Mechanisms of local adaptation in Daphnia
Species belonging to the Daphnia genus are a prime example for phenotypic plasticity, occupying a wide range of habitats throughout the world and able to colonize rapidly and thrive in new locations thanks to their cyclical parthenogenetic mode of reproduction. However, Daphnia populations of the same species can be highly divergent in their behavior, life history traits and genetic polymorphism. An obvious cause (although not the only one) for the differentiation observed between populations is local adaptation.
Simultaneously phenotype and genotype, gene expression profiles are heritable, thus providing a substrate for evolution. In species with little or no gene flow between populations such as permanent Daphnia populations, local adaptation can lead to distinctive gene expression profiles within a single species. Selection drivers as diverse as parasitism, predation and heavy metal pollution were already shown to cause rapid genetic shifts in Daphnia.
The intra-specific variation at the transcriptome level in Daphnia galeata was inferred by conducting a large scale RNAseq experiment on four European populations. Genes exhibiting differential expression patterns between populations were relatively few, partly due to the high variation within populations. A QST/FST approach and DRIFTSEL were used to identify candidate genes departing from the neutral expectations. The extensive body of literature available for Daphnia species allows linking environmental stressors with the observed gene expression patterns.
The sequence information allows searching for signs of selection at the sequence level in the transcriptome of Daphnia galeata, by using a genomic scan approach. Furthermore, expression levels of the candidate genes will be correlated with fitness measurements and sequence variation, in order to establish a link between phenotype and genotype. The outcomes of this study will allow us to understand the genetic background of rapid adaptation to environmental changes in a key species of aquatic ecosystems.