It has long been hypothesized that chromosomal rearrangements play a central role in different evolutionary processes, particularly in speciation and adaptation. Interchromosomal rearrangements have been extensively mapped using chromosome painting. However, intrachromosomal rearrangements have only been described using molecular cytogenetics in a limited number of mammals, including a few rodent species. This situation is unfortunate because intrachromosomal rearrangements are more abundant than interchromosomal rearrangements and probably contain essential phylogenomic information. Significant progress in the detection of intrachromosomal rearrangement is now possible, due to recent advances in molecular biology and bioinformatics. We investigated the level of intrachromosomal rearrangement in the Arvicolinae subfamily, a species-rich taxon characterized by very high rate of karyotype evolution. We made a set of region specific probes by microdissection for a single syntenic region represented by the p-arm of chromosome 1 of Alexandromys oeconomus, and hybridized the probes onto the chromosomes of four arvicolines (Microtus agrestis, Microtus arvalis, Myodes rutilus, and Dicrostonyx torquatus). These experiments allowed us to show the intrachromosomal rearrangements in the subfamily at a significantly higher level of resolution than previously described. We found a number of paracentric inversions in the karyotypes of M. agrestis and M. rutilus, as well as multiple inversions and a centromere shift in the karyotype of M. arvalis. We propose that during karyotype evolution, arvicolines underwent a significant number of complex intrachromosomal rearrangements that were not previously detected.

A review of studies on the chromosome sets of Palaearctic mammals carried out by Russian karyologists and an important contribution to improving the taxonomy of mammals were made for the first time. For many mammal species, the process of speciation was associated with variability in the number and morphology of the chromosomes; therefore, the karyotypes are often used as diagnostic peculiarities of morphologically similar cryptic species (sibling species). The prospects for cytogenetic studies in the field of speciation (particularly, an enhancement of reproductive isolation, initiated by chromosomal rearrangements, by selection) are discussed.

The species number has been strikingly increasing in recent current works on Ethiopian endemic rodents, which to a great extent is due to genomic studies including the karyotype discoveries. Here, the diploid number 2N = 52 is defined anew for the genus Stenocephalemys, endemic to the Ethiopian Highlands, where four karyotypes with 2N values of 54, 50 and 46 were so far described for four taxa. The new karyotype (S. “pseudogriseicauda”) combines into a nearly full series with other 2Ns (except the not yet found 2N = 48) within the genus and shows one interspecific chromosome rearrangement associated with the tandem fusion, in the direction 2N = 54 to 2N = 52. Together with the earlier detection of tandem fusions between karyotypes with 2N = 46 (S. albipes) and 2N = 54 (S. albocaudatus, S. griseicauda), this rodent taxonomic group presents an interesting case for the phenomenon of karyotypic orthoselection in an intrageneric phyletic line via the reoccurence of such characteristic chromosomal rearrangements as tandem fusions.

MAMMALIAN CYTOGENETICS AND ITS CONTRIBUTION TO THE DEVELOPMENT OF CHROMOSOMAL DIAGNOSES AND THE SPECIES SYSTEM Цитогенетика млекопитающих и ее вклад в разработку хромосомных диагнозов и системы видов

The name of Academician Ivan Ivanovich Schmalhausen (1884–1963), an outstanding evolutionist, Director of the A.N. Severtsov Institute of Evolutionary Morphology (IEM, 1936–1948)—now the A.N. Severtsov Institute of Ecology and Evolution of the Russian Academy of Sciences (IEE)—is strongly associated not only with evolutionary morphology, the classical section of evolutionary teaching, in which he proved himself a brilliant experimenter and a major theorist. Schmalhausen is also recognized as one of the creators of evolutionary synthesis, although without personal participation in genetic work. He was interested in everything new that would accelerate the development of science, and, in particular, noted the “dizzying success” (his assessment) of cytogenetics, a new trend that arose in the synthesis of cytology with genetics and flourished literally before his eyes at a nearby institute headed by Academician N.I. Vavilov. The cytogenetic method is included in the modern practice of evolutionary and ecological work at IEE, and in this one can see the legacy of Academician I.I. Schmalhausen. The possibilities of comparative cytogenetics in relation to evolutionary issues are in the focus of this compilation using the example of a well-cytogenetically studied zoological object, a species of Palearctic mammal, the common shrew Sorex araneus L. and a group of related species. The participation of one of the categories of chromosomal rearrangements—whole arm fusions and fissions—is discussed as an inexhaustible source of evolutionary diversity of karyotypes of this supraspecific group. This allows us to correct the traditional point of view of early cytogeneticists and their contemporary evolutionists, who expressed doubt about the evolutionary novelty of interchromosomal rearrangements, which, as summarizes this paper, combined variability in fused chromosome arm combinations with genetic conservatism of the initial evolutionary elements.