Low parasitism rates in parthenogenetic bagworm moths do not support the parasitoid hypothesis for sex

The parasite hypothesis for sex is one of the many theories that have been suggested to solve the mystery of the widespread occurrence of sex despite its high short‐term costs. It suggests that sexual lineages have an evolutionary advantage over parthenogens because they can frequently generate new genotypes that are temporarily less prone to coevolving parasites. In this study, we looked for further supporting evidence for the parasite hypothesis of sex in an attempt to understand the coexistence of sexual and parthenogenetic bagworm moths (Naryciinae). The bagworm moths and their parasitoids form one of the few natural host–parasite systems where sexual and parthenogenetic hosts are apparently not separated by ecological or geographical barriers. Furthermore, in support of the parasite hypothesis for sex, parthenogenetic presence is negatively correlated with parasitism rate. We specifically tested, by identifying the reproductive mode of the parasitized individuals, whether parasitoids preferentially attack the parthenogens in sites with both sexual and parthenogenetic forms, as predicted by the parasite hypothesis. We collected hosts from sites with different frequencies of parthenogenetic and sexual moths. A DNA barcoding approach was used to determine the reproductive mode of the parasitized hosts. Furthermore, we investigated whether differences in host and parasitoid phenology could provide an alternative explanation for the variation in parasitism rates between parthenogens and sexuals. Our results contradict the prediction of the parasite hypothesis because parthenogenetic bagworm moths were less parasitized than sexuals in sympatric sites. Our findings can be explained by differences in phenology between the parthenogenetic and sexual moths rather than genetic incompatibility between parthenogenetic hosts and parasitoids. The stable coexistence of sexual and parthenogenetic Naryciinae despite the many apparent costs of sex in this system remains a mystery. Our work adds to the list of studies were the assumptions of the parasite hypothesis for sex are not all met.     

REDUCED MALE ALLOCATION IN THE PARTHENOGENETIC HERMAPHRODITE DUGESIA POLYCHROA

Parthenogenetic lineages that arise in a hermaphroditic, sexual population will inherit the male function from their sexual progenitors. Natural selection then acts to reduce male allocation of the parthenogens, freeing resources presumably for the female function. Depending on age and the available genetic variation, one therefore expects to find reduced male allocation in naturally occurring parthenogenetic lineages. We investigated the allocation to sperm production in the hermaphroditic flatworm Dugesia polychroa in three lakes containing a sexual (S), a (pseudogamous) parthenogenetic (P), and a mixed sexual-parthenogenetic population (M). Parthenogenetic lineages from M were assumed to be relatively young due to recurrent origins from the coexisting sexuals, whereas those from P were assumed to be older on biogeographical grounds. As predicted, we found drastically reduced sperm production in parthenogens compared to sexuals, even in the parthenogenetic lineages from M, which may be younger. M parthenogens did not have more testes, but produced more sperm than individuals from the purely parthenogenetic population (P). However, the latter result could not be reproduced with laboratory-raised animals and therefore may be a consequence of different ecological conditions in the different lakes, for example, differences in mating rates. To study the behavioral component of male allocation, copulation frequencies were recorded for sexuals from M and for parthenogens from P. Compared to the drastic reduction in sperm production, copulation frequency was less reduced in parthenogens. This may be a consequence of allosperm limitation in pseudogamous parthenogenetic populations.     

Comparing fecundity in parthenogenetic versus sexual populations of the freshwater snail Campeloma limum: is there a two‐fold cost of sex?

Abstract. The predominance of sexuality in eukaryotes remains an evolutionary paradox, given the "two-fold cost of sex" also known as the "cost of males." [Correction added after online publication 29 January 2009: in the preceding sentence, extraneous words were deleted.] As it requires two sexual parents to reproduce and only one parthenogenetic parent, parthenogens should have twice the reproductive rate compared with their sexual counterparts and their genes should spread twice as fast, if all else is equal. Yet, parthenogenesis is relatively rare and considered an evolutionary dead-end, while sexuality is the dominant form of reproduction in multicellular eukaryotes. Many studies have explored short-term benefits of sex that could outweigh its two-fold cost, but few have compared fecundity between closely related sexuals and parthenogens to first verify that "all else is equal" reproductively. We compared six fecundity measures between sexual and parthenogenetic populations of the freshwater snail, Campeloma limum , during a brooding cycle (1 year) across two drainages. Drainages were analyzed separately because of a significant drainage effect. In the Savannah drainage, fecundity was not significantly different between sexuals and parthenogens, even though parthenogens had significantly more empty egg capsules per brood. In the Ogeechee drainage, parthenogens had significantly more egg capsules with multiple embryos and more hatched embryos than sexuals. Taken over 1 year, embryo size was not significantly different between parthenogens and sexuals in either drainage. Given these results and the close proximity of sexual and parthenogenetic populations, it is perplexing why parthenogenetic populations have not completely replaced sexual populations in C. limum.     

