Abstract
Transitions between reproductive systems are very frequent in eucaryotes. Getting a comprehensive view of the actual evolutive advantages and costs of the different reproductive systems requires the understanding of the selective forces shaping such transitions. Over the last decades, empirical studies on the ecology and genetics of reproductive systems focused on long-term consequences and were conducted on natural populations. My PhD thesis aims at showing how early steps during transitions between reproductive systems are a key component to understand their evolution. To this end, I used the water flea; Daphnia spp. as a model system and study the genetic consequences of new reproductive systems. First, I investigated in the literature of asexual animals, whether the traditional view of asexuality as clonality (producing identical offspring) is realistic. This project showed that asexuals retain many features associated with sexuality from which they evolved so that strict clonality is not preeminent. While secondary evolution seems to favor clonality-like reproduction, the first steps of asexual evolution are certainly not clonal, particularly due to recombination. Second, I performed sex-asex crosses in a Daphnia species where obligate asexuals lineages producing “rare males” co-occur with sexuals. I studied the recombination rate of these asexual males and found that asexual males recombine as much as sexual ones, while asexual females recombine much less than sexual females. These results showed that the evolution of suppression of recombination is female-specific in this species and that meiosis modifications are also probably female-specific. The two projects showed that recombination is not exclusive to sexuals. Third, because males transmit asexuality genes via such crosses (a process called contagious asexuality), I also studied the reproductive modes and fitness of lab-generated asexuals compared to natural lineages. Interestingly, whereas natural asexuals are clonal, I found that new asexuals are in majority not clonal and less fit than natural ones. These results suggest that asexual lineages evolve relatively quickly to acquire the characteristics of the asexual lineages observed in natura. Fourth, using another Daphnia species, we investigated the gene expression levels of individuals with an incipient sex chromosome compared to closely related lineages whose sex is environmentally determined. I found that the evolution of genetically determined females that lost the ability to produce males is not determined by a “loss-of-function" mutation but rather by a more complex molecular mechanism. This work illustrates the relevance of using species with polymorphic reproductive systems to investigate the early evolutionary transitions between reproductive systems found in nature.