Robertsonian polymorphism in the common shrew (Sorex araneus L.) and selective advantage of heterozygotes indicated by their higher maximum metabolic rates.

Some cases of Robertsonian (Rb) polymorphism in the common shrew (Sorex araneus L.) are believed not to be associated with hybrid zones. One of the hypotheses explaining the persistence of such Rb polymorphism is that they are maintained by some form of selection for Rb heterozygotes. To test this hypothesis, we compared several parameters between homozygotes and Rb heterozygotes for the mp chromosome pair. We used shrews from Jurowce population in Poland, situated within the range of the Bialowieza race, where Rb polymorphism persists far from any known hybrid zone. We found no differences between the two karyotypic classes in maximum metabolic rate during running (forced activity). However, the Rb heterozygotes showed significantly higher maximum metabolic rate during swimming (forced activity combined with thermal stress). The levels of fluctuating asymmetry (FA) of homozygous and Rb heterozygous shrews were indistinguishable, indicating no effect of chromosomal heterozygosity on developmental stability of shrews. We suggest that selective advantages, such as the higher metabolic performance in activity combined with cold stress, may outweigh the expected negative effects of Rb heterozygosity upon fertility, and help to maintain huge areas of the Rb polymorphism in this species.

Preferential segregation of metacentric chromosomes in simple Robertsonian heterozygotes of Sorex araneus.

One of the hypotheses explaining preferential transmission of metacentrics among simple Robertsonian (Rb) heterozygotes of the common shrew (Sorex araneus L.) invokes the existence of meiotic drive. Thus far, evidence that metacentrics are favoured at meiosis has been obtained indirectly, on the basis of crosses made under controlled conditions. The aim of the present work was to test the hypothesis in a direct study. We analysed products of chromosome segregation among 12 simple heterozygote male subjects from a wild population, with regard to jl, io, nr and mn Rb fusions. We were able to demonstrate significant segregation distortion in favour of all four metacentrics. The level of preferential segregation was independent either of the composition of chromosome arms or the dimensions of metacentrics. We also found that X chromosomes were favoured over Y1Y2 chromosomes during segregation. We discuss the role of meiotic drive in the evolutionary success of metacentric chromosomes in S. araneus, as well as in the emergence of post-hybridization modifications in the zones of contact between races.

Meiotic studies of male common shrews (Sorex araneus L.) from a hybrid zone between chromosome races.

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.

The evolutionary history of the two karyotypic groups of the common shrew, Sorex araneus, in Poland.

Genetic variability within and among two karyotypic groups and five chromosome races of the common shrew (Sorex araneus) in Poland was assayed by sequencing a 1023 bp part of the cytochrome b gene (mtDNA) from 28 individuals. Thirty-four variable positions defined 21 distinct haplotypes with a maximum sequence divergence of 0.88%. No significant differentiation in the cytochrome b gene between Western and Eastern Karyotypic groups was found. Haplotype diversity estimates within the races and groups sampled were high (h = 0.800-0.928), while nucleotide diversity estimates were low (pi = 0.0034-0.0053). The distribution of pairwise nucleotide differences fits well with expectations of a "sudden expansion" model. High haplotype diversity was accompanied by relatively high expected heterozygosity (H(E)) values in nuclear genes (calculated over 47 enzyme loci: H(E) = 0.031 - 0.049), giving no evidence for a recent bottleneck after the process of post-Pleistocene recolonization of Poland by the shrews. Thus, for S. araneus chromosome races in Poland, the data on the cytochrome b gene variability support the hypothesis assuming the Robertsonian fusions having spread into an ancestral acrocentric distribution.

Study of a contact between two chromosomally monomorphic races of Sorex araneus L. (common shrew).

One hundred and twelve specimens of the common shrew from 21 populations in NE Poland were studied from an area of contact between chromosomal race II (metacentric chromosomes: hi, ko, gm, np) and race VII (ki, hn, gr, mp). No direct contact was found between the races in the study area; there were no mixed or hybrid populations. The shortest distance between populations of the two races was 1.2-1.6 km. Possible explanations for this pattern of the races' disjunction are discussed.