Phylogenetics and evolution of a circumarctic species complex (Cladocera :Daphnia pulex)

The evolutionary history of freshwater zooplankton is still relatively unkn own. However, studies of the microcrustacean Daphnia have revealed interest ing patterns; the daphniids that dominate ponds and lakes in the northern h emisphere may have recent origins, likely associated with the glacial advan ces and retreats during the Pleistocene. Moreover, they form species comple xes that actively engage in hybridization and introgression. The present st udy examines the phylogenetic relationships among circumarctic members of t he Daphnia pulex complex, through the analysis of sequence diversity in 498 nt of the ND5 mitochondrial gene. Our results suggest that the complex is composed of three major clades, two of which are subdivided into at least e ight different lineages. Clearly, species in the complex shown genetic disc ontinuity. Many lineages originated during the Pleistocene, but at least th ree lineages diverged during the Pliocene. Two taxa (D. pulex, D. pulicaria ), thought to be broadly distributed in the northern hemisphere, are shown to be endemic to single continents. In general, the diversification of the pulex complex is characterized by rapidly dispersed lineages spanning enorm ous distances and also by endemism in temperate areas. Gene flow among line ages from the temperate region of different continents are restricted to ra re intercontinental migrations across a polar bridge followed by convergent morphological evolution. (C) 1998 The Linnean Society of London.