The phylogeographic relationships of common hamster (Cricetus cricetus) populations in Poland were determined by the analysis of three partial mtDNA sequences: control region, cytochrome b and 16S rRNA. A phylogenetic tree as well as parsimony network, consistently separate Polish common hamsters into two groups: E1 being so far specific for the area of Poland, and P3 which clusters inside a Pannonian lineage, previously described from the Carpathian Basin. Polish hamsters do not share any haplotypes with the ÔNorthÕ -lineage from Germany and Western Europe, although Poland most likely represents the main migration corridor from the eastern distribution centre to the western boundary of the species range. Fossil and DNA data indicate a very recent appearance of the E1 lineage in the Polish Uplands, probably at the very end of the last glaciation. On the other hand, the Pannonian group entered southern Poland as early as the second stadial of the last glaciation (Middle Vistulian 53.35 ka). The hamster lineages in Poland seem to show different population structures and demographic histories.

Although the European hamster is probably the fastest-declining Eurasian mammal, its IUCN RedList status is still Least Concern. In addition to the huge distribution area, this categorization is based on the assumptions (1) that the decline affects only Western Europe, where (2) modern agriculture has led to (3) an increase in the mortality of the species. Since mortality-reducing protection measures in Western Europe have been unable to stop the decline, we reviewed the literature from 1765 to the present and reappraised the situation. We found support for none of these assumptions. The species has also vanished from more than 75% of its range in Central and Eastern Europe. In 48 of 85 Russian, Belarussian, Ukrainian and Moldovan provinces, its relative occurrence has decreased. It is now rare in 42 provinces and extinct in 8. Mortality has not increased, but the reproduction rate has shrunk since 1954 throughout the distribution area. Today the reproduction rate is only 23% of that between 1914 and 1935. Taking into account the mortality of this prey species, 1 female today raises only 0.5 females for next year's reproduction. The extrapolation of the literature data points to an extinction of the species between 2020 and 2038. We strongly recommend (1) changing the status of the European hamster on the IUCN Red List from Least Concern at least to Vulnerable or even Endangered and (2) supporting scientific research on the reproduction of European hamsters as a protection measure. Global threats such as climate change, light pollution or (in the past) fur trapping are more likely to be the ultimate reason for the decline of this species than modern agriculture.

Two parapatric chromosomal races of the common shrew (Sorex araneus) in Poland differ in their complement of metacentric arm combinations: hk, io, gr, nm (race IV), and hi, ko, gm, np (race II). In hybrids, these eight race-diagnostic metacentrics form two randomly segregating complexes. The first complex (C ) occurs in the form of a ring configuration ok/kh/hi/io, or a chain o/ok/kh/hi/i (when there is Robertsonian polymorphism of the element io). The second complex (C ) always takes the form of a six-element chain configuration r/rg/gm/mn/np/p. The C complex may be shortened to five or even four elements, when acrocentrics g, m and n are present. In the contact zone we found shrews of pure races (race II or IV), as well as hybrids with C or C complexes, and recombinants hi, ko, gr, nm. Complex heterozygotes are likely to suffer reduced fertility due to malsegregation at meiosis. However, the C hybrids with ring configurations occur with a high frequency throughout the contact zone. This suggest that their fitness is only slightly lowered relative to pure race individuals, in contrast to the hybrids with C or C chain configurations, which presumably have a more heavily reduced fertility. On the other hand, at the center of the zone there is a high proportion of recombinants, which, being chromosomal homozygotes, should display normal meiotic segregation. Furthermore, the high frequencies of recombinants within the contact zone should facilitate gene flow between the races. The occurrence of recombinants plays a similar role as the appearance of the maximum frequencies of acrocentric homozygotes described in several contact zones of S. araneus.

Chromosomal rearrangements are proposed to promote genetic differentiation between chromosomally differentiated taxa and therefore promote speciation. Due to their remarkable karyotypic polymorphism, the shrews of the Sorex araneus group were used to investigate the impact of chromosomal rearrangements on gene flow. Five intraspecific chromosomal hybrid zones characterized by different levels of karyotypic complexity were studied using 16 microsatellites markers. We observed low levels of genetic differentiation even in the hybrid zones with the highest karyotypic complexity. No evidence of restricted gene flow between differently rearranged chromosomes was observed. Contrary to what was observed at the interspecific level, the effect of chromosomal rearrangements on gene flow was undetectable within the S. araneus species.

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