SexChange

The evolution and maintenance of sexual reproduction has been one of the major questions in evolutionary biology for the last decades: although biparental sex entails many costs, pure asexuality is rare in the eukaryotic kingdom, and a substantial number of organisms are obligate sexuals. Understanding why this costly mode of transmission of genetic material (as opposed to clonal propagation) is so widespread still represents of one the greatest challenge for evolutionary biology. On the theoretical side, important progress has been achieved over recent years: in particular, several plausible scenarios generating indirect selection for genetic mixing have been proposed, such as temporal fluctuations of the environment (generated for example by host-parasite interactions) or stochastic effects due to finite population size, generating strong interference between selected loci in non-recombining populations. However, it is still not clear to what extent the different theories proposed can explain the evolution of high rates of sex in the presence of strong direct costs. Furthermore, most of these models make simplistic assumptions about the genetic architecture of fitness: epistatic interactions between genes are either neglected, or assumed to be the same between all pairs of loci. Finally, experimental tests of theories on the possible benefits of sex remain scarce. Although experimentation in natural populations is technically difficult, experimental evolution on laboratory populations emerged as a promising approach to test theoretical predictions: in particular, experiments on different model species showed that sex accelerates adaptation to new conditions. However, in most of these experiments the environment stayed constant in time and space. The SexChange project proposes to explore selective forces acting on sex in different types of environments (stable vs. changing in time or space) using a combination of theoretical and experimental approaches. The theoretical part will consist in using adaptive landscape models representing selection acting on a number of quantitative phenotypic traits. Interestingly, these models capture different aspects of the complexity of interactions between genes, such as distributions of epistatic effects and possible compensatory effects among mutations ("sign epistasis"). They have been increasingly used over recent years to explore the dynamics and adaptation and generate predictions for the distribution of fitness effects of mutations (which have been validated by experimental data), but have rarely been used to study the evolution of reproductive systems. We will use a combination of analytical and simulation methods to explore selection for sex under (i) stable environmental conditions, or (ii) temporally or (iii) spatially varying environments. These models will allow us to investigate the effect of the genetic architecture of adaptation on selection for sex, and quantify the relative importance of stochastic and deterministic forces. The second objective of the project is to test theoretical predictions by evolution experiments on facultatively sexual rotifers (Brachionus plicatilis) in different types of environments. Mongonont rotifers represent a particularly interesting system to explore selective forces acting on sex: they are easy to maintain in the lab, have short generation times and become sexual in response to an environmental stimulus. We will use these organisms to explore the consequences of sexual reproduction on the mean and variance in fitness among offspring in different sets of conditions: stable environment with different population sizes, temporally or spatially changing environment (considering different degrees of complexity of environmental change). This combination of experimental and theoretical approaches is expected to yield new insights on one of the most challenging questions in evolutionary biology (why sex).