Asexual Fitness Research Articles

Warming increases survival and asexual fitness in a facultatively sexual freshwater cnidarian with winter diapause.

Temperature is a key abiotic factor controlling population dynamics. In facultatively sexual animals inhabiting the temperate zone, temperature can regulate the switch between asexual and sexual modes of reproduction, initiates growth or dormancy, and acts together with photoperiod to mediate seasonal physiological transitions. Increasing temperature due to recent global warming is likely to disrupt population dynamics of facultatively sexual animals because of the strong temperature dependence of multiple fitness components. However, the fitness consequences of warming in these animals are still poorly understood. This is unfortunate since facultatively sexual animals-through their ability for asexual reproduction resulting in quick population growth and sexual reproduction enabling long-term persistence-are key components of freshwater ecosystems. Here, I studied the fitness effects of warming in Hydra oligactis, a freshwater cnidarian that reproduces asexually throughout most of the year but switches to sexual reproduction under decreasing temperatures. I exposed hydra polyps to a simulated short summer heatwave or long-term elevated winter temperature. Since sexual development in this species is dependent on low temperature, I predicted reduced sexual investment (gonad production) and elevated asexual fitness (budding) in polyps exposed to higher temperatures. The results show a complex effect of warming on sexual fitness: While gonad number decreased in response to warming, both male and female polyps exposed to high winter temperature were capable of multiple rounds of gamete production. Asexual reproduction and survival rate, on the contrary, clearly increased in response to higher temperature, especially in males. These results predict increased population growth of H. oligactis in temperate freshwater habitats, which will likely affect the population dynamics of its main prey (freshwater zooplankton), and through that, the whole aquatic ecosystem.

Aborting meiosis allows recombination in sterile diploid yeast hybrids

Hybrids between diverged lineages contain novel genetic combinations but an impaired meiosis often makes them evolutionary dead ends. Here, we explore to what extent an aborted meiosis followed by a return-to-growth (RTG) promotes recombination across a panel of 20 Saccharomyces cerevisiae and S. paradoxus diploid hybrids with different genomic structures and levels of sterility. Genome analyses of 275 clones reveal that RTG promotes recombination and generates extensive regions of loss-of-heterozygosity in sterile hybrids with either a defective meiosis or a heavily rearranged karyotype, whereas RTG recombination is reduced by high sequence divergence between parental subgenomes. The RTG recombination preferentially arises in regions with low local heterozygosity and near meiotic recombination hotspots. The loss-of-heterozygosity has a profound impact on sexual and asexual fitness, and enables genetic mapping of phenotypic differences in sterile lineages where linkage analysis would fail. We propose that RTG gives sterile yeast hybrids access to a natural route for genome recombination and adaptation.

Periodic, Parasite-Mediated Selection For and Against Sex.

Asexual lineages should rapidly replace sexual populations. Why sex then? The Red Queen hypothesis proposes that parasite-mediated selection against common host genotypes could counteract the per capita birth rate advantage of asexuals. Under the Red Queen hypothesis, fluctuations in parasite-mediated selection can drive fluctuations in the asexual population, leading to the coexistence of sexual and asexual reproduction. Does shifting selection by parasites drive fluctuations in the fitness and frequency of asexuals in nature? Combining long-term field data with mesocosm experiments, we detected a shift in the direction of parasite selection in the snail Potamopyrgus antipodarum and its coevolving parasite, Microphallus sp. In the early 2000s, asexuals were more infected than sexuals. A decade later, the asexuals had declined in frequency and were less infected than sexuals. Over time, the mean infection prevalence of asexuals equaled that of sexuals but varied far more. This variation in asexual infection prevalence suggests the potential for parasite-mediated fluctuations in asexual fitness. Accordingly, we detected fitness consequences of the shift in parasite selection: when they were less infected than sexuals, asexuals increased in frequency in the field and in paired mesocosms that isolated the effect of parasites. The match between field and experiment argues that coevolving parasites drive temporal change in the relative fitness and frequency of asexuals, potentially promoting the coexistence of reproductive modes in P. antipodarum.

Intraspecific competition facilitates the evolution of tolerance to insect damage in the perennial plant Solanum carolinense

Tolerance to herbivory (the degree to which plants maintain fitness after damage) is a key component of plant defense, so understanding how natural selection and evolutionary constraints act on tolerance traits is important to general theories of plant-herbivore interactions. These factors may be affected by plant competition, which often interacts with damage to influence trait expression and fitness. However, few studies have manipulated competitor density to examine the evolutionary effects of competition on tolerance. In this study, we tested whether intraspecific competition affects four aspects of the evolution of tolerance to herbivory in the perennial plant Solanum carolinense: phenotypic expression, expression of genetic variation, the adaptive value of tolerance, and costs of tolerance. We manipulated insect damage and intraspecific competition for clonal lines of S. carolinense in a greenhouse experiment, and measured tolerance in terms of sexual and asexual fitness components. Compared to plants growing at low density, plants growing at high density had greater expression of and genetic variation in tolerance, and experienced greater fitness benefits from tolerance when damaged. Tolerance was not costly for plants growing at either density, and only plants growing at low density benefited from tolerance when undamaged, perhaps due to greater intrinsic growth rates of more tolerant genotypes. These results suggest that competition is likely to facilitate the evolution of tolerance in S. carolinense, and perhaps in other plants that regularly experience competition, while spatio-temporal variation in density may maintain genetic variation in tolerance.

