A Phylogenetic View on Species Radiation in Apodemus Inferred from Variation of Nuclear and Mitochondrial Genes

Species of field mice (genus Apodemus) are the most common rodents inhabiting woodlands and forests of the Palaearctic region. We examined the cytochrome b (cyt b) gene in mitochondrial DNA (1140 bp) and the interphotoreceptor retinoid binding protein (IRBP) gene in nuclear DNA (1152 bp) in nine species of Apodemus. Based on the genetic variation, the nine species were grouped into four lineages: (1) Agrarius group (A. agrarius, A. peninsulae, A. semotus, and A. speciosus), (2) Argenteus group (A. argenteus), (3) Gurkha group (A. gurkha), and (4) Sylvaticus group (A. alpicola, A. flavicollis, and A. sylvaticus). It was shown that these four lineages diverged within a short period of evolutionary time, suggestive of a radiation event. Soon after the radiation, the Agrarius group was likely to have differentiated again into the species lineages simultaneously. In contrast, the European clade, the Sylvaticus group, radiated rather recently. The relative ratio of the extent of sequence divergence among the four main lineages to that among the members of the subfamily Murinae (including Mus and Rattus) was calculated to be 72.4% in the cyt b gene with transversional substitutions, and 58.5% in the IRBP gene with all substitutions, using the Kimura two-parameter method. The value for the three European lineages was 27.6% in the cyt b gene and 12.3% in the IRBP gene. These results may have a correlation with the notion that deciduous broadleaf forests remained in Central East Asia through the late Tertiary to the present, while those in Europe to a large extent had disappeared by the Pliocene.