Ecological genetics of the benthic feeding habits of "Daphnia"

The environment of most organisms varies over different scales of space and time. Examples of evolutionary responses to environmental heterogeneity are well investigated in cladoceran crustaceans of the genus Daphnia. While, traditionally, studies of Daphnia have largely focused on its planktonic lifestyle, few authors have highlighted an important role of the benthic environment for the ecology of some species and for the evolutionary history of the genus. In this perspective, my thesis investigated the behavioural and morphological feeding adaptation of Daphnia to benthic environments mostly using the traditional model Daphnia magna which, despite being primarily pelagic, often dwells in benthic habitats.
In the first part of my thesis, I presented my work on a poorly studied feeding behaviour of D. magna, termed sediment browsing. I demonstrated genetic variation and high heritability for the behaviour among D. magna genotypes and began to investigate the ecological determinants of such variation. While local pelagic predation in the original habitats of the clones did not appear to influence browsing behaviour, clones from big lakes and ponds were shown to differ in browsing levels, suggesting how the proximity of the benthic environment might influence the evolution of benthic feeding preferences. Next, I described the genetic architecture of the behaviour by QTL analyses and identified three genomic regions associated with its variation. In another study, I analysed how genetic variation in browsing behaviour influences the establishment of microbial associations in D. magna. This study showed how genetic variation in behaviour might play a role in determining the genotype specific microbiota observed in a particular environment.
In the second part of my thesis, I focused on morphological variation in a previously poorly studied limb of Daphnia, trunk limb II, which has been proposed to serve to collect food by scraping. This study was conducted at different levels: plastic responses to food treatments within D. magna genotypes, genetic variation between D. magna clones spanning the geographical and habitat range of the species and morphological comparisons between species of the genus. The analyses did not detect a plastic response in setal morphology to the feeding treatments applied. However, I found high heritability for trunk limb II setal morphology and that variation is partially explained by geographic genetic lineage differences between clones. Finally, a preliminary comparison of trunk limb II among eleven Daphnia species found a phylogenetic distribution suggestive of convergent evolution of setal morphology in some species with similar ecologies.
Together, my work on Daphnia benthic feeding functional morphology and on the ecological genetics and functional aspects of sediment browsing behaviour highlighted the interactions with the benthic environment as an important, yet often overlooked, aspect of the ecology of Daphnia. Recently, this line of research has gained momentum in the light of a novel focus of ecological studies considering the coupling of benthic and pelagic lentic habitats. In this perspective, the work presented in my thesis might contribute to a better integration of the benthic habitats into Daphnia eco-evolutionary models.