Variation in the frequency of B chromosomes in small isolated populations of Apodemus flavicollis

Abstract

Background Long-term studies on B chromosome (Bs) dynamics in natural populations of yellow-necked mice, Apodemus flavicollis, have unveiled intriguing seasonal variations influenced by environmental factors. On average, one third of animals possessed Bs in large natural populations. Urbanization, a potent catalyst for genetic diversity alterations, brings about habitat fragmentation and degradation, producing patches of small forested areas within urban landscapes. These fragmented habitats pose significant challenges for small mammal populations as migration and gene flow are severely restricted. To comprehensively grasp the repercussions of urbanization on B chromosome frequency, we embarked on a study within the Belgrade region. Materials and Methods The frequency of individuals with B chromosomes in five urban isolated forests populations of A. flavicollis has been studied. Employing live animal traps, we collected samples, and chromosome preparation was done directly from the bone marrow. Each animal’s karyotype was analysed through examination of twenty metaphase figures. Results A total of 246 animals were sampled across five forest sites within the Belgrade territory and underwent karyotyping. The prevalence of animals with Bs exhibited remarkable variability, ranging from 3% to 50%. Intriguingly, at four of the sites, the incidence of Bs was significantly lower (ranging from 3% to 13%) compared to the average frequencies observed in natural populations. Conversely, at one site, the frequency soared to an exceptionally high 50%. Since all studied localities are at short distances from each other, climatic factors could be considered as the same. However, the studied sites differ in size and the level of adaptation to people’s urban life. In a bid to elucidate the driving factors behind frequency variation, we developed a model simulating the influence of genetic drift, the predominant evolutionary force in small, isolated populations. Conclusions Beyond investigations focused on the molecular structure and origin of B chromosomes, it becomes evident that a comprehensive understanding necessitates population-level research, shedding light on the intricate mechanisms governing their persistence across diverse environments.