Long-Term Dynamics of Spatial and Genetic Structure of a Narrow Hybrid Zone between Russet (Spermophilus major Pallas, 1778) and Yellow (Spermophilus fulvus Lichtenstein, 1823) Ground Squirrels

The research into the structure, population and genetic patterns of formation and long-term existence of natural hybrid zones is of paramount importance for modern biology and the theory of evolution. The article presents the research results on the 22-year monitoring (2002-2023) of a hybrid zone and hybrid colonies of russet (Spermophilus major Pallas, 1778) and yellow (Spermophilus fulvus Lichtenstein, 1823) ground squirrels. This hybrid zone arose as a result of active dispersal of S. major to the south in the Saratov Trans-Volga region in the early 2000s. Twelve rodent colonies attributed to different research periods were studied. A total of 405 individuals (including 103 individuals of S. major, 52 individuals of S. fulvus, and 250 genetically confirmed interspecific hybrids) were trapped and sampled. Based on evidence, the article examines the structure and factors for formation of the hybrid zone between russet and yellow ground squirrels. Hybrid colony features and the evolution of hybrid population are considered. Introgressive hybridization between russet and yellow ground squirrels clearly manifests itself towards actively dispersing S. major. The revealed structural and genetic features of the hybrid zone between S. major and S. fulvus have enabled it to be termed as a bimodal hybrid zone of an ecological type with introgressive hybridization and stable hybrid colonies.