Abstract
The XY chromosomes of bone marrow metaphases and the XY-synapses of pachytene spermatocytes of six taxa of Clethrionomys and Eothenomys from Japan were examined using C-banding and surface-spreading techniques. Light, and electron, microscopy revealed that in the red-backed vole, the XY size ratios of the metaphase sex chromosomes and the SC-axes of pachytene XY-synapses show a similar pattern of variation. The X chromosomes of these vole taxa were classified, on the basis of their size and morphology, as one of two types, that is they were either acrocentric or sub-telocentric. Similarly, two types of Y chromosome, small and medium, were recorded. According to these criteria, C. rufocanus, C. rutilus and Eothenomys andersoni carry an acrocentric X chromosome and a small Y chromosome, whereas the two local forms of E. smithii, the so-called “smithii-type” and “kageus-type”, carry a subtelocentric X chromosome and a medium Y chromosome. In contrast to these XY combinations, E. imaizumii showed a composite combination, with a subtelocentric X chromosome and a small Y chromosome. In view of earlier findings on the genetic background of E. imaizumii (Suzuki 1994; Suzuki et al. 1999), such a composite combination of the sex chromosomes suggests that E. imaizumii may have inherited an X chromosome from a female E. smithii and a Y chromosome from a male E. andersoni during the course of speciation through hybridisation.
