Chromosomal Relationships between Two Species of Water Shrew, Chimarrogale platycephalus and Neomys fodiens

The chromosomal relationships between the Japanese water shrew Chimarrogale platycephalus and the Eurasian water shrew Neomys fodiens were investigated by conventional and differential staining. From the side-by-side analysis of matching G-banding patterns from the two species, a high degree of G-band homology was recognized in most of the chromosomes that were compared, although the extent of such homology was unclear in several small chromosomes. In C. platycephalus, large blocks of C-heterochromatin (C-blocks) were found in pair nos. M1, M8, M10 and ST23 and in the Y chromosome as conspicuous C-bands, while N. fodiens had no such C-blocks, having centromeric C-bands on most chromosomes with the exception of several small pairs, which included ST1 and ST23. Silver-stained nucleolus organizers (Ag-NORs) were found only on the secondary constriction of pair no. 1 in C. platycephalus and on the minute short arms of ST23 and ST24 in N. fodiens. A discussion of interspecific differences in C-banding patterns, as well as in the locations of Ag-NORs, is presented, focusing on the direction of alterations in these cytogenetic features. We propose that Chimarrogale might have differentiated from a fodiens-like ancestor through chromosomal alterations, such as formation of C-blocks, translocation of NORs, and other rearrangements.