Genetic differentiation among Oregon lake populations of the Daphnia pulexspecies complex

Gene flow among invertebrate populations inhabiting bodies of nonflowing fr eshwater such as ponds or lakes must at some stage involve transport across habitat unsuitable for adult stages. Consequently the potential for interp opulational differentiation is high in these species, yet empirical studies of lake populations of Cladocerans such as Daphnia have failed to reveal h igh levels of genetic distinctiveness among populations and have led to muc h speculation about how these populations exchange genes and remain cohesiv e evolutionary units. In this study we surveyed 42 Oregon lake populations of Daphnia from the D. pulex species complex for genetic variation within t he mitochondrial DNA control region. We have used this data to test the rel ative abilities of various ecological factors to explain the observed patte rns in genetic differentiation among lakes. Despite limited genetic variati on detected among our samples-11 very similar RFLP-defined mtDNA genotypes from 388 individuals-analyses of nucleotide variance using analogs to Wrigh t's F statistics indicate that when multilake populations are defined in te rms of the river drainage basin to which they belong, strong and significan t amounts of among-population genetic variation can be detected at this loc us (F-ST estimates between 0.5 and 0.6). In contrast, we fail to detect con sistent significant among-population variation when populations are defined on the basis of regional physical geography, bird migratory flyways, or la ke trophic status. The manner in which the data are compiled, that is, whet her RFLPs or nucleotide sequences are used, has little effect on the overal l conclusions, yet it is clear that nucleotide sequence data would lower th e standard errors of F-ST estimates. We propose that periodic wide-scale fl ooding during the late Pleistocene may be an important mechanism to homogen ize genetic differences among lake Daphnia continent-wide south of the sout hern-most extent of Pleistocene glaciation.