenetic offspring but predominantly produced offspring by sexual reproduction. Both mated parthenogenetic females and their parthenogenetic-sexual descendants were capable of parthenogenesis. The alleles responsible for parthenogenesis can be propagated through both parthenogenesis and sexual reproduction. As diploidy is restored predominantly by gamete duplication, heterozygosity would be very low in parthenogenetic individuals. Hence, genetic variation in parthenogenetic genomes would result from sexual reproduction. The mating propensity of females after more than 20 years of isolation from males was decreased. If mutations reducing mating propensities could occur under male-limited conditions in natural populations, decreased mating propensity might accelerate tychoparthenogenesis through a positive feedback mechanism. This process provides an opportunity for the evolution of obligate parthenogenesis. Therefore, the persistence of facultative parthenogenesis may be an adaptive reproductive strategy in Drosophila when a few founders colonize a new niche or when small populations are distributed at the edge of a species'' range, consistent with models of geographical parthenogenesis.     

Daphnia females adjust sex allocation in response to current sex ratio and density

Cyclical parthenogenesis presents an interesting challenge for the study of sex allocation, as individuals’ allocation decisions involve both the choice between sexual and asexual reproduction, and the choice between sons and daughters. Male production is therefore expected to depend on ecological and evolutionary drivers of overall investment in sex, and those influencing male reproductive value during sexual periods. We manipulated experimental populations, and made repeated observations of natural populations over their growing season, to disentangle effects of population density and the timing of sex from effects of adult sex ratio on sex allocation in cyclically parthenogenetic Daphnia magna. Male production increased with population density, the major ecological driver of sexual reproduction; however, this response was dampened when the population sex ratio was more male‐biased. Thus, in line with sex ratio theory, we show that D. magna adjust offspring sex allocation in response to the current population sex ratio.     

EFFECTS OF PARTHENOGENESIS AND GEOGRAPHIC ISOLATION ON FEMALE SEXUAL TRAITS IN A PARASITOID WASP

Population divergence in sexual traits is affected by different selection pressures, depending on the mode of reproduction. In allopatric sexual populations, aspects of sexual behavior may diverge due to sexual selection. In parthenogenetic populations, loss‐of‐function mutations in genes involved in sexual functionality may be selectively neutral or favored by selection. We assess to what extent these processes have contributed to divergence in female sexual traits in the parasitoid wasp Leptopilina clavipes in which some populations are infected with parthenogenesis‐inducing Wolbachia bacteria. We find evidence consistent with both hypotheses. Both arrhenotokous males and males derived from thelytokous strains preferred to court females from their own population. This suggests that these populations had already evolved population‐specific mating preferences when the latter became parthenogenetic. Thelytokous females did not store sperm efficiently and fertilized very few of their eggs. The nonfertility of thelytokous females was due to mutations in the wasp genome, which must be an effect of mutation accumulation under thelytoky. Divergence in female sexual traits of these two allopatric populations has thus been molded by different forces: independent male/female coevolution while both populations were still sexual, followed by female‐only evolution after one population switched to parthenogenesis.     

Sexual Conflict over the Maintenance of Sex: Effects of Sexually Antagonistic Coevolution for Reproductive Isolation of Parthenogenesis

Sexual reproduction involves many costs. Therefore, females acquiring a capacity for parthenogenetic (or asexual) reproduction will gain a reproductive advantage over obligately sexual females. In contrast, for males, any trait coercing parthenogens into sexual reproduction (male coercion) increases their fitness and should be under positive selection because parthenogenesis deprives them of their genetic contribution to future generations. Surprisingly, although such sexual conflict is a possible outcome whenever reproductive isolation is incomplete between parthenogens and the sexual ancestors, it has not been given much attention in the studies of the maintenance of sex. Using two mathematical models, I show here that the evolution of male coercion substantially favours the maintenance of sex even though a female barrier against the coercion can evolve. First, the model based on adaptive-dynamics theory demonstrates that the resultant antagonistic coevolution between male coercion and a female barrier fundamentally ends in either the prevalence of sex or the co-occurrence of two reproductive modes. This is because the coevolution between the two traits additionally involves sex-ratio selection, that is, an increase in parthenogenetic reproduction leads to a female-biased population sex ratio, which will enhance reproductive success of more coercive males and directly promotes the evolution of the coercion among males. Therefore, as shown by the individual-based model, the establishment of obligate parthenogenesis in the population requires the simultaneous evolution of strong reproductive isolation between males and parthenogens. These findings should shed light on the interspecific diversity of reproductive modes as well as help to explain the prevalence of sexual reproduction.     

