Natural hybridization between extremely divergent chromosomal races of the common shrew (Sorex araneus, Soricidae, Soricomorpha): hybrid zone in Siberia

Abstract

Chromosomal races of the common shrew differ in sets of metacentric chromosomes and on contact may produce hybrids with extraordinarily complex configurations at meiosis I that are associated with reduced fertility. There is an expectation that these may be some of the most extreme tension zones available for study and therefore are of interest as potential sites for reproductive isolation. Here, we analyse one of these zones, between the Novosibirsk race (characterized by metacentrics go, hn, ik, jl, mp and qr) and the Tomsk race (metacentrics gk, hi, jl and mn and acrocentrics o, p, q and r), which form hybrids with a chain‐of‐nine (CIX) and a chain‐of‐three (CIII) configuration at meiosis I. At the Novosibirsk–Tomsk hybrid zone, the CIX chromosomes form clines of 8.53 km standardized width on average, whereas the cline for the CIII chromosomes was 52.83 km wide. The difference in these cline widths fits with the difference in meiotic errors expected with the CIX and CIII configuration, and we produce estimates of selection against hybrids with these types of configurations, which we relate to dispersal and age of the hybrid zone. The hybrid zone is located at the isocline at 200 m altitude above sea level; this relationship between the races and altitude is suggested at both coarse and fine scales. This indicates adaptive differences between the races that may in turn have been promoted by the chromosome differences. Thus, the extreme chromosomal divergence between the Novosibirsk and Tomsk may be associated with genic differentiation, but it is still striking that, despite the large chromosomal differences, reproductive isolation between the Novosibirsk and Tomsk races has not occurred.

Intra-species structuring in the common shrew Sorex araneus (Lipotyphla: Soricidae) in European Russia: morphometric variability could give evidence of limitation of interpopulation migration

“…In the common shrew simple heterozygotes cause marginal reductions in fitness, while complex heterozygotes, especially with large rings or long chains in meiosis, are substantially unfit (Searle, 1993;Searle & Wójcik, 1998). In accordance with theoretical predictions, the width of the hybrid zones in the common shrew is related to the degree of difference in karyotypes of the hybridizing races and to the complexity of their meiotic alignment (Bulatova et al, 2011;Polyakov et al, 2011). However hierarchical analyses of morphological differentiation in 24 chromosomal races and two sibling species (Sorex antinorii Bonaparte and Sorex coronatus Millet) show that it does not appear to be closely linked to karyotypic diversity.…”

Evidence for a differentiated chromosomal race north of classical south European refuge areas in the garden dormouse Eliomys quercinus

“…This is the first time that a hybrid between two chromosomal races of the garden dormouse has been reported. The existence of fertile hybrids between chromosomal races was reported in several other species including the house mouse (Mus domesticus) in Madeira (Britton-Davidian et al 2007), Mus minutoides in South Africa (Veyrunes et al 2010) and the common shrew (Sorex araneus) in Siberia (Polyakov et al 2011). The existence of a hybrid with 2N049 shows that the two races 2N050 and 2N048 can interbreed.…”

Chromosome Synapsis and Recombination in Male Hybrids Between Two Chromosome Races of the Common Shrew (<em>Sorex araneus</em> L., Soricidae, Eulipotyphla)

“…Thus, correct segregation of chromosomes is possible in the male hybrids, though direct studies are needed to estimate the frequency of meiotic non-disjunction in this hybrid zone. Certainly the narrowness of the Novosibirsk-Tomsk hybrid zone indicates that there is a degree of infertility associated with presence of the CVIII or CIX configuration in hybrids [11], either attributable to germ cell death (reduced reproductive lifespan of females, greater frequency of males with insufficient numbers of sperm) or non-disjunction (greater embryo death) [35]. Figure 1 shows the balanced segregation of the CIX, when all chromosomes derived from Tomsk race move to one pole and all Novosibirsk derived chromosomes to another pole.…”

“…The races differ for a series of chromosome rearrangements ( Figure 1, Supplementary Figure S1). They form a narrow hybrid zone and generate hybrids with both simple and complex synaptic configurations [10,11]. The simple configurations are trivalents, i.e.…”

“…The chromosomes involved in a CIX chain form clines of about nine km width on average, while the cline for the CIII chromosomes is 53 km wide. Thus, the cline widths are inverse to the complexity of synaptic configurations and the probability of meiotic errors [11].…”