Structure and movement of the hybrid zone between two divergent lineages of the Japanese newt Cynops pyrrhogaster (Amphibia: Urodela) in central Japan

Abstract

The maintenance mechanisms of hybrid zones between two lineages/species and their gene flow dynamics are major topics for speciation studies. Elucidating these dynamics can help us to understand the process of speciation. The newt Cynops pyrrhogaster is distributed on the main islands of Japan and comprises five mitochondrial DNA (mtDNA) lineages. In this study, we surveyed the genetic relationship between two lineages (northern and central) around their boundary. We genotyped 202 individuals from 24 populations using the mitochondrial cytochrome b gene and 12 microsatellite loci. MtDNA analysis revealed that the two lineages were distributed parapatrically to each other without coexistence, whereas the microsatellite analysis indicated the presence of a hybrid zone. The estimated dispersal rates were higher from both parental lineages to the hybrid populations than those in the opposite directions. These data indicate that the hybrid zone is a tension zone, which is maintained by an equilibrium between selection against hybrid individuals and the invasion of both parental individuals from adjacent areas. The two parental lineages are genetically moderately isolated from each other and appear to be biologically independent species. The overall dispersal rate was more pronounced in the south-to-north direction. More than 10 individuals having central type of mtDNA retained some genetic characteristics derived from the northern lineage, which can be regarded as genetic footprint of their northern lineage ancestor. These results suggest that the hybrid zone has shifted from south to north, resulting in replacing the range of the northern lineage to that of the central lineage.