Some Aspects of Robertsonian Karyotype Variation in European Wild Mice

Abstract

Karyotypically the genus Mus is highly conservative and the most prevalent karyotype consists of 40 acrocentric chromosomes with pericentromeric heterochromatin. In addition, many species of the genus Mus exhibit a homosequential G-band pattern along their chromosomes (Hsu et al. 1978). The 40 all acrocentric karyotype can be considered as the standard karyotype of the house mouse Mus musculus within their world-wide distribution area. In Western Europe, however, several populations with varying chromosome numbers from 2n=39 to 2n=22 are found. The reduction in chromosome number is always associated with a gain of biarmed (metacentric) chromosomes that are equivalent to two acrocentric autosomes. This type of structural rearrangement — the translocation of two acrocentric autosomes on top of each other — is well known as Robertsonian (Rb) translocation, fusion, or exchange. It is generally assumed that prior to whole arm exchange, breaks are simultaneously occurring in both partners. By an analysis of C-banded Rb chromosomes the position of the breakpoints can be ascertained more precisely. Since on both sides of the primary constriction of the rearranged chromosome similar sized blocks of constitutive heterochromatin are present, the breaks must have occurred very close to the centromeric tip of the ancestral acrocentric autosomes (Fig. 1).