N. Sh. Bulatova

[Cytogenetic study of the parapatric contact zone between two 46-chromosomal forms of the common vole in European Russia].

Since the first description of sympatric sibling species and allopatric chromosomal forms of the common vole (Microtus arvalis group) in 1969, the search of their range boundaries has been continually going on up to the present time. Based on the cytogenetic material presented in the present study, the geographical distribution of two karyoforms (2n = 46: NF = 84 and NF = 72) was examined in 20 geographic localities of European Russia (within the interval between the longitude 34°–56° East and latitude 51°–61° North). The places of findings of two karyoforms, “arvalis” and “obscurus”, in Russia have been supplemented and were shown to be significantly closer; whereas the well-known distribution boundaries for the “obscurus” form were specified in the northern and southern parts of Russia. A direct evidence of parapatric distribution and hybrid zone formation between the “arvalis” and “obscurus” karyoforms was obtained, which suggests the possibility of studying the evolutionary relationships between two genomes in the Russian part of the group range under the natural experimental conditions—the hybrid zone at the boundary between the “arvalis” and “obscurus”.

Distribution of two chromosome races of the common shrew (Sorex araneus L.) in the hybrid zone: Can a change of the dispersal mode maintain independent gene frequencies?

Combination of different dispersal modes may itself, without external obstacles, lead to the appearance of subdivided populations and maintain the existence of independent population systems. The common shrew, a mammal convenient for studying different levels of intraspecific differentiation, was the object of the study. Empirical data have been used for simulation taking into account the change of dispersal modes in the population area. The obtained results agree with empirical data on the distribution of races and hybrids in the hybrid zone of chromosome races Moscow and Seliger. Change of the dispersal mode may maintain independent population dynamics and, in the case of chromosome races, prevent the migration of parental individuals into the territory of the other race.

Cytogenetic control of a hybrid zone between two Sorex araneus chromosome races before breeding season

Two chromosome races of common shrew, Moscow and Seliger, differ in the arm combination in 11 diagnostic chromosomes (Robertsonian metacentrics/acrocentrics). Homozygotes of both pure races, simple Robertsonian heterozygotes of Seliger race, and complex heterozygotes (F1 hybrids) were detected in the found earlier hybrid zone of these races, in the spring before the breeding season. The g/o heterozygote was first discovered in race Seliger, whose chromosome formula typically contains acrocentrics g and o. The m/q heterozygote was recorded for the second time. Meiosis was studied in 16 males representing five detected karyotypic categories. No abnormal in pairing of homologs in either sex trivalent common for the species (XY1Y2) or autosome trivalents (g/o and m/q) was detected at diakinesis-metaphase I. Two hybrids displayed a theoretically expected and unimpaired meiotic configuration in a form of a very long chain comprising 11 monobrachial homologs (g/gm/mq/qp/pr/rk/ki/ih/hn/no/o). The results are discussed in terms of hypotheses on fertility of complex heterozygotes and limited gene flow in hybrid zone.

Polyploid races of earthworms (Lumbricidae, Oligochaeta) in the East European plain and Siberia

The distribution of polyploid races of earthworms in the East European plain and Siberia was analyzed. Amphimictic autopolyploid races of two species of the Asian genus Eisenia, E. nordenskioldi and E. atlaviniteae, are widespread in Siberia, from its southern boundary to the arctic region, while polyploid Lumbricidae in the East-European plain, except for the Volga region, are represented mainly by parthenogenetic forms of other genera. This is presumably related to differences in the Pleistocene environmental history between the two adjacent geographic regions of Eurasia.

Intraspecific Phylogenetic Relationships in Sorex araneus L.: The Southern Baltic Subgroup of Chromosome Races

Results of chromosome G-banding were used to identify two new karyotypic races of the common shrew (Sorex araneus L.) in the European part of Russia. The chromosomal diagnosis of race St. Petersburg included five metacentrics (hk, ip, jl, mq, and nr) and two acrocentrics (g and o) (2Na = 20); the diagnosis of race West Dvina included six metacentrics (gm, hk, ip, jl, no, and qr) (2Na = 18). The phylogenetic significance of the chromosomal markers gm, hk, and ip is considered and the possibility of reticulate evolution in the species S. araneus is discussed.

