The Costs Of Sex: Facing Real-world Complexities

Abstract

Understanding the maintenance of sexual reproduction constitutes a difficult problem for evolutionary biologists because of the immediate costs that sex seems to incur. Typically, general benefits to sex and recombination are investigated that might outweigh these costs. However, several factors can strongly influence the complex balance between costs and benefits of sex; these include constraints on the evolution of asexuality, ecological differentiation, and certain lif-history traits. We review these factors and their empirical support for the first time in a unified framework and find that they can reduce the costs of sex, circumvent them, or make them inapplicable. These factors can even tip the scales to a net benefit for sex. The reviewed factors affect species and species groups differently, and we conclude consequently that understanding the maintenance of sex could turn out to be more species-specific than commonly assumed. Interestingly, our study suggests that, in some species, no general benefits to sex and recombination might be needed to understand the maintenance of sex, as in our case study of dandelions.

Can resource costs of polyploidy provide an advantage to sex?

“…Because only females contribute directly to the rate of population growth, the production of males creates a twofold cost of sexual reproduction that should theoretically result in the selective elimination of sex (Maynard Smith, 1978). Why sex remains so common despite this and other costs has been termed the 'queen of questions' in evolutionary biology (Bell, 1982; recently reviewed in Meirmans et al, 2012) and continues to be the focus of much theoretical and empirical research (Otto, 2009;Zimmer, 2009).…”

“…Although recombinationfocused hypotheses have received considerable support, it is likely that no single hypothesis will provide a general explanation for the persistence of sex in natural populations. Instead, there is a growing sense that the maintenance of sex may often rely on complex or multiple mechanisms involving ecological variables (Butlin et al, 1999;West et al, 1999;Meirmans and Neiman, 2006;Scheu and Drossel, 2007;Otto, 2009;Zimmer, 2009;Meirmans et al, 2012).…”

Does the avoidance of sexual costs increase fitness in asexual invaders?

“…Physical structures such as flower size are rarely reduced in asexual plants (e.g., ref. 47). Evidence for increased vegetative growth (i.e., reallocation) in clonal vs. sexual plants is equally scarce (37).…”

“…This has been done, for instance, in the case of dandelions (47). The most notable cost of sex in this system was a combination of a cost of males and genome dilution.…”

“…However, the magnitude of this cost was much lower than the theoretical expectation, which was mainly caused by a higher incidence of male sterility in asexuals and low fertilization success of asexual pollen. Other costs of sex (e.g., mating costs) were either irrelevant or negligible (47). The authors concluded that weak ecological differentiation between sexuals and asexuals in combination with the low costs of sex were sufficient to explain coexistence of both reproductive modes.…”