The Seliger – Moscow hybrid zone between chromosome races of common shrews – an initial description

Abstract

We provide the first description of a hybrid zone between chromosome races of Sorex araneus in Central European Russia. In the area near Lake Seliger (Upper Volga), the locally-distributed Seliger race (diagnostic chromosomes hn, ik, mq, pr, g, o) meets and hybridises with the more widespread Moscow race (gm, hi, kr, no, pq). This hybrid zone appears to be narrow and constrained to the inter-lake isthmus between Lake Seliger and the River Volga system. The hybrid zone may be centred on a minor road. Hybrids occur between the two races but at low frequency. There were three F 1 hybrids out of a total of 87 specimens. These were expected to form the maximally long meiotic chain that can be found in the common shrew: a chain-of-eleven (CXI). Such a configuration was observed in meiotic preparations from one adult male F 1. Two other complex heterozygote karyotypes were found. The existence of these backcross individuals reflects Robertsonian polymorphism for certain race-specific chromosomes. It is presumed that these various hybrids have reduced fertility, consistent with the narrowness of the hybrid zone and bimodal structure. This is likely to be a strong hybrid zone with a greater probability of reduced gene flow and evolution of reproductive isolation than other hybrid zones in the common shrew.

Phenotypic Variation across Chromosomal Hybrid Zones of the Common Shrew (Sorex araneus) Indicates Reduced Gene Flow

“…The specimens represent two Sorex araneus hybrid zones that were the subject of previous chromosomal studies: (1) the Moscow-Seliger (M-S) zone in European Russia, where individuals were allocated to 18 sublocalities derived from five parallel trap lines crossing the hybrid zone ( Figure 2A ) ; and (2) the Novosibirsk-Tomsk (N-T) hybrid zone in Siberia, where individuals were collected from 20 sublocalities scattered on both sides of the metacentric hybrid zone center ( Figure 2B ) and two additional pure-race localities farther removed from the hybrid zone (see Table S1 for locality details for both hybrid zones). The chromosomal clines at these two hybrid zones were recently published by some of us [10], [11], [34]; the methods used to extract chromosomes and to estimate clines are described in full there. Briefly, however, karyotypes for each individual were determined using G-banding procedures [54].…”

“…We measured clines in skull and mandible shape using geometric morphometrics across the Moscow-Seliger (M-S) hybrid zone in European Russia [10], [34] and the Novosibirsk-Tomsk (N-T) zone in Siberia [11], [35], [36]. Heterozygote offspring from the two zones produce especially long meiotic chains of eleven and nine and three chromosomes respectively ( Figure 1 ).…”

New data on the geographic distribution of chromosome races of Sorex araneus (Soricidae, Eulipotyphla) in European Russia

“…It is notable that this race (#14 in Fig. 1) was found close to where the Moscow and Seliger races form a hybrid zone (near the type locality of the Seliger race #8; see Bulatova et al, 2007). Thus, it is likely that the Moscow race (#7) forms a continuous hybrid zone with two other races, the West Dvina (#14) and the Seliger (#8), along its western edge.…”

“…1) along the Upper Volga and its right tributary the Oka and show, first, that it crosses the Volga to the north of the type locality, and second, that it occurs further to the south-east than described previously, where it comes very close to the Neroosa race (#6). While the contact between the Moscow and Neroosa races is unconfirmed, samples of the Moscow race to the north-west of its range have helped to define a hybrid zone between the Moscow and Seliger races (Bulatova et al, 2007). West Dvina race.…”

“…Now the focus is shifting to a search for the contacts between these races (Bystrakova et al, 2003). The first description of a hybrid zone in European Russia is given in this volume (Bulatova et al, 2007). Here we report distribution data on six previously described chromosome races and use these data to suggest locations for new contact zones.…”