Phylogeographical Patterning in Daphnia ambigua: Regional Divergence and Intercontinental Cohesion

Daphnia ambigua, one of the most broadly distributed members of its genus, occurs in Europe as well as in North and South America. This investigation examines diversity in two mitochondrial genes, COI and 12S rDNA, to ascertain the geographical distribution of variation in this species. The results establish that North American D. ambigua are separable into four phylogroups with an average of 4% mtDNA sequence divergence. One of these groups is dominant, occupying Mexico and the central United States, whereas the other groups are restricted to the Atlantic or Pacific coasts. Their distributional centroids coincide with refugial areas identified in prior studies, reinforcing evidence that the Appalachian and West Coast ranges have been important in the isolation of zooplankton lineages. Gene exchange, mediated by migratory birds whose flyways are structured by these mountains, is likely instrumental in maintaining north-south cohesion within each phylogroup. European populations of D. ambigua are closely allied to those from the eastern United States, a result concordant with their presumed recent introduction from North America. By contrast, South American populations last shared an ancestor with North American lineages approximately 2 million yr ago. The phylogeographic information now available suggests that topographic barriers generate predictable patterns of population divergence in zooplankton species and that allopatric speciation has played an important role in their diversification.