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

Abstract

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.

Restricted Gene Flow at Specific Parts of the Shrew Genome in Chromosomal Hybrid Zones

“…3). This category of hybrids has been shown to be as fit as homozygotes in these shrews Fredga 1997, 1998;Banaszek et al 2002) and several backcross hybrids have been detected in this zone . In this situation, rearranged chromosomes can recombine independently and should form a much weaker linkage block than in HT.…”

“…Data from the S. araneus group suggest that Robertsonian heterozygotes do not suffer from infertility as substantially as other taxa (Searle 1993;Fredga 1997, 1998;Banaszek 2000). Nevertheless, Banaszek et al (2002) observed levels of nondisjunction high enough to affect fertility of complex heterozygotes. These authors highlighted the difference between simple and complex heterozygotes because they did not find any evidence that simple heterozygotes are less fit than homozygotes.…”

“…These models were mainly developed thinking about inversions, which affect recombination especially strongly. However, they can also be applied to other categories of rearrangements (such as Robertsonian fusions), which are also known to affect the frequency and distribution of crossovers (e.g., Bidau et al 2001;Banaszek et al 2002;Merico et al 2003). Although each model has unique features (for recent reviews, see Spirito 2000;Rieseberg 2001;Coyne and Orr 2004;Ayala and Coluzzi 2005;Butlin 2005), all suggest an interesting possibility: gene exchange frequencies should be differentiated according to the chromosome on which they are located.…”

“…3). This category of hybrids has good survival and does not seem to suffer a reduction in fertility Fredga 1997, 1998;Banaszek et al 2002), and species-specific chromosomes can almost freely introgress into heterospecific populations (Brü nner et al 2002b). In HT, most of the F 1 hybrids with S. antinorii present a long chain of 11 elements (complex heterozygous, C-XI; Fig.…”

Chromosomal rearrangements and gene flow over time in an inter-specific hybrid zone of the Sorex araneus group

“…This hybrid zone occurs between Sorex antinorii, which is characterized by the metacentrics gi, hj, kn and l/o (notation: l/o, the slash indicating that the Robertsonian fusion is polymorphic), and the Vaud race of S. araneus, which is characterized by mg, hi, jl, kr and no, according to the nomenclature by Searle et al (1991) (Figure 1). F1 hybrids between these taxa should be relatively infertile because their chromosomes would assemble into a long chain of 11 elements during meiosis ('complex' heterozygotes, CXI; Figure 1) Fredga, 1997, 1998;Banaszek et al, 2002). To obtain balanced gametes (with a complete haploid set of chromosome arms) during meiosis, the species-specific chromosomes have to segregate in the same way as when they formed the F 1 zygote (Figure 1).…”