Behavioral differentiation and hybridization of the European and Asian forms of Harting’ vole Microtus hartingi (Rodentia, Arvicolinae)

Abstract

The final isolation of Europe from Asia took place comparatively not long ago-in the early Holocene-late Pleistocene which led to the division of Microtus hartingi area into two isolated parts (Asian-Central and West Anatolia and European-southern Balkans). The Northern Mediterranean was influenced by considerable climatic and habitat fluctuations that caused fragmentation of a potential area of Microtus hartingi and promoted microevolutionary processes. The purpose of the work was evaluation of the range disjunction influence on diversification of the species. We analyzed (1) copulatory behavior, (2) exploratory activity and ability to overcome the stress, (3) growth and development of pups, and also (4) hybridization success in the two nominal forms of M. hartingi-M. h. lydius and M. h. strandzensis. Considerable changes in behavior of individuals of two subspecies were obtained. As a result of the hybridization of the two forms, viable and prolific posterity of F1 was received, but the subsequent cross breeding including backcrossing sterility of males and high mortality of posterity began to be established. The testes and the epididymis of the hybrids were with smaller weight in comparison with parents' subspecies, especially M. h. lydius, reduced quantity of spermatozoa and small percent mobile of them which confirmed the lower level of spermatogenesis. We assume that evolution of the phenotypic characteristics of the voles was supported by gene drift and probably by the selection, induced by the stress more strongly, than changes in the genotypic characteristics, e.g. distinctions on cytochrome b and the karyotype are not significant. This can be explained as an influence of specific ecological conditions and fragmentation of the habitats in the Northern Mediterranean Region. Until recently M. h. lydius and M. h. strandzensis were considered as subspecies. However, they reached considerable degree of a divergence and the question of their specific status has to be considered additionally.

Consequences of Hybridization in Mammals: A Systematic Review

“…Moreover, in some cases the reproductive output differs between different generations of hybrids. For example, hybridization between Microtus hartingi lydius and Microtus hartingi strandzensis produces viable and prolific F1 hybrids, while in the F2 generation, males are sterile and the mortality rate is high [ 164 ]. Falling fertility rates and loss of reproductive outputs may lead to severe demographic declines in parental species and even a rapid extinction of local populations involved in cross-breeding.…”

Voles and Lemmings (Arvicolinae) of the Palaearctic Region

“…The hybrids, however, showed lower spermatogenetic activity and males obtained in backcrossing trials were sterile and less viable. On these grounds Zorenko et al (2016) called for taxonomic separation of the European voles (as strandzensis) from the Asiatic ones (lydius). The name lydius was occasionally used for the Asiatic Harting's social voles (Yiğit & Colak 2002, Grimmberger et al 2009.…”

Evaluation of the “Bottleneck” Effect in an Isolated Population of Microtus hartingi (Rodentia, Arvicolinae) from the Eastern Rhodopes (Bulgaria) by Methods of Integrative Analysis

“…On the other hand, hybridization between Rhodopean and Anatolian voles is much more successful than that between M. h. strandzensis from Strandzha and M. h. ankaraensis. Overall, for all crossing options, in the former case, 86.5% of pairs breed, while in the latter, only 43.7% do [5]. Mortality of hybrid offspring in the former case is 2.2 times lower than in the latter (25.5% and 55.7%, respectively).…”

“…European and Asian forms of Harting's vole, separated by the Sea of Marmara and narrow straits-the Bosphorus and the Dardanelles-diverged substantially during the Pleistocene and Holocene. When the European subspecies M. h. strandzensis was hybridized with the Asia Minor subspecies, M. h. ankaraensis from central Anatolia, viable and fertile F1 offspring were obtained; however, during subsequent crossing, including backcrossing, partial sterility of males and high mortality of pups began to manifest themselves [5]. For the first time, two males and three females of M. hartingi were captured in the Eastern Rhodopes in the 1990s and were studied karyologically [6] The present authors (T. Zorenko and N. Atanasov) surveyed the Eastern Rhodopes (Bulgaria) in 2014 and 2016 and documented the presence of an isolated population of M. hartingi, represented by small habitats that are distant from each other.…”