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.

A comparison of spermatogenesis in homozygotes, simple Robertsonian heterozygotes and complex heterozygotes of the common shrew (Sorex araneus L.).

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 differed 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 five-element chains. There was a significant effect of karyotype on the level of germ-cell death and chain-forming complex heterozygotes suffered the greatest germ-cell loss. However, the estimated level of germ-cell death is probably insufficient to influence the fertility of these males.

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.