Evolutionary genetics and ecology of sperm-dependent parthenogenesis

Sperm-dependent (or pseudogamous) forms of parthenogenetic reproduction occur in a wide variety of animals. Inheritance is typically clonal and matroclinous (of female descent), but sperm are needed to initiate normal development. As opposed to true parthenogenesis (i.e., sperm-independent reproduction), pseudogamous parthenogenetic lineages must coexist with a ‘sperm donor’— e.g., males from a conspecific sexual lineage, conspecific hermaphrodites, or males from a closely related sexual species. Such sperm donors do not contribute genetically to the next generation. The parasitic nature of sperm-dependent parthenogenesis raises numerous ecological and evolutionary questions. How do they arise? What factors help stabilize coexistence between the pseudogamous parthenogens and their sperm donors (i.e., ‘sexual hosts’)? Why do males waste sperm on the asexual females? Why does true parthenogenesis not evolve in pseudogamous lineages and free them from their dependency on sperm donors? Does pseudogamous parthenogenesis provide compensatory benefits that outweigh the constraints of sperm-dependence? Herein, we consider some genetic, ecological, and geographical consequences of sperm-dependent parthenogenesis in animals.     

Population structure, parasitism, and survivorship of sexual and autodiploid parthenogenetic Campeloma limum

Two theories for the maintenance of sexual reproduction, the Red Queen hypothesis and mutation accumulation, suggest that the dispersal rates of sexuals and asexuals may determine the elimination or persistence of asexuals. Under higher dispersal rates of asexuals, asexuals may temporarily escape virulent parasites and reduce the effects of deleterious mutations. In the present study, I examine the population structure, parasite loads, and juvenile survivorship of Campeloma limum sexuals and autodiploid parthenogens from the southeastern U.S. Atlantic coastal plain. Using mtDNA sequence variation, it is shown that parthenogenetic haplotypes with limited sequence divergence are geographically widespread throughout this region and there is no significant population differentiation over a broad geographical scale. Sexual C. limum populations show significant mtDNA differentiation among and within river drainages and there is significant isolation by distance. These patterns are consistent with a recent origin and range expansion of parthenogens. Prevalence of infection by digenetic trematodes is significantly higher in autodiploid parthenogens, and the variance of prevalence is also higher in autodiploid parthenogens. I argue that the latter pattern indicates that unparasitized parthenogens have temporarily escaped these virulent parasites, but recolonization of these populations by trematodes results in high infection levels (> 40%), possibly due to reduced variation in resistance genes. I also examined whether the survivorship of juvenile sexuals and parthenogens varied under different stress levels. Sexual juveniles had twofold higher survivorship in all environments. Compared to polyploid parthenogens, autodiploid parthenogens may be less buffered against the effects of deleterious recessive alleles. I propose that the combined effects of higher parasitism and reduced juvenile survivorship of these autodiploid parthenogens accounts for the spatial distribution of sexual and parthenogenetic C. limum in the Atlantic coastal plain. Parthenogens may persist by higher dispersal rates into marginal habitats where there is a temporary escape from digenetic trematodes and competition with sexuals.     

Lower fecundity in parthenogenetic geckos than sexual relatives in the Australian arid zone

Theoretical models of the advantage of sexual reproduction typically assume that reproductive output is equal in sexual and parthenogenetic females. We tested this assumption by comparing fecundity between parthenogenetic and sexual races of gekkonid lizards in the Heteronotia binoei complex, collected across a 1200 km latitudinal gradient through the Australian arid zone. Under laboratory conditions, parthenogenetic geckos had approximately 30% lower fecundity when compared with their sexual progenitors, irrespective of body size. Reflecting clutch-size constraints in gekkonids, this fecundity difference was mainly because of fewer clutches over a shorter period. When parthenogens were compared more broadly with all coexisting sexual races across the latitudinal gradient, parthenogens had lower fecundity than sexuals only when corrected for body size. Differences in fecundity between parthenogens and coexisting sexual races depended on which sexual race was considered. There was no significant relationship between fecundity and parasite (mite) load, despite significantly higher mite loads in parthenogens than in sexual races.     

