The evolution and maintenance of sexual reproduction has been one of the major questions in evolutionary ecology for the last decades. Sex and reproduction are not synonymous, although they are often linked since the vast majority of animals reproduce sexually. Sexual reproduction, which combines meiosis and fertilisation, is associated with profound costs. This means that sex must provide significant benefits that can compensate for its costs. In addition, the balance between relative costs and benefits of sex may vary according to different factors, such as life-histories or ecology of species. A collection of theoretical models has studied variables costs and benefits of sex, but empirical studies are scarce.
Although less frequent than sex in animals, several organisms have evolved towards a reproduction without sex, such as parthenogenesis. Studying such alternative forms of reproduction is of fundamental importance for identifying the balance between costs and benefits of sex that could tip towards a selection for parthenogenesis, and thus help understanding the evolution of reproductive polymorphisms in the wild.
Ephemeroptera (mayflies) are one of the few animal groups where sexual reproduction co-occurs with different types of parthenogenesis at the inter- and intra-species levels, including facultative parthenogenesis. In addition, their typically low dispersal ability and short and fragile adult life may frequently generate situations of mate limitation in females, which may strongly select for reproductive assurance. Mayflies thus provide ideal conditions to progress in understanding the maintenance of sexual reproduction in natural populations, and the transition towards parthenogenesis.
This thesis aims to investigate the frequency of parthenogenesis in mayfly species overall, as a first step towards developing this group as a model system for investigating the balance between the benefits of sexual reproduction compared to parthenogenesis in natural populations. Then, this work focuses on the mayfly species Alainites muticus to better understand the evolution of reproductive polymorphism in the wild, and to identify ecological correlates of different reproductive strategies in natural populations.
We find that the indirect benefits of sex do not always compensate for its costs, which leads to a transition from sexual reproduction to parthenogenesis, more often than previously thought given the high frequency of parthenogenesis in mayflies. We find that life-history traits, such as body sizes, population growth rates and dispersal abilities are contributing to the transition rate from sexual to parthenogenetic reproduction. Such transition is also influenced by demographic and environmental factors, such as population density and community diversity.
Overall, this thesis improves our knowledge on life-histories and ecologies of mayflies, and gives novel insights into why sexual reproduction is the dominant reproductive mode in the wild. Mayflies are currently clearly underappreciated for their value as an outstanding model system to help resolving the “paradox of sex”.