Phylogeography and ecological niche modelling of the New Zealand stick insect Clitarchus hookeri (White) support survival in multiple coastal refugia

Aim Increasing our understanding of the effects of the Last Glacial Maximum (LGM) and determining the location of refugia requires studies on widely distributed species with dense sampling of populations. We have reconstructed the biogeographic history of Clitarchus hookeri (White), a widespread species of New Zealand stick insect that exhibits geographic parthenogenesis, using phylogeographic analysis and ecological niche modelling. Location New Zealand. Methods We used DNA sequence data from the mitochondrial cytochrome c oxidase subunit I gene to reconstruct phylogenetic relationships among haplotypes from C. hookeri and two undescribed Clitarchus species. We also used distribution data from our own field surveys and museum records to reconstruct the geographic distribution of C. hookeri during the present and the LGM, using ecological niche modelling. Results The ecological niche models showed that the geographic distribution of C. hookeri has expanded dramatically since the LGM. Our model predicted large areas of suitable LGM habitat in upper North Island, and small patches along the east coast of South Island. The phylogeographic analysis shows that populations in the northern half of North Island contain much higher levels of genetic variation than those from southern North Island and South Island, and is congruent with the ecological niche model. The distribution of bisexual populations is also non-random, with males completely absent from South Island and very rare in southern North Island. Main conclusions During the LGM C. hookeri was most likely restricted to several refugia in upper North Island and one or more smaller refugia along the east coast of South Island. The unisexual populations predominate in post-glacial landscapes and are clearly favoured in the recolonization of such areas. Our study exemplifies the utility of integrating ecological niche modelling and phylogeographic analysis.     

Exploring the relationship between tychoparthenogenesis and inbreeding depression in the Desert Locust,Schistocerca gregaria

Tychoparthenogenesis, a form of asexual reproduction in which a small proportion of unfertilized eggs can hatch spontaneously, could be an intermediate evolutionary link in the transition from sexual to parthenogenetic reproduction. The lower fitness of tychoparthenogenetic offspring could be due to either developmental constraints or to inbreeding depression in more homozygous individuals. We tested the hypothesis that in populations where inbreeding depression has been purged, tychoparthenogenesis may be less costly. To assess this hypothesis, we compared the impact of inbreeding and parthenogenetic treatments on eight life‐history traits (five measuring inbreeding depression and three measuring inbreeding avoidance) in four laboratory populations of the desert locust, Schistocerca gregaria, with contrasted demographic histories. Overall, we found no clear relationship between the population history (illustrated by the levels of genetic diversity or inbreeding) and inbreeding depression, or between inbreeding depression and parthenogenetic capacity. First, there was a general lack of inbreeding depression in every population, except in two populations for two traits. This pattern could not be explained by the purging of inbreeding load in the studied populations. Second, we observed large differences between populations in their capacity to reproduce through tychoparthenogenesis. Only the oldest laboratory population successfully produced parthenogenetic offspring. However, the level of inbreeding depression did not explain the differences in parthenogenetic success between all studied populations. Differences in development constraints may arise driven by random and selective processes between populations.     