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.     

The cost of sex and competition between cyclical and obligate parthenogenetic rotifers

The ubiquity of sexual reproduction is an evolutionary puzzle because asexuality should have major reproductive advantages. Theoretically, transitions to asexuality should confer substantial benefits in population growth and lead to rapid displacement of all sexual ancestors. So far, there have been few rigorous tests of one of the most basic assumptions of the paradox of sex: that asexuals are competitively superior to sexuals immediately after their origin. Here I examine the fitness consequences of very recent transitions to obligate parthenogenesis in the cyclical parthenogenetic rotifer Brachionus calyciflorus. This experimental system differs from previous animal models, since obligate parthenogens were derived from the same maternal genotype as cyclical parthenogens. Obligate parthenogens had similar fitness compared with cyclical parthenogens in terms of the intrinsic rate of increase (calculated from life tables). However, population growth of cyclical parthenogens was predicted to be much lower: sexual female offspring do not contribute to immediate population growth in Brachionus, since they produce either males or diapausing eggs. Hence, if cyclical parthenogens constantly produce a high proportion of sexual offspring, there is a cost of sex, and obligate parthenogens can invade. This prediction was confirmed in laboratory competition experiments.     

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.     

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.     

Selection of low investment in sex in a cyclically parthenogenetic rotifer

Cyclical parthenogens, which combine asexual and sexual reproduction, are good models for research into the ecological and population processes affecting the evolutionary maintenance of sex. Sex in cyclically parthenogenetic rotifers is necessary for diapausing egg production, which is essential to survive adverse conditions between planktonic growing seasons. However, within a planktonic season sexual reproduction prevents clonal proliferation. Hence, clones with a low propensity for sex should be selected, becoming dominant in the population as the growing season progresses. In this context, we studied the dynamics of the heritable variation in propensity for sexual reproduction among clones of a Brachionus plicatilis rotifer population in a temporary Mediterranean pond during the period the species occurred in plankton. Clonal isolates displayed high heritable variation in their propensity for sex. Moreover, the frequency of clones with low propensity for sex increased during the growing season, which supports the hypothesized short‐term selection for low investment in sex within a growing season. These results demonstrate (1) the inherent instability of the cyclical parthenogenetic life cycle, (2) the cost of sexual reproduction in cyclical parthenogens where sex produces diapausing eggs and (3) the role of the association between sexual reproduction and diapause in maintaining sex in these cyclical parthenogens.     

Fitness of alternative modes of reproduction: developmental constraints and the evolutionary maintenance of sex

Parthenogenesis, including facultative parthenogenesis, is common among orthopteroid insects. We investigated the fitness associated with sexual and asexual reproduction within a population of the facultatively parthenogenetic cockroach Nauphoeta cinerea. There is significantly reduced fitness for females reproducing parthenogenetically compared to sexually. Fewer than half of all females can reproduce parthenogenetically. In addition, tenfold fewer offspring are produced by parthenogenesis due to reductions in both the number of offspring produced per clutch and the number of clutches produced. Development and brooding of sexually or parthenogenetically produced first instar nymphs does not differ, although the production of the first parthenogenetic clutch is delayed relative to the first sexually produced clutch. The fitness of parthenogens is also lower than the fitness of sexually produced offspring. Parthenogens are less viable than sexually produced offspring even in the benign conditions of the laboratory. Development to adulthood of parthenogens is slower. Fewer parthenogens survive to adulthood and the adult life span of parthenogens is reduced. Individuals produced by parthenogenetic reproduction are unlikely to reproduce parthenogenetically themselves. Finally, parthenogenetically produced females produce fewer offspring by sexual reproduction than do sexually produced females. Since parthenogenetic reproduction is apomictic in N. cinerea and parthenogens are diploid, we suggest that asexual reproduction is developmentally constrained. Once meiosis has evolved, returning to a mitotic mode of reproduction may be difficult. Nauphoeta cinerea offers a system for testing how asexuality is constrained as modes of reproduction can be compared within a facultative parthenogen.     

