Abstract:
Explaining organismal diversity and adaptation are the two central aims of evolutionary biology. In this thesis, I address these questions using a comparative evolutionary approach and nematodes of the family Diplogastridae as a study system. Systematic sampling of diplogastrid nematodes resulted in the discovery of numerous nematode species, their novel traits and ecological associations, providing reference points for consequent comparative evolutionary analyses. I first studied forces that drive evolutionary radiation and speciation in a host-symbiont system, specifically in a diverse group of predatory nematodes of the genus Micoletzkya, which are associated with bark beetles. I showed that nematode evolutionary radiation resulted from parallel divergence with their beetle hosts and was also shaped by preferential hosts shifts among closely related hosts, thus highlighting the roles of isolation and adaptation in the evolution of host-symbiont systems. I then built a new phylogenetic framework for the family Diplogastridae and used it to investigate the evolutionary implications of discrete phenotypic plasticity (polyphenism), specifically its impact on evolutionary tempo and the evolution of novelties. As a result, I identified that the origin of polyphenism coincided with a sharp increase in phenotypic complexity, which surprisingly decreased after secondary loss of plasticity; the rates of evolution, however, became even higher after phenotype fixation, which provided evidence for developmental character release. These results gave original support for a role of developmental plasticity in evolutionary diversification. Finally, I have discovered several new species of Pristionchus, which live in symbiosis with figs and fig wasps and show an extreme polyphenism with up to five discrete eco-morphotypes per species. This study revealed that adaptive radiation, or the rapid filling of contrasting ecological niches need not to be associated with diversifying selection on genotypes and can be based on discontinuous phenotypic plasticity.
developmental plasticity