B chromosomes of the bat species Nyctalus leisleri from Serbia

Abstract

Background B chromosomes (Bs) are infrequent in bat species, with only four known so far, including Lesser Noctule, Nyctalus leisleri (Kuhl, 1817). This western Palearctic species is widely distributed, though seldom captured, due to its late emergence from roosts or the fact they are tree dwellers, thus difficult to track. As a known long-distance migrant, it covers substantial distances between summer and winter roosts. Previously, a cytogenetical study investigated chromosome sets of four males from two Serbian locations, revealing karyotypes with two to five micro Bs. Notably, studies on female N. leisleri are scarce. Materials and Methods Recently, karyotypes and B's presence of additional specimens from southwestern Serbia (two males), and Belgrade (one female) were examined. Chromosome slides were made from the primary fibroblast cell cultures established from a piece of skin from the wing membrane. At least 20 metaphase plates per individual were assessed. Results The species’ standard chromosome complement counts 44 (42 autosomes and a sex chromosome pair), with extra chromosomes classified as Bs. In two males, counts were 2n=44+1B and 2n=44+2B, while the Belgrade female had 2n=44+2B. Notably, the X chromosome was a medium-sized metacentric and the Y chromosome was a small acrocentric, while all Bs were microchromosomes. A previous study from Poland from 1970 reported a female N. leisleri karyotype of 2n=46, but Bs were not mentioned due to the analysis of a single individual. Our findings of a female with 2n=44 and two additional Bs suggest that the Polish study likely shared the same scenario. Conclusions The paucity of B chromosomes in bats might be attributed to their small genome size, possibly linked to the energy demands of flight. Additionally, bats’ low reproductive rates could hinder Bs establishment and maintenance, as opposed to rodents, which exhibit the highest occurrence of Bs among mammals. This research marks the first confirmation of Bs in female N. leisleri, and supplements the limited understanding of additional chromosomes in bats with two more male karyotypes. Acknowledgements This work was supported by the Ministry of Science, Technological Development and Innovation of the Republic of Serbia (Grants No. 451-03-47/2023-01/200007).