An uncoupling of male and sexual egg production leads to reduced inbreeding in the cyclical parthenogen Daphnia

Cyclical parthenogenesis involves an alternation of parthenogenetic and sexual reproduction. In cyclical parthenogens with a short generation time, seasonal succession of clones switching to sexual reproduction at different periods of the growing season entails the risk of severe inbreeding with associated fitness costs. We show, however, that differences in genotype frequencies between males and sexual females result in a substantial reduction in the probability of selfing in natural Daphnia populations. This suggests that responses to male- and sexual egg-inducing stimuli may be uncoupled at the level of individual clones as a mechanism to avoid severe inbreeding.     

Environmental stability and the distribution of the sexes: insights from life history experiments with the geographic parthenogen Eucypris virens (Crustacea: Ostracoda)

In many species with mixed reproduction, parthenogens cover a wider geographic range than sexuals. In freshwater ostracods this pattern referred to as geographic parthenogenesis is traditionally explained by ascribing a higher potential for dispersal to parthenogens. For example, the postglacial invasion hypothesis states that the lack of males in northern Europe is caused by the relatively slow range expansion of sexual lineages after deglaciation. An alternative explanation for the contemporary distribution of the sexes is based on spatial and temporal variations in ecological habitat stability. To test this hypothesis, we compared life history data of Eucypris virens individuals originating from bisexual and all-female populations. Populations with males are only found around the Mediterranean, whereas parthenogens cover most of Europe. The animals were hatched and grown in environments mimicking temperature and photoperiod conditions observed in Belgium and Spain. The data confirm the higher potential for population growth in parthenogens. In particular their faster hatching response, possibly higher fecundity (as derived from a difference in body height) and the absence of a cost of males should allow them to out-compete sexuals under stable conditions. However, the comparison of the hatchling accumulation curves of bisexual and all-female populations suggests that sexuals have an advantage in highly unpredictable environments. Indeed, under conditions mimicking those in southern Europe, bisexual populations exhibit a bet-hedging strategy, while parthenogenetic resting eggs hatch on average earlier and more synchronously. Overall, the life history data stress the importance of short term environmental fluctuations for the distribution of the sexes in E. virens , and probably many other inhabitants of ephemeral water bodies.     

Parasites, sex, and clonal diversity in natural snail populations

Under the Red Queen hypothesis, host-parasite coevolution selects against common host genotypes. Although this mechanism might underlie the persistence of sexual reproduction, it might also maintain high clonal diversity. Alternatively, clonal diversity might be maintained by multiple origins of parthenogens from conspecific sexuals, a feature in many animal groups. Herein, we addressed the maintenance of overall genetic diversity by coevolving parasites, as predicted by the Red Queen hypothesis. We specifically examined the contribution of parasites to host clonal diversity and the frequency of sexually reproducing individuals in natural stream populations of Potamopyrgus antipodarum snails. We also tested the alternative hypothesis that clonal diversity is maintained by the input of clones by mutation from sympatric sexuals. Clonal diversity and the frequency of sexual individuals were both positively related to infection frequency. Surprisingly, although clones are derived by mutation from sexual snails, parasites explained more of the genotypic variation among parthenogenetic subpopulations. Our findings thus highlight the importance of parasites as drivers of clonal diversity, as well as sex.     

Evolution of clonality and polyploidy in a weevil system

The increased interest in asexual organisms calls for in-depth studies of asexual complexes that actively give rise to new clones. We present an extensive molecular study of the Otiorhynchus scaber (Coleoptera, Curculionidae) weevil system. Three forms have traditionally been recognized: diploid sexuals, triploid, and tetraploid parthenogens. All forms coexist in a small central area, but only the polyploid parthenogens have colonized marginal areas. Analyzing the phylogenetic relationship, based on three partial mitochondrial genes, of 95 individuals from 19 populations, we find that parthenogenesis and polyploidy have originated at least three times from different diploid lineages. We observe two major mitochondrial lineages, with over 2.5% sequence divergence between the most basal groups within them, and find that current distribution and phylogenetic relationships are weakly correlated. Quite unexpectedly, we also discover diploid clones that coexist with, and are morphologically indistinguishable from, the diploid sexual females. Our results support that these diploid clones are derived directly from the diploid sexuals. We also find that it is mainly an increase in ploidy level and not the benefits of asexual reproduction that confers to polyploid parthenogens the advantage over their diploid sexual relatives.     

