Hereditas 125: zyxwvutsrqp 169-176 (1996) zyxwvutsrqp A hybrid zone between two chromosome races of the common shrew, Sorex araneus, in eastern Poland: preliminary results z KATARZYNA A. SZALAJ, STANISLAW FEDYK, AGATA BANASZEK, WLODZIMIERZ CHFTNICKI and MIROSLAW RATKIEWICZ Institute zyxwvutsrqp of Biology, University of Warsaw, Branch in Biatystok, Poland SZALAJ, K. A,, FEDYK, S., BANASZEK, A,, CHBTNICKI, W. and RATKIEWICZ, M. 1996. A hybrid zone between two chromosome races of the common shrew, zyxwv Sorex araneus, in eastern Poland: preliminary results. zyxwvutsrq ~ Hereditus 125 169- 176. Lund, Sweden. ISSN 0018-0661. We established the location of the contact zone between the Drnholec (diagnostic metacentrics hi, ko, gm, nr) and Bialowieia (hn, zyxwvuts ik, gr, mp) races of the common shrew in eastern Poland. The hybrids in this zone form meiotic chain configurations consisting of a maximum of zyxw 10 elements. Moreover, we caught 13 different types of hybrids with shorter chain complexes, resulting because of a polymorphism for metacentrics ko, nr, gr. The Drnholec/BiaIowieza contact zone is stabilized on an environmental barrier (a railway embankment). The sharp change in frequencies of diagnostic metdcentrics across the railtrack showed that it was a strong barrier to migration of the shrews. At present it is difficult to say whether there is any mechanism enhancing fertility in the hybrid populations. Katarzynu A. Szulaj, Institute of Biology, Swierkowa St. 208, 15-950 Biatystok, Poland The common shrew, Sorex araneus Linnaeus 1758, is one of the most variable species of mammals with respect to karyotype. Up to now about 50 chromo- somal races of the common shrew from Europe and Asia have been described (see ZIMA et al. 1996). Chromosomal races coming into contact give rise to hybrid zones. Hybrids (Robertsonian complex heterozygotes) of S. araneus form chain (SEARLE 1986a, 1990; ANISKIN and LUKIANOVA 1989; FEDYK et al. 1991; BRUNNER et al. 1993; BA- NASZEK 1994) or ring (FEDYK et al. 1991; BA- NASZEK 1994; NARAIN and FREDGA 1995, 1996) complexes at meiosis I. Hybrids producing both ring and chain complexes have also been found (FEDYK et al. 1991). Detailed studies have been carried out in the contact zone near Oxford in England (SEARLE 1986a, 1990, 1993; HATFIELD et al. 1992) and in two zones in Poland: near Olsz- tynek (FEDYK et al. 1991; FEDYK 1995; SZALAJ et al. 1995) and near Paslqk (BANASZEK 1994; RATKIEWICZ et al. 1994). The results of those studies suggest that hybrids have lower fertility than individuals of pure races. Hence, Robertso- nian complex heterozygotes are direct targets of natural selection. However, the strength of selec- tion varies according to the type of meiotic configu- ration formed by the hybrids. It has been suggested that ring configurations are more advantageous than chain ones, and shorter chain configurations than longer ones (FEDYK et al. 1991; BANASZEK 1994; FEDYK 1995). The chain complexes consist- ing of many elements should be the least fa- vourable. These conclusions corroborate studies on heterozygous mice, which showed that the proba- bility of producing chromosomally balanced gametes is inversely proportional to the size of meiotic multivalent (for review, see SEARLE 1993). In eastern Poland, the strongest selection should be found in the contact zone between monomor- phic regions of the Stobnica and Bialowieia races of S. maneus. Hybrids forming 10-element chain configurations at meiosis I should occur there. In order to localize that hybrid zone, studies were carried out in the lower Bug River valley. How- ever, direct contact was not found there (FEDYK et al. 1993). We caught shrews with 10-element chain configurations about 100 km south from Bug River valley, near Dqblin town. At the 12th International Chromosome Conference this area was described incorrectly as the StobnicalBiaiowieia contact zone (SZALAJ1995). NOW,it is known that the Drnholec and Bialowieza races meet in the region. The difference is that metacentric nr (diagnostic for the Drnholec race) is present instead of metacen- tric np (diagnostic for the Stobnica one). The chro- mosome arms r and p are very difficult to distinguish (SEARLE et al. 1991). In the present paper we describe the location and first results in our study of the structure of the Drnholec/Bialowieia contact zone.