title: The long-term consequences of hybridization between the two Daphnia species, D. galeata and D. dentifera, in mature habitats.
abstract: BACKGROUND
  Ecological specializations such as antipredator defense can reinforce morphological and distributional divergence within hybridizing species. Two hybridizing species of Daphnia (D. galeata and D. dentifera) are distributed in both Japan and North America; however, these populations have a longer history in Japan than in North America due to the differing impact of the last glaciation on these two regions. We tested the hypothesis that this longer coexistence in Japan would lead to extensive genetic admixture in nuclear and mitochondrial DNA whilst the distinct morphological traits and distributional patterns would be maintained.
  
  
  RESULTS
  The high level of correspondence among morphological traits, distribution, and mitochondrial and nuclear DNA types for the specimens with D. dentifera mtDNA indicated that the species distinction has been maintained. However, a discordance between mtDNA and nuclear ITS-1 types was observed for most specimens that had D. galeata mtDNA, consistent with the pattern seen between the two species in North America. This observation suggests nuclear introgression from D. dentifera into D. galeata without mitochondrial introgression.
  
  
  CONCLUSIONS
  The separation of morphological traits and distribution ranges of the two hybridizing species in Japan, as well as in North America, has been maintained, despite large differences in climatic and geographical histories of these two regions. Variations in environmental factors, such as predation pressure, might affect maintenance of the distribution, although the further studies are needed to confirm this.
authors.affiliation: Division of Ecology and Evolutionary Biology, Department of Environmental Life Sciences, Graduate School of Life Sciences, Tohoku University, Aoba, Sendai, Miyagi, Japan.
authors.forename: Seiji
authors.initials: S
authors.lastname: Ishida
authors.forename: Akiko
authors.initials: A
authors.lastname: Takahashi
authors.forename: Noe
authors.initials: N
authors.lastname: Matsushima
authors.forename: Jun
authors.initials: J
authors.lastname: Yokoyama
authors.forename: Wataru
authors.initials: W
authors.lastname: Makino
authors.forename: Jotaro
authors.initials: J
authors.lastname: Urabe
authors.forename: Masakado
authors.initials: M
authors.lastname: Kawata
mesh_terms: D000818:Animals; D002678:Chimera / Q000145:classification / Q000235:genetics*; D004272:DNA, Mitochondrial / Q000235:genetics; D003621:Daphnia / Q000145:classification / Q000235:genetics*; D017753:Ecosystem; D006824:Hybridization, Genetic; D007564:Japan; D008969:Molecular Sequence Data; D009656:North America; D010802:Phylogeny; D013045:Species Specificity
chemical_list: D004272:DNA, Mitochondrial