Superspecies complex Cricetulus barabensis sensu lato: Karyotype divergence and interrelation in natural contact zones

Abstract

Three different chromosomal forms of striped hamsters that belong to the superspecies complex Cricetulus barabensis sensu lato (Cricetidae, Rodentia), namely, “barabensis”, “pseudogeiseus”, and “griseus”, were studied to determine their relationships based on different biological and genetic species concepts and using the morphological, karyomorphs, molecular, and hybridization data. The results of craniometry suggest that differences between these karyotypes are rather of subspecies level. Cytogenetic differences also proved to be insignificant and correspond to the level of chromosomal races within a single species. An analysis of cytochrome b mitochondrial gene (cytb) differences of about 2.3–4.1% between the forms studied, which suggest intraspecific distinctions or those between allied species. Hybridization experiments, which produced various litters from mating between three karyomorphs, also confirmed both male and female fertility. In nature, there are several zones where contact is possible between the “barabensis” and “pseudogeiseus” karyomorphs. In one of these zones, in Central Mongolia, atypical karyotypes were found in two male specimens using chromosomal analysis. Taking into account the chromosome number and shape, this karyotype could be attributed to a hybrid between the second and subsequent generations. However, typing the cytb gene and intron localized to Y chromosome (DBY1) did not reveal any traces of recent or ancient hybridization. In another zone, in South Buryatia, where the contact could be expected between the karyomorphs studied, the “barabensis” and “pseudogeiseus” populations are separated by the Chikoi River and the distance between them does not exceed 5 km. Nevertheless, neither karyological nor molecular analysis detected any hamsters with nonstandard karyotypes or any traces of gene flow. An analysis of the entire cytb nucleotide sequence, as well as the Tajima’s test, the distribution of pairwise differences, and the level of nucleotide diversity suggest that both contact zones have formed long ago; the absence of gene flow cannot be explained by their recent formation. Thus, the evolutionary formation of each of the three karyomorphs that were the subject of our study appears to be independent.