Figure 3
The types of hybrids present in the Dn/Łg and Gu/Łg hybrid zones of Sorex araneus. (a) Schematic representation of the most common crosses giving rise to hybrids in the Dn/Łg hybrid zone. The individuals of the Dn, Łg race and recombinants are shown as simple Rb heterozygotes with respect to polymorphic pairs of chromosomes. The recombinants are the most common category of individuals in the centre of the hybrid zone. The crosses between recombinants and shrews of the Łg race give rise to hybrids with C1 meiotic complex. C1 hybrids are homozygotes or simple Rb heterozygotes with respect to gr and mn pairs of chromosomes that are specific for both recombinants and shrews of the Łg race. The crosses between recombinants and shrews of the Dn race give rise to hybrids with C2 meiotic complex. C2 hybrids are homozygotes with respect to hi and ko chromosome pairs. The crosses between shrews of the Łg and Dn races give rise to hybrids with two independent meiotic complexes C1 and C2. These hybrids are extremely rare in the hybrid zone due to high frequencies of recombinants (Fedyk et al., 1991). (b) chromosome composition of C1 complex. (c) chromosome composition of C2 complex. The C2 chromosome complex is highly variable due to polymorphism of all chromosome pairs forming this complex. In the case of CHV and CHIV configurations of C2 complex different chromosome arms may occur as acrocentrics. If the C2 complex is of CHIV configuration, one pair of acrocentrics is excluded from the complex and forms independent bivalent in meiosis I. (d) meiotic configurations of C1 and C2 complexes at pachytene stage: RIV — four-element ring configuration or CHV — five-element chain configuration or CHIV — four-element chain configuration. The chromosomal structure of the Gu/Łg hybrid zone generally corresponds to the part of the Dn/Łg hybrid zone, e.g. the crosses between the recombinants (the Gu race) and shrews of the Łg race. The only difference is the polymorphism of ko chromosome pair which results in the presence of one more type of C1 hybrid with CHV configuration — k/kh/hi/io/o (Banaszek, 1994).
