A unique accumulation of chromosomal variants and coat colors in a small population of the Norway rat at Kanagawa Prefecture, central Japan

We researched coat coloration, chromosomal polymorphism, and mitochondrial DNA in a small population of the Norway rat, Rattus norvegicus, in Fujisawa, Kanagawa Prefecture of central Honshu, Japan. In the research area, higher frequency of the non-agouti coat coloration (29.4%), and heteromorphic pairs of subtelocentrics (ST) and telocentrics (T) in the Nos. 3 (57.2%) and 12 (14.3%) homologues were observed. Previously, such coat coloration and chromosomal heterozygosity have been confirmed at lower frequencies in some localities in Japan. In addition, according to the Hardy–Weinberg test for the current zygosities for both homologues, both FIS values statistically indicating denial of inbreeding (p＞0.05) were apparently different and Fisher’s test revealed the rejection of independent inheritance of both homologues (p＞0.1). Moreover, the non-agouti individuals frequently carried the No. 3 chromosome of ST. These facts would be caused by a certain bias of some combinations of zygosities of both homologues. Furthermore, the mitochondrial DNA results revealed quite lower nucleotide diversity indicating the occurrence of a recent bottleneck. Considering the higher incidence of the non-agouti coat with frequent No. 3 chromosome of ST and such a bias of combinations of both homologue zygosities, it is concluded that inbreeding has been progressed after a bottleneck event in the current research area and a certain No. 3 chromosome of ST carrying a mutant allele causing the non-agouti seems to be accumulated there. Accordingly, the coat and chromosomal tendencies of the research area seem to be unique, that are different from those of other localities.