Founder effects determine the genetic structure of the water flea Daphnia in Ethiopian reservoirs

Founder effects introduce stochasticity in the genetic structure of species at the regional scale. To the extent that founder effects are important that they will result in a reduced signature of space, time and environmental variation in landscape genetic data. We studied the metapopulation genetic structure of recently founded populations of the microcrustacean Daphnia sinensis in ten Ethiopian water reservoirs. We used three different approaches of estimating the number of effective founders applied to two independent genetic marker sets to investigate the role of founder effects and to estimate effective size of the founding population. Estimates of founding sizes rarely exceeded eight individuals but were most often limited to less than four individuals. No associations of genetic identities, gene frequencies, measures of genetic diversity or differentiation with environmental and spatial variables were found. Age and size of the reservoirs were not correlated with genetic diversity measures or number of founders in these reservoirs. These findings indicate that neither strong selection, nor dispersal limitation are responsible for the observed pattern of genetic variation. Our results suggest a regional population structure that is strongly impacted by founder events, reflecting colonization by just a few founders per water body, and not noticeably influenced by subsequent dispersal and gene flow. Our results show that rapid colonization of empty habitats and fast population growth by a handful of founders can result in strong founder effects, even in relatively large habitats (estimated populations sizes of several million individuals) that are likely regularly reached by new immigrants.