Molecular systematics of European Hyalodaphnia: the role of contemporary hybridization in ancient species

We examined phylogenetic relationships among Daphnia using mitochondrial DN A (mtDNA) sequences from the small subunit ribosomal RNA (12S), cytochrome c oxidase subunit I and nuclear DNA sequences from the first and second int ernal transcribed spacer representing 1612 base positions. Phylogenecic ana lyses using several species of the three main Daphnia subgenera, Ctenodaphn ia, Hyalodaphnia and Daphnia, revealed that the Hyalodaphnia are a monophyl etic sister group of the Daphnia. Most Hyalodaphnia species occur on one co ntinent, whereas only three are found in North America and Europe. Endemici ty of species is associated with variation in thermal tolerance and habitat differentiation. Although many species of the Hyalonaphnia are known to hy bridize in nature, mtDNA divergence is relatively high (ca. 9%) compared to other hybridizing arthropods (ca. 3%). Reproductive isolation in Daphnia s eems to evolve significantly slower than genetic isolation. We related thes e findings to what is known about the ecology and genetics of Daphnia in or der to better understand the evolutionary diversification of lineages. The relationship of these data to phylogenetic patterns is discussed in the con text of speciation processes in Daphnia.