Abstract
Many hermaphroditic organisms, either plants or animals, are able to reproduce by self-fertilization, at least alternatively with cross fertilization. Theoretical models predict several important consequences linked to this mating system. The first prediction is that a selfing population is less sensitive to inbreeding depression than an outcrossing one, because part of the depression can be « purged » meaning that the recessive deleterious alleles are easier to eliminate by natural selection under selfing. This purge creates a positive feedback to favour self fertilization. In these circumstances, we also expect the evolution of traits facilitating self fertilization (for example closed flowers) and a reallocation of resources from the male to the female function, because sexual selection is reduced in the male function. Self-fertilization also affects standing variation, as the effective population size is divided by two, enhancing the effects of drift. In addition, recombination becomes inefficient, increasing the extent of selective interference among loci (background selection, selective sweep) and decreasing the probability to fix several advantageous mutations in the same genome. In other words, self-fertilization decreases the adaptive potential and the efficiency of natural selection. We then predict that autogamous species have a higher probability of extinction, this is called the “dead end hypothesis”. Some of these predictions have been tested mainly in plants or not at all. The aim of this thesis is to test them in animals, using freshwater snails as model systems. To this end, we followed an experimental evolution approach using laboratory populations of Physa acuta a preferentially outcrossing snail able to reproduce by self-fertilization. These populations were maintained for 20 to 30 generations either under pure outcrossing or under alternating generations of outcrossing and selfing. In a first experiment we show that inbreeding depression is largely purged after only ten generations of selfing, but also that the waiting time, (a trait positively correlated to the outcrossing rate) decreased largely. We did not observe however any reallocation in favour of the female function. In a second experiment we compared the response to artificial selection on a morphological trait under selfing and outcrossing. We observed that when an outbred population switches to self-fertilization the response to selection is initially enhanced as alleles are progressively made homozygous. However this advantage is quickly offset by selective interference and after no more than three generations selfing populations start to respond to selection more slowly than outcrossing onesThis work brings new elements for the understanding of the evolution of mating systems, and provides empirical support for the lower adaptability of selfing species.