Genomic analyses of population structure reveal metabolism as a primary driver of local adaptation in Daphnia pulex.

Elucidating population structure is important for understanding evolutionary features of an organism. In the freshwater microcrustacean Daphnia pulex, an emerging model system in evolutionary genomics, previous studies using a small number of molecular markers indicated that genetic differentiation among populations is high. However, the dispersal ability of D. pulex is potentially high, and evolutionary forces shaping genetic differentiation among populations are not understood well. In this study, we carried out genomic analyses using high-throughput sequencing to investigate the population structure of D. pulex. We analyzed 10 temporary-pond populations widely distributed across the midwestern United States, with each sample consisting of 71 to 93 sexually reproducing individuals. The populations are generally in Hardy-Weinberg equilibrium and have relatively large effective sizes. The genetic differentiation among the populations is moderate and positively correlated with geographic distance. To find outlier regions showing significantly high or low genetic differentiation, we carried out a sliding-window analysis of the differentiation estimates using the bootstrap. Genes with significantly high genetic differentiation show striking enrichment of gene ontology terms involved in food digestion, suggesting that differences in food quality and/or quantity among populations play a primary role in driving local adaptation of D. pulex.