Spermatogenesis was studied in 56 shrews (Sorex araneus L.) from two chromosomal hybrid zones in Poland. The hybrid zones were formed between chromosome races that diered in Robertsonian metacentrics. Shrews were compared in four classes: homozygotes, simple Robertsonian heterozygotes, complex heterozygotes forming four-element rings in meiosis I, and complex heterozygotes forming four-or ®ve-element chains. There was a signi®cant eect of karyotype on the level of germ-cell death and chain-forming complex heterozygotes suered the greatest germ-cell loss. However, the estimated level of germ-cell death is probably insucient to in¯uence the fertility of these males.Keywords: chromosome races, common shrew, complex heterozygotes, hybrid zone, Sorex araneus, spermatogenesis. IntroductionThe common shrew (Sorex araneus L. 1758) shows exceptional chromosome variation at both the intraand interpopulation level. The variability of the common shrew karyotype is based on the occurrence of Robertsonian (centric) fusions and whole-arm reciprocal translocations (see Searle, 1993). Interpopulation chromosome variation manifests itself as the presence of chromosome races, that dier either in the chromosome arm combination of the metacentrics, or the number of these metacentrics relative to the ancestral acrocentric complement (for the list of races see Zima et al., 1996). Metacentrics speci®c to neighbouring races may show monobrachial homologies relative to one another (i.e. metacentrics have one arm in common). In consequence, interracial hybridization gives rise to hybrids that form chromosome multivalents at prophase I of meiosis. Chromosome multivalents consisting of meta-and acrocentrics form chain (CH) con®gurations, whereas ring (R) con®gurations are formed by the metacentrics only.It is commonly thought that a balance between dispersal and selection against hybrids maintains chromosomal hybrid zones. Selection against hybrids results from their lowered fertility caused by the presence of atypical con®gurations at meiosis I. Meiotic disorders, such as a failure of homologous chromosomes to pair accurately at prophase I and an increased level of nondisjunction at anaphase I, may lead to the greater germ-cell death and postzygotic loss of aneuploid embryos. Pairing irregularities may lead to germ-cell death as a result of inappropriate expression of genes at unpaired regions of autosomes (Miklos, 1974; Burgoyne & Baker, 1984) or as a consequence of interactions between the sex chromosomes in males and unsynapsed regions of autosomes (Forejt, 1984).Although the lower ®tness of hybrids may be inferred from the structure of many chromosomal hybrid zones (see Searle, 1993; Searle & Wo jcik, 1998), direct data on the fertility of hybrids of the common shrew are still scarce. This paper presents results of studies on spermatogenesis in male common shrews aimed to establish the relationship between level of germ-cell death and karyotype. The males come from two hybrid zones: one between the Drnholec (Dn) and èeË gucki Møyn (...

Thirty-three adult male common shrews (Sorex araneus L.) were collected from a hybrid zone between two chromosomal races that differed in Robertsonian metacentrics. Anaphase I nondisjunction frequencies were estimated on the basis of metaphase II counts. RIV and CV complex heterozygotes (four-element rings and five-element chains at meiosis I, respectively) had substantially higher nondisjunction rates than homozygotes and simple Robertsonian heterozygotes. However, at least in the case of RIV-forming hybrids, increased nondisjunction frequency did not result from malsegregation of the heterozygous complex. Extra elements found in hyperploid spreads were most frequently acrocentrics, that could not originate from a fully metacentric multivalent. Complex heterozygotes were also characterized by higher frequencies of univalents observed at diakinesis I. However, univalents did not originate from complex configurations, which were regularly formed with usually one chiasma per chromosome arm. Hence, we suppose that the presence of multivalents in the cell affects pairing and segregation of other elements at meiosis I.

We established the location of the contact zone between the Drnholec (diagnostic metacentrics hi, ko, gm, nr) and Bialowieza (hn, 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 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/Bialowieza 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.

Common shrews are subdivided into numerous chromosome races which make contact and form hybrid zones. To date, no strong differences in morphology have been found between hybrid and pure race individuals from the vicinity of such hybrid zones. To investigate this further, we carried out studies in Poland on three hybrid zones between races belonging to the West and East European Karyotypic Groups using material collected from 1989 to 1994. Shrews were measured (head and body, tail and hindfoot length) and weighed. Statistically significant differences in morphology were only observed in the Stobnica/Legucki Mlyn hybrid zone. The results indicated that both adult and immature animals belonging to the Stobnica race were smaller than those belonging to the Legucki Miyn race. The hybrids were longer than individuals of pure race karyotype but the mean body mass of the hybrids was the lowest. At the population level, the size of the shrews varied with value of a hybrid index. The absence of similar results for the Drużnol/Łȩgucki Mtyn and Drnholec/Bialowieza hybrid zones may reflect their narrowness compared with the Stobnica/Łȩgucki Mlyn zone.