Does the silver moss Bryum argenteum exhibit sex-specific patterns in vegetative growth rate, asexual fitness or prezygotic reproductive investment?

Expected life history trade-offs associated with sex differences in reproductive investment are often undetected in seed plants, with the difficulty arising from logistical issues of conducting controlled experiments. By controlling genotype, age and resource status of individuals, a bryophyte was assessed for sex-specific and location-specific patterns of vegetative, asexual and sexual growth/reproduction across a regional scale. Twelve genotypes (six male, six female) of the dioecious bryophyte Bryum argenteum were subcultured to remove environmental effects, regenerated asexually to replicate each genotype 16 times, and grown over a period of 92 d. Plants were assessed for growth rates, asexual and sexual reproductive traits, and allocation to above- and below-ground regenerative biomass. The degree of sexual versus asexual reproductive investment appears to be under genetic control, with three distinct ecotypes found in this study. Protonemal growth rate was positively correlated with asexual reproduction and sexual reproduction, whereas asexual reproduction was negatively correlated (appeared to trade-off) with vegetative growth (shoot production). No sex-specific trade-offs were detected. Female sex-expressing shoots were longer than males, but the sexes did not differ in growth traits, asexual traits, sexual induction times, or above- and below-ground biomass. Males, however, had much higher rates of inflorescence production than females, which translated into a significantly higher (24x) prezygotic investment for males relative to females. Evidence for three distinct ecotypes is presented for a bryophyte based on regeneration traits. Prior to zygote production, the sexes of this bryophyte did not differ in vegetative growth traits but significantly differed in reproductive investment, with the latter differences potentially implicated in the strongly biased female sex ratio. The disparity between males and females for prezygotic reproductive investment is the highest known for bryophytes.

Divergent adaptation promotes reproductive isolation among experimental populations of the filamentous fungus Neurospora

BackgroundAn open, focal issue in evolutionary biology is how reproductive isolation and speciation are initiated; elucidation of mechanisms with empirical evidence has lagged behind theory. Under ecological speciation, reproductive isolation between populations is predicted to evolve incidentally as a by-product of adaptation to divergent environments. The increased genetic diversity associated with interspecific hybridization has also been theorized to promote the development of reproductive isolation among independent populations. Using the fungal model Neurospora, we founded experimental lineages from both intra- and interspecific crosses, and evolved them in one of two sub-optimal, selective environments. We then measured the influence that initial genetic diversity and the direction of selection (parallel versus divergent) had on the evolution of reproductive isolation.ResultsWhen assayed in the selective environment in which they were evolved, lineages typically had greater asexual fitness than the progenitors and the lineages that were evolved in the alternate, selective environment. Assays for reproductive isolation showed that matings between lineages that were adapted to the same environment had greater sexual reproductive success than matings between lineages that were adapted to different environments. Evidence of this differential reproductive success was observed at two stages of the sexual cycle. For one of the two observed incompatibility phenotypes, results from genetic analyses were consistent with a two-locus, two-allele model with asymmetric (gender-specific), antagonistic epistasis. The effects of divergent adaptation on reproductive isolation were more pronounced for populations with greater initial genetic variation.ConclusionDivergent selection resulted in divergent adaptation and environmental specialization, consistent with fixation of different alleles in different environments. When brought together by mating, these alleles interacted negatively and had detrimental effects on sexual reproductive success, in agreement with the Dobzhansky-Muller model of genetic incompatibilities. As predicted by ecological speciation, greater reproductive isolation was observed among divergent-adapted lineages than among parallel-adapted lineages. These results support that, given adequate standing genetic variation, divergent adaptation can indirectly cause the evolution of reproductive isolation, and eventually lead to speciation.

The Role of Pleiotropy in the Maintenance of Sex in Yeast

In facultatively sexual species, lineages that reproduce asexually for a period of time can accumulate mutations that reduce their ability to undergo sexual reproduction when sex is favorable. We propagated Saccharomyces cerevisiae asexually for approximately 800 generations, after which we measured the change in sexual fitness, measured as the proportion of asci observed in sporulation medium. The sporulation rate in cultures propagated asexually at small population size declined by 8%, on average, over this time period, indicating that the majority of mutations that affect sporulation rate are deleterious. Interestingly, the sporulation rate in cultures propagated asexually at large population size improved by 11%, on average, indicating that selection on asexual function effectively eliminated most of the mutations deleterious to sporulation ability. These results suggest that pleiotropy between mutations' effects on asexual fitness and sexual fitness was predominantly positive, at least for the mutations accumulated in this experimental evolution study. A positive correlation between growth rate and sporulation rate among lines also provided evidence for positive pleiotropy. These results demonstrate that, at least under certain circumstances, selection acting on asexual fitness can help to maintain sexual function.

