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The contact zone between Moscow and Western Dvina chromosomal races of common shrew Sorex araneus L. at the south of the Valdai Hights was traced over a distance of 20 km. Within this, close to parapatric, contact zone of chromosomal races the width of sympatry zone was about 500 m (the narrowest among currently known hybrid zones), and the proportion of hybrids was 24.3%. It was shown that in bimodal hybrid zones between chromosomal races of common shrew the width of sympatry zones varied from 0.5 to 13 km. This width does not correlate with the cytogenetic features of the hybrids, and seems to be determined by competitive relations between the races. The hybrid proportion is determined by the type of hybrid heterozygosity, and decreased in the race sympatry zone from 33-40 to 21.5-25.2%. The decrease of the hybrid proportion can be associated with the abnormal fertility of either the first generation, or the backcross hybrids.

The electron microscopic analysis of synaptonemal complexes of the Robertsonian translocation heterozygotes (Rb/T) and heterozygotes of the reciprocal translocation (Rb(8, 17)/T(16, 17) 43H revealed three types of translocation configurations (TC). The observed TC were subdivided into three types according to the manner of synapsis: 1) full synapsis of homologues forming trivalent with interstitial centromere in 15%; 2) the centromeric asynapsis (8 and 17-16 chromosomes forming trivalent with the extension region of asynapsis and breakpoint of the reciprocal translocation on chromosome 17 in 70%; 3) centromeric asynapsis (only single chromosome 17-16 of trivalent, when the break point of reciprocal translocation and the absence of centromeric asynapsis of another chromosome 8 formed trivalent in 15%). The frequency of association between trivalents and the XY bivalents is 20% and close proximity is 15% for heterozygotes Rb/T. Which are fertile, while in the sterile heterozygotes Rb(8, 17)/T(16, 17) 43H it is 45% for association and 15% close proximity (total 60%). These data show the significant differences in the contact of translocation configurations with the XY bivalents of fertile heterozygotes, in comparison with the sterile. Thus, it is shown that the chromosome pairing of fertile and sterile mice is clearly distinguished.

A review of studies on the chromosome sets of Palaearctic mammals carried out by Russian karyologists and an important contribution to improving the taxonomy of mammals were made for the first time. For many mammal species, the process of speciation was associated with variability in the number and morphology of the chromosomes; therefore, the karyotypes are often used as diagnostic peculiarities of morphologically similar cryptic species (sibling species). The prospects for cytogenetic studies in the field of speciation (particularly, an enhancement of reproductive isolation, initiated by chromosomal rearrangements, by selection) are discussed. [ABSTRACT FROM AUTHOR]

In this review, we discuss the processes of fixation of Robertsonian chromosome fusions in populations of the common shrew Sorex araneus L. Various Robertsonian fusions, accumulating in populations, create an illusion of large chromosomal rearrangements, reciprocal translocations of complete chromosome arms. The use of these rearrangements for phylogenetic reconstructions results in false conclusions. Robertsonian fusions accumulate in populations at such stages of the species evolution, when large open or subdivided populations prevail (populations of warm periods of Pleistocene and many present-day populations) and are fixed in small isolated populations and glacial refugia. The formation of monomorphic chromosome races requires a long time, several glaciation epochs during the whole Pleistocene. [ABSTRACT FROM AUTHOR]