Evolutionary Dynamics of ‘the’ Bdelloid and Monogonont Rotifer Life-history Patterns

Substantial differences in both life-table characteristics and reproductive patterns distinguish bdelloid from monogonont rotifers. Bdelloids reproduce only asexually, whereas most monogononts are cyclical parthenogens. We explore some of the adaptive consequences of these life-history differences using a computer model to simulate the evolutionary acquisition of new beneficial mutations. A one-locus mutation-selection regime based on the life-history characteristics of bdelloids indicates that asexuals can maintain higher levels of both allelic and genotypic diversity over a longer time period than obligate sexuals. These results are produced by differences in the magnitude of random genetic drift (RGD) associated with the different types of reproduction. Cyclical parthenogens have significantly higher evolutionary rates than sexual forms in a single-locus model, but incorporate beneficial mutations more slowly than sexuals in a two-locus simulation. Our results are therefore strongly influenced by the number of loci being evaluated as well as the pattern of reproduction. The asexual life history was found to maintain higher levels of allelic diversity than any pattern including sexual reproduction. This intriguing finding is amplified as the number of loci undergoing selection is increased. We end by considering the adaptive consequences of the remarkably divergent life histories found in typical bdelloid and monogonont rotifers.     

Mode of origin differentially influences the fitness of parthenogenetic freshwater snails

How parthenogenetic lineages arise from sexual ancestors may strongly influence their persistence over evolutionary time. Hybrid parthenogens often have elevated heterozygosity and ploidy, thus making it difficult to disentangle the influence of reproductive mode, hybridity and ploidy on their relative fitness. By comparing the relative fitness of both hybrid and non-hybrid parthenogens to their sexual ancestors, further insight may be gained into how these three factors influence the maintenance of sexual and parthenogenetic reproduction. In the present study, hybrid and non-hybrid parthenogenetic and sexual snails (Campeloma sp.) were compared for the following characteristics: female size-fecundity curves, offspring size, survivorship, and growth. Compared to nearby sexual populations, triploid hybrid parthenogens from the Florida Gulf coast have similar fecundity and offspring size, five-times higher survivorship, and 60% higher growth. Relative to nearby sexual populations, non-hybrid parthenogenetic C. limum from the Atlantic coast have significantly higher fecundity, smaller offspring size, similar survivorship and slightly lower growth. Given the considerable fitness advantages of parthenogens, especially hybrid parthenogens, it is enigmatic as to why these parthenogens occupy marginal natural habitats.     

Paternal sex in parthenogenetic planarians: a tool to investigate the accumulation of deleterious mutations

Clonally reproducing organisms are expected to accumulate slightly deleterious mutations, and this has been demonstrated in RNA viruses, bacteria and unicellular algae. In this paper we present evidence for increased embryo mortality in obligate parthenogenetic strains of the freshwater flatworm Schmidtea polychroa, possibly indicating the action of deleterious mutations. The inheritance of this fitness defect was tested by crossing parthenogens with sexuals. This is possible because both forms are simultaneous hermaphrodites that produce fertile sperm. The resulting sexual offspring showed significantly increased embryo mortality in comparison to offspring of a sexual × sexual cross. Alternatives to a mutation explanation of these results, like the degeneration of male traits in parthenogens, are being discussed. In conclusion, these results lend support to the hypothesis that sex is advantageous to a multicellular organism because it prevents the accumulation of deleterious mutations.     

Physiological implications of genomic state in parthenogenetic lizards of reciprocal hybrid origin

Parthenogenesis often evolves in association with hybridization, but the associated ecological consequences are poorly understood. The Australian gecko Heteronotia binoei is unusual because triploid parthenogenesis evolved through reciprocal crosses between two sexual lineages, resulting in four possible cytonuclear genotypes. In this species complex, we compared the performance of these parthenogenetic genotypes with their sexual progenitors for a suite of physiological traits (metabolic rate, thermal tolerance, locomotor performance, and in vitro activity and gene sequence divergence of a cytonuclear metabolic pathway, cytochrome C oxidase). Mass‐specific metabolic rate scaled differently with body mass for parthenogens and sexuals, while heat tolerance provided the only evidence for cytonuclear incompatibility in hybrid parthenogens. The most prominent phenotypic effects were attributable to nuclear genome dosage. Overall, our results suggest that the hybrid/polyploidy origin of parthenogenetic H. binoei has had surprisingly few negative fitness consequences and may have produced a broader overall niche for the species.