We examined the gene sequences of mitochondrial cytochrome b (cyt b ) in two Japanese wood mouse species, Apodemus speciosus (n = 89) and A. argenteus (n = 46), which are distributed on the four main islands of Japan (Hokkaido, Honshu, Shikoku, and Kyushu) and on the small islands surrounding them. Apodemus speciosus , the larger of the two species, showed substantial genetic variation, with a maximum of 3% sequence divergence, and remarkable phylogenetic subdivision with two major clades. One clade represents haplotypes from a central region, including Honshu, Shikoku, Kyushu, and their adjacent islands; the other clade includes haplotypes from Hokkaido and the peripheral islands, forming four subclades: a) Hokkaido, b) Sado Island, c) Satsunan Islands, and d) the Izu Islands. Sequence divergence among the four subclades was 1.0 to 1.5%, implying that A. speciosus colonized these geographic regions 0.2 to 0.3 million years ago, assuming a substitution rate of 2.4% per million years. The population on the Izu Islands has preserved haplotypes that are distinct from those in any other region, providing good evidence for the natural colonization of the volcanic islands of the Izu Islands. The cyt b sequence variation had no relation to the karyotypic dimorphism for the eastern (2n = 48) and western (2n = 46) geographic groups, between which a strict border exists at central Honshu. On the other hand, Apodemus argenteus , the smaller of the two species, showed a similar level of sequence divergence (maximum of 3%) but no substantial geographic differentiation: populations in Hokkaido, Sado, and Yakushima shared similar haplotypes with each of the central populations, suggesting that genetic exchanges occurred among the localities in the last 0.15 million years. The apparent genetic structure of the mitochondrial DNA found in the A. speciosus population might be caused solely by longterm existence in insular regions, presumably due to ecological superiority relative to A. argenteus .

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Abstract:We examined sequence variation in the mitochondrial cytochrome b gene (1140 bp, n = 73) and control region (842-851 bp, n = 74) in the Eurasian harvest mouse (Micromys minutus (Pallas, 1771)), with samples drawn from across its range, from Western Europe to Japan. Phylogeographic analyses revealed region-specific haplotype groupings combined with overall low levels of inter-regional genetic divergence. Despite the enormous intervening distance, European and East Asian samples showed a net nucleotide divergence of only 0.36%. Based on an evolutionary rate for the cytochrome b gene of 2.4%·(site·lineage·million years) -1 , the initial divergence time of these populations is estimated at around 80 000 years before present. Our findings are consistent with available fossil evidence that has recorded repeated cycles of extinction and recolonization of Europe by M. minutus through the Quaternary. The molecular data further suggest that recolonization occurred from refugia in the Central to East Asian region. Japanese haplotypes of M. minutus, with the exception of those from Tsushima Is., show limited nucleotide diversity (0.15%) compared with those found on the adjacent Korean Peninsula. This finding suggests recent colonization of the Japanese Archipelago, probably around the last glacial period, followed by rapid population growth.Résumé : Nous avons étudié la variation des séquences du gène mitochondrial du cytochrome b (1140 pb, n = 73) et de la région de contrôle (842-851 pb, n = 74) dans des échantillons de rats des moissons (Micromys minutus (Pallas, 1771)) répartis sur toute l'aire de répartition, de l'Europe de l'Ouest jusqu'au Japon. Des analyses phylogéographiques révèlent l'existence de regroupements régionaux d'haplotypes, ainsi que des niveaux globaux faibles de divergence gé-nétique entre les régions. Malgré les distances considérables entre eux, les échantillons de l'Europe et de l'Asie de l'Est possèdent une divergence nette des nucléotides de seulement 0,36 %. Avec un taux d'évolution du gène du cytochrome b de 2,4 %·(site·lignée·million d'années) -1 , la divergence initiale estimée de ces populations se situe à il y a environ 80 000 années. Nos résultats concordent avec les données disponibles sur les fossiles qui indiquent des cycles répétés d'extinction et de recolonisation de M. minutus en Europe durant le quaternaire. Les données moléculaires laissent aussi croire que la recolonisation s'est faite à partir de refuges des régions centrales et orientales de l'Asie. Les haplotypes de M. minutus du Japon ont une faible diversité des nucléotides (0,15 %), à l'exception de ceux des îles Tsushima, par comparaison à ce qui prévaut dans la péninsule adjacente de Corée. Cette information laisse croire à une colonisation récente de l'archipel japonais, probablement aux environs de la dernière période glaciaire, suivie d'une croissance rapide des populations.[Traduit par la Rédaction] Yasuda et al. 1420

Phylogenetic relationships and divergence times among dormice (Rodentia, Gliridae) based on three nuclear genes.-Zoologica Scripta , 36 , 537-546. We examined phylogenetic relationships among six species representing three subfamilies, Glirinae, Graphiurinae and Leithiinae with sequences from three nuclear protein-coding genes (apolipoprotein B, APOB; interphotoreceptor retinoid-binding protein, IRBP; recombination-activating gene 1, RAG1). Phylogenetic trees reconstructed from maximumparsimony (MP), maximum-likelihood (ML) and Bayesian-inference (BI) analyses showed the monophyly of Glirinae ( Glis and Glirulus ) and Leithiinae ( Dryomys , Eliomys and Muscardinus ) with strong support, although the branch length maintaining this relationship was very short, implying rapid diversification among the three subfamilies. Divergence time estimates were calculated from ML (local clock model) and Bayesian-dating method using a calibration point of 25 Myr (million years) ago for the divergence between Glis and Glirulus , and 55 Myr ago for the split between lineages of Gliridae and Sciuridae on the basis of fossil records. The results showed that each lineage of Graphiurus , Glis , Glirulus and Muscardinus dates from the Late Oligocene to the Early Miocene period, which is mostly in agreement with fossil records. Taking into account that warm climate harbouring a glirid-favoured forest dominated from Europe to Asia during this period, it is considered that this warm environment triggered the prosperity of the glirid species through the rapid diversification. Glirulus japonicus is suggested to be a relict of this ancient diversification during the warm period.