Effects of Local Plant Density and Incomplete Dichogamy on the Reproductive Success of the Rare Neotropical Palm Geonoma epetiolata

Density dependent processes are known to influence reproduction and establishment of plant populations. In this study, we evaluated the effects of local density and sexual expression on the reproductive success of the rare palm species Geonoma epetiolata in Costa Rica. We classified individuals in two density categories based on distances to the two nearest neighbors and recorded the occurrence of sexual overlap on each individual. Overlap between pistillate and staminate flowers in monoecious plants allows geitonogamous pollination, potentially reducing dependence on mates for reproduction. We measured plant size and light availability, and evaluated the influence of these variables on pollination success, fruit production, fruit abortion and seed mass. Pollination success significantly increased with density but there was no effect of sexual overlap. In contrast, there was no effect of density on the probability of initiated fruit, and a negative effect on fruit set. Fruit abortion was lowest in isolated plants with no sexual overlap. Plant size and light did not contribute to variation in reproductive success. In conclusion, the pollination advantage of plants in dense neighborhoods does not compensate reproductive losses incurred by fruit abortion, possibly due to mating among genetically related individuals. Geonoma epetiolata is threatened by habitat loss and poaching of seeds for the horticultural market. High fruit abortion rates associated with density in G. epetiolata suggest that seed collections from the remnant wild populations of this palm species may cause bottlenecks that further threaten population viability.     

Population genetic structure of parthenogenetic flatworm populations with occasional sex

1. Sexual populations are expected to perform better in fluctuating environments than asexuals because recombination provides the potential to adapt to changing environments due to increased genetic variation. Nevertheless, some asexual species show comparably high levels of genotypic diversity. Such diversity might be achieved through gene flow between coexisting sexual and asexual populations or through sexual events within asexual populations. 2. Evidence for occasional sex in the flatworm Schmidtea polychroa was previously found at one specific site that is inhabited by parthenogenetic forms. There, varying rates of sex between subpopulations, reaching up to 12%, were observed. Past recurrent sexual processes left a significant genetic signature in the population genetic structure of this population. In the present study, we examined the population genetic structure of six independent metapopulations (lakes) of the freshwater planarian flatworm S. polychroa, to confirm the presence of occasional sex and that its population genetic consequences can be generalised. 3. Using microsatellites, we found varying rates of occasional sex among subpopulations. Metapopulations showed medium to high levels of genotypic diversity that correlated with the rate of sex. 4. We conclude that occasional sex has considerable consequences for population genetic structure of parthenogenetic species and promotes diversity that might allow response to the particular type of selection that is usually predicted to favour sexual reproduction. This reproductive strategy provides genetic characteristics required for selection to act on, and might, therefore, explain the success of this parthenogenetic species.     

The origin and evolution of parthenogenesis in theHeteronotia binoei complex: Synthesis

TheHeteronotia binoei complex includes several cryptic species of sexually reproducing lizards and parthenogenetic lineages derived from them. This paper synthesizes analyses of distribution and variation of allozymes, chromosomes, mitochondrial DNA and ribosomal DNA genes in order to make inferences about the origins of the parthenogenetic lineages, the extent and source of their genetic diversity, their current and historical biogeography and their ecological properties. The parthenogens appear to have arisen recently (relative to geographic differentiation within the sexual taxa) via episodes of repetitive hybridization between two of the sexual taxa. These events probably occurred within one or two small geographic areas of western Australia, after which some of the parthenogenetic lineages rapidly expanded their ranges to central Australia. The parthenogenetic form has extraordinarily high genetic diversity, mostly derived from the repetitive origins, but with some contribution from mutation and biased gene conversion/recombination being apparent. The rapid and extensive range expansion of the parthenogenetic lineages from western to central Australia attests to the short-term success of this reproductive strategy, in this case perhaps reinforced by the parthenogenetic females having higher fecundity than their smaller sexual relatives. However, the parthenogens are orders of magnitude more susceptible to infection by ectoparasitic mites, suggesting that they could be at a long-term disadvantage. The detailed characterization of this system provides a basis for critical evaluation of hypotheses about the evolutionary advantage of sexual reproduction derived from broad comparative studies.     