A genetic study of natural hybridization between two forms of the common vole (Microtus arvalis) with the use of molecular and cytogenetic methods

222 Natural hybridization zones between earlier diverged populations provide broad opportunities for studying the gene flow dynamics and the consequences of disturbance of coadapted genomes [1]. The use of chromosome analysis methods in taxonomic studies of common voles of the Microtus arvalis group led to distinguishing karyologically discrete sibling species-the sibling vole M. rossiaemeridionalis (2 n = 54) and the common vole M. arvalis (2 n = 46) [2, 3]. The latter includes two karyomorphs differing both in the proportion of small acrocentric and metacentric chromosomes and the Y chromosome morphology: arvalis ( NF = 84) and obscurus ( NF = 72) [4–6]. These forms are widespread in the European part of Russia. They geographically replace one another [7] and are regarded by some authors as independent species [8]. A hybrid zone between them has been recently discovered in Vladimir oblast [9]. Until the end of 1990s, the karyotype remained the only criterion on the basis of which differential diagnostics of sibling species and karyomorphs of common voles could be performed. Recent data on the mitochondrial gene of cytochrome b (based primarily on samples of M. arvalis s. lat., collected outside of Russia) are indicative of a significant level of genetic differentiation between both M. rossiaemeridionalis and M. arvalis and the two karyomorphs of 46-chromosome voles [10]. The results of analysis of this gene showed that forms arvalis and obscurus are clearly distinct throughout their ranges, despite a certain level of their intraspecific differentiation [11]. The goal of this study was the search for a form-specific nuclear marker for common voles and the study of natural hybridization between forms obscurus and arvalis using cytogenetic and molecular-genetic methods. For the first time, the structure of the hybrid zone between these forms was studied using chromosome, nuclear, and mitochondrial markers. The results of this work revealed a fairly unusual combination of characteristics of this zone, such as an almost complete absence of “pure” representatives of initial parental forms and an obvious deficit of first-generation hybrids.

Ural chromosomal Serov race of common shrew Sorex araneus L. (Insectivora, Mammalia) that inhabits polydominant deep coniferous taiga in the north of European Russia.

The chromosomal race Serov of the common shrew ( Sorex araneus ) known in the Urals and Western Siberia was found for the first time in ten areas studied in the European Cis-Ural region at least 300 km west of the Ural Mountains. Diagnostic metacentrics (go, hn, ip, km, qr) were stable; karyotyping of 16 animals showed no polymorphism (2 n A = 18). This common shrew race proved to move through the European territory within an area of Siberian stone pine (the major edificator of the polydominant Siberian taiga) which suggests that this race is involved in the relationships with Siberian taiga biota. The closest Eastern European chromosomal races, Manturovo and Pechora, were beyond the boundaries of the territory occupied by this community.

[Identification of a novel WART-like chromosome rearrangement in complex heterozygotes in an interracial hybrid zone of the common shrew Sorex araneus L].

Karyotypes uncharacteristic of pure races or hybrids were identified in the interracial hybrid zones of the common shrew Sorex araneus L. that were recently discovered in the European part of Russia. This suggests the actual existence in natural populations of WART-like rearrangements (whole-arm reciprocal trans-ocations) along with Robertsonian fusions of acrocentrics. Demonstration of new and still rare chromosome variants is the aim of this communication.

The role of hybrid zones in speciation: A case study on chromosome races of the house mouse Mus domesticus and common shrew Sorex araneus

Although diverse complexes of chromosome races are of rather rare occurrence in mammals, this does not reduce their importance to an insignificant phenomenon unworthy of studying as a unique case without direct analogy. Moreover, these complexes represent virtually ideal models for estimation of the impact of hybridization on the process of microevolution. The chromosome races are characterized by an almost zero level of molecular genetic differentiation and well-defined distinctions between them, which are usually associated only with chromosome rearrangements. This review shows the valuable contribution of the studies on Sorex araneus and Mus domesticus chromosome Robertsonian systems to our understanding of the varied impacts of hybridization on the speciation process. It particularly promotes a better understanding of evolutionary phenomena such as the “reinforcement” of reproductive isolation in secondary contact zones between divergent populations, speciation without geographic separation (“divergence with gene flow”), and “zonal raciation.”

Nucleolar organizing regions (NORs) of common vole chromosomes as nuclear markers of genome differentiation in data from a hybrid zone of two karyoforms, arvalis and obscurus

Contrasting chromosome location of nucleolar organizing regions (NORs), a poorly studied differentiating feature of two karyoforms of the 46-chromosome common voles, arvalis and obscurus, is demonstrated in the heterozygous karyotype of a natural hybrid from the arvalis/obscurus contact zone in European Russia. It was deduced from the karyotype analysis that the obscurus form was parental in this case and the arvalis form was defined as maternal via the markers of the mitochondrial genome. The similarities in the levels of chromosome polytypy and the lineage differentiation of the arvalis/obscurus and obscurus mitochondrial genomes according to the new and earlier published data is noticeable.