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.

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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a b s t r a c tThe common hamster is endangered in Europe due to changes in agricultural practice, industrialization, and direct persecution. In Poland, two main areas of the current species distribution have become separated. Moreover, theoretical possibilities of the gene flow have been further complicated by the presence of two ancient phylogeographic lineages. To describe the differentiation of the populations and levels of gene flow in this complex pattern, we examined the current genetic variability using a set of 17 nuclear DNA microsatellites in 195 hamsters representing 12 sampling sites. The levels of genetic diversity are fairly high within phylogeographic lineages, but most of the variation is partitioned between populations. The common hamster populations probably function in a metapopulation structure and the connectivity of habitats is crucial for diversity preservation. Presently the possibilities of the gene flow are low or absent, which already becomes evident in lowered diversity measures of one lineage. Although the common hamster populations are genetically resilient to fluctuations in numbers, the fragmentation of agricultural habitats and isolation of suitable patches directly endangers the species.