Parthenogenetic female populations in the brown alga Scytosiphon lomentaria (Scytosiphonaceae,Ectocarpales): decay of a sexual trait and acquisition of asexual traits

In isogamous brown algae, the sexuality of populations needs to be tested by laboratory crossing experiments, as the sexes of gametophytes are morphologically indistinguishable. In some cases, gamete fusion is not observed and the precise reproductive mode of the populations is unknown. In the isogamous brown alga Scytosiphon lomentaria in Japan, both asexual (gamete fusion is unobservable) and sexual populations (gamete fusion is observable) have been reported. In order to elucidate the reproductive mode of asexual populations in this species, we used PCR‐based sex markers to investigate the sex ratio of three asexual and two sexual field populations. The markers indicated that the asexual populations consisted only of female individuals, whereas sexual populations are composed of both males and females. In culture, female gametes of most strains from asexual populations were able to fuse with male gametes; however, they had little to no detectable sexual pheromones, significantly larger cell sizes, and more rapid parthenogenetic development compared to female/male gametes from sexual populations. Investigations of sporophytic stages in the field indicated that alternation of gametophytic and parthenosporophytic stages occur in an asexual population. These results indicate that the S. lomentaria asexual populations are female populations that lack sexual reproduction and reproduce parthenogenetically. It is likely that females in the asexual populations have reduced a sexual trait (pheromone production) and have acquired asexual traits (larger gamete sizes and rapid parthenogenetic development).     

Life-history changes that accompany the transition from sexual to parthenogenetic reproduction in Drosophila mercatorum

In spite of the predicted genetic and ecological costs of sex, most natural populations maintain sexual reproduction, even those capable of facultative parthenogenesis. Unfertilized eggs from natural populations of Drosophila mercatorum occasionally develop into viable adults, but obligately parthenogenetic populations are unknown in this species. To evaluate the microevolutionary forces that both favor and constrain the evolution of parthenogenesis in D. mercatorum, we have measured parthenogenetic rates across a natural, sexually reproducing population and characterized the life-history changes that accompany the transition from sexual to parthenogenetic reproduction in laboratory strains. A highly significant difference in parthenogenetic rate was found between two populations in close geographic proximity, with increased rate found with lower population density. Laboratory strains of parthenogenetic females suffered increased mortality and reduced egg viability relative to their virgin counterparts from a sexual strain. Lifetime egg production was similar across all strains, but a shift in peak egg production to an earlier age also occurred. The combination of these life-history traits resulted in a higher net reproductive value for sexual females, but because they also had a longer generation time, intrinsic rate of increase was not as dramatically different from parthenogenetic females. In environments with high early mortality, there may be no fitness disadvantage to parthenogenesis, but the predicted ecological advantage of a twofold increase in intrinsic rate of increase was not realized. These results support the theory of Stalker (1956) that parthenogenesis is favored in environments in which sexual reproduction is difficult or impossible.     

Contrasting reproductive strategies of triploid hybrid males in vertebrate mating systems

The scarcity of parthenogenetic vertebrates is often attributed to their ‘inferior’ mode of clonal reproduction, which restricts them to self‐reproduce their own genotype lineage and leaves little evolutionary potential with regard to speciation and evolution of sexual reproduction. Here, we show that for some taxa, such uniformity does not hold. Using hybridogenetic water frogs (Pelophylax esculentus) as a model system, we demonstrate that triploid hybrid males from two geographic regions exhibit very different reproductive modes. With an integrative data set combining field studies, crossing experiments, flow cytometry and microsatellite analyses, we found that triploid hybrids from Central Europe are rare, occur in male sex only and form diploid gametes of a single clonal lineage. In contrast, triploid hybrids from north‐western Europe are widespread, occur in both sexes and produce recombined haploid gametes. These differences translate into contrasting reproductive roles between regions. In Central Europe, triploid hybrid males sexually parasitize diploid hybrids and just perpetuate their own genotype – which is the usual pattern in parthenogens. In north‐western Europe, on the other hand, the triploid males are gamete donors for diploid hybrids, thereby stabilizing the mixed 2n‐3n hybrid populations. By demonstrating these contrasting roles in male reproduction, we draw attention to a new significant evolutionary potential for animals with nonsexual reproduction, namely reproductive plasticity.