DENTAL VARIATION IN AN INVASIVE SPECIES AT THE DISPERSAL STAGE: <i>MICROTUS ROSSIAEMERIDIONALIS</i> (ARVICOLINAE, RODENTIA) IN THE IRKUTSK OBLAST’, SOUTHERN CIS-BAIKAL REGION

Abstract

The East European vole is a facultative synanthropic species that actively spreads in the eastern part of northern Eurasia. The distribution area of the species in the Irkutsk Oblast’ has been known as a disjunct part of the range since the 1980s. Our dataset includes 98 individuals caught in 2016–2017 and 2021–2023 in the eastern segment of the species’ dispersal area, where the East European vole was first discovered in the early 2010s. Animals were caught in anthropogenically transformed areas spanning from the outskirts of the city of Irkutsk to kilometer 23 of the Goloustnensky Tract; trapping in natural biotopes revealed no M. rossiaemeridionalis. Species identification was based on molecular genetic markers. Morphological and fine structural characteristics of molar teeth were studied using both light and scanning electron microscopy. The complexity of the occlusal surface, the alternation patterns of the enamel cutting edges (occlusal regularity), and the presence or absence of atypical extra elements such as prismatic folds and prisms in places of the early cricetid dental features were assessed. The estimates of complexity and regularity patterns fell within the range of dental variability known for the continuous distribution area of the species. The atypical extra elements on molars were described for the first time in M. rossiaemeridionalis based on material coming from two trapping sites. The extra elements were confined to the posterior lobes of the lower molars, where an additional reentrant angle separated an extra prism in place of a hypoconid. Enamel walls of the extra prism showed an inversion of the enamel types (a thin layer of radial enamel on the anterior wall and a thick layer of lamellar plus radial enamel on the posterior wall) that was contrary to the evolutionary patterns and functional requirements known for Microtus. The local micropopulation studied is concluded to show phenotypic consequences of a reduced number of founders. Apparently, the species could have faced sequential establishment events in the study area. Our results indicate that dental features are useful for monitoring the arvicoline species, the distribution limits of which are subject to significant spatial changes in the relatively short time intervals available for observation.