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Sexual conflict and the evolution of asexuality at low population densities.

ABSTRACT: Theories for the evolution of sex rarely include facultatively sexual reproduction. Sexual harassment by males is an underappreciated factor: it should at first sight increase the relative advantage of asexual reproduction by increasing the cost of sex. However, if the same females can perform either sexual or asexual life cycles, then females trying to reproduce asexually may not escape harassment. If resisting male harassment is costly, it might be beneficial for a female to accept a mating and undertake a sexual life cycle rather than 'insist' on an asexual one. We investigate the effects of sexual harassment on the maintenance of sex under different population densities. Our model shows that resisting matings pays off at low population densities, which leads to the complete extinction of males, and thus to the evolution of completely asexual populations. Facultative sex persists in a narrow range of slightly higher densities. At high densities, selection favours giving up resisting male mating attempts and thus sexual reproduction takes over. These interactions between the outcomes of sexual conflict and population density suggest an explanation for the rarity of facultative sex and also patterns of geographical parthenogenesis, where marginal environments with potentially low densities are associated with asexuality.

SUBMITTER: Gerber N

PROVIDER: S-EPMC5095377 | biostudies-literature | 2016 Oct

REPOSITORIES: biostudies-literature

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Sexual conflict and the evolution of asexuality at low population densities.

Gerber Nina N Kokko Hanna H

Proceedings. Biological sciences 20161001 1841

Theories for the evolution of sex rarely include facultatively sexual reproduction. Sexual harassment by males is an underappreciated factor: it should at first sight increase the relative advantage of asexual reproduction by increasing the cost of sex. However, if the same females can perform either sexual or asexual life cycles, then females trying to reproduce asexually may not escape harassment. If resisting male harassment is costly, it might be beneficial for a female to accept a mating an ...[more]

PMID: 27798298

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Project description:Existing models explaining the evolution of sexual dimorphism in the timing of emergence (SDT) in Lepidoptera assume equal mortality rates for males and females. The limiting assumption of equal mortality rates has the consequence that these models are only able to explain the evolution of emergence of males before females, i.e. protandry-the more common temporal sequence of emergence in Lepidoptera. The models fail, however, in providing adaptive explanations for the evolution of protogyny, where females emerge before males, but protogyny is not rare in insects. The assumption of equal mortality rates seems too restrictive for many insects, such as butterflies. To investigate the influence of unequal mortality rates on the evolution of SDT, we present a generalised version of a previously published model where we relax this assumption. We find that longer life-expectancy of females compared to males can indeed favour the evolution of protogyny as a fitness enhancing strategy. Moreover, the encounter rate between females and males and the sex-ratio are two important factors that also influence the evolution of optimal SDT. If considered independently for females and males the predicted strategies can be shown to be evolutionarily stable (ESS). Under the assumption of equal mortality rates the difference between the females' and males' ESS remains typically very small. However, female and male ESS may be quite dissimilar if mortality rates are different. This creates the potential for an 'evolutionary conflict' between females and males. Bagworm moths (Lepidoptera: Psychidae) provide an exemplary case where life-history attributes are such that protogyny should indeed be the optimal emergence strategy from the males' and females' perspectives: (i) Female longevity is considerably larger than that of males, (ii) encounter rates between females and males are presumably low, and (iii) females mate only once. Protogyny is indeed the general mating strategy found in the bagworm family.

Project description:Sexual selection and sexual conflict play central roles in driving the evolution of male and female traits. Experimental evolution provides a powerful approach to study the operation of these forces under controlled environmental and demographic conditions, thereby allowing direct comparisons of evolutionary trajectories under different treatments such as mating systems. Despite the rapid progress of experimental and statistical techniques that support experimental evolution studies, we still lack clear theoretical predictions on the effects of different mating systems beyond what intuition suggests. For example, polygamy (several males and females in a mating group) and polyandry (one single female and multiple males in a mating group) have each been used as treatments that elevate sexual selection on males and sexual conflict relative to monogamy. However, polygamy and polyandry manipulations sometimes produce different evolutionary outcomes, and the precise reasons why remain elusive. In addition, the softness of selection (i.e., scale of competition within each sex) is known to affect trait evolution, and is an important factor to consider in experimental design. To date, no model has specifically investigated how the softness of selection interacts with different mating systems. Here, we try to fill these gaps by generating clear and readily testable predictions. Our set of models were designed to capture the most important life cycle events in typical experimental evolution studies, and we use simulated changes of sex-specific gene expression profiles (i.e., feminization or masculinization) to quantify trait evolution under different selection schemes. We show that interactions between the softness of selection and the mating system can produce results that have been identified as counterintuitive in previous empirical work such as polyandry producing stronger feminization than monogamy. We conclude by encouraging a stronger integration of modelling in future experimental evolution studies and pointing out remaining knowledge gaps for future theoretical work.

Dataset Information

Sexual conflict and the evolution of asexuality at low population densities.

Publications

Sexual conflict and the evolution of asexuality at low population densities.

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