ANTAGONISM BETWEEN SEXUAL AND NATURAL SELECTION IN EXPERIMENTAL POPULATIONS OF SACCHAROMYCES CERVISIAE

Trade-offs between life-history components are a central concept of evolution and ecology. Sexual and natural selection seem particularly apt to impose antagonistic selective pressures. When sex is not integrated into reproduction, as in Saccharomyces cerevisiae, natural selection can impair or even eliminate it. In this study, a genetic trade-off between the sexual and asexual phases of the yeast life cycle was suggested by sharp declines in the mating and sporulation abilities of unrelated genotypes that were propagated asexually in minimal growth medium and in mice. When sexual selection was applied to populations that had previously evolved asexually, sexual fitness increased but asexual fitness declined. No such negative correlation was observed when sexual selection was applied to an ancestral strain: sexual and asexual fitness both increased. Thus, evolutionary history affected the evolution of genetic correlations, as fitness increases in a population already well adapted to the environment were more likely to come at the expense of sexual functions.

ANTAGONISM BETWEEN SEXUAL AND NATURAL SELECTION IN EXPERIMENTAL POPULATIONS OF SACCHAROMYCES CEREVISIAE

Trade-offs between life-history components are a central concept of evolution and ecology. Sexual and natural selection seem particularly apt to impose antagonistic selective pressures. When sex is not integrated into reproduction, as in Saccharomyces cerevisiae, natural selection can impair or even eliminate it. In this study, a genetic trade-off between the sexual and asexual phases of the yeast life cycle was suggested by sharp declines in the mating and sporulation abilities of unrelated genotypes that were propagated asexually in minimal growth medium and in mice. When sexual selection was applied to populations that had previously evolved asexually, sexual fitness increased but asexual fitness declined. No such negative correlation was observed when sexual selection was applied to an ancestral strain: sexual and asexual fitness both increased. Thus, evolutionary history affected the evolution of genetic correlations, as fitness increases in a population already well adapted to the environment were more likely to come at the expense of sexual functions.

Maintenance of sexually dimorphic preadult traits in Marchantia inflexa (Marchantiacae)

Marchantia inflexa, a dioecious thallose liverwort, is sexually dimorphic in clonal expansion traits. We used selection analyses to measure the magnitude and direction of selection on clonal fitness to uncover possible mechanisms for the maintenance of preadult sexually dimorphic characters. We planted replicates of genotypes of female and male M. inflexa in two light environments in a greenhouse and measured morphological and phenological characters associated with growth and asexual reproduction. Timing to onset of asexual reproduction and plant size early in development were under sex-specific selection in a low light environment. Additionally, females exhibited a sex-specific cost of plasticity in the timing of their onset of asexual reproduction in high light. Selection on asexual fitness tended to shift traits toward monomorphism rather than sexual dimorphism, whereas the expressed phenotype of females was congruent with patterns of selection acting on sexual fitness. We detected negative trade-offs between asexual and sexual fitness components in females in one light environment. Opposing selective forces acting on asexual and sexual fitness components may explain how sexual dimorphisms persist in the face of selection for monomorphism in the preadult phase.

ECOLOGICAL AND EVOLUTIONARY STABILITY OF SPERM-DEPENDENT PARTHENOGENESIS: EFFECTS OF PARTIAL NICHE OVERLAP BETWEEN SEXUAL AND ASEXUAL FEMALES.

Pseudogamous females reproduce parthenogenetically but require sperm. We analyze a density- and frequency-dependent model for the ecological and evolutionary stability of bisexual populations exposed to invasion by pseudogamous clones. In particular, we examine the effects of partial niche overlap and asymmetric competition between sexual and asexual forms. The model predicts that for a variety of relative fitness values for asexual females, pseudogamous forms can successfully invade bisexual populations. The probability of successful invasion increases as niche overlap decreases. Furthermore, invaded populations are often likely to be stable; for the parameter values analyzed, only combinations of nearly complete niche overlap and high asexual fitness will lead to extinction. Even such combinations will be stable under pronounced asymmetric competition. Asymmetric competition does not, however, affect the invadability of bisexual populations. The model predicts that stable populations cannot have more than three or four females per male; populations with more biased sex ratios are expected to be unstable. We analyze available sex ratio data for pseudogamous insects, fish, and salamanders, and find significant changes in roughly one-half of the asexual-dominated populations, but in only one sexual-dominated population. This analysis includes previously unpublished data on population sex ratios in a pseudogamous bark beetle, Ips acuminatus. Some asexual-dominated populations have far more than four females per male, contrary to predictions of the model.