B chromosomes in populations of Apodemus flavicollis – never ending story

Abstract

B chromosomes (Bs) are uncommon in mammals, featuring less than 2% of species, but six out of 22 species possess them in the genus Apodemus. In Serbia, over 40 populations of yellow-necked mice (Apodemus flavicollis) with varying habitat quality were studied. The frequency of B carriers (range: 0.09 to 0.67) increased with elevation, correlated with sub-zero days, and inversely with average temperature. B carrier frequency remained stable over eight years despite population density fluctuations, while seasonal variations linked to population size were observed. Overcrowding stress reduced B carriers among pre-reproductive individuals but enhanced survival in suboptimal conditions. Phenotypic traits and B frequency were correlated, influencing cranial morphometric development. B carriers showed distinct developmental pathways for cranial traits, suggesting environment-specific benefits. Bs’ presence did not affect carrier fecundity or fertility and was evenly distributed across age groups. B chromosomes (up to five) displayed common structures across populations in Serbia and Eastern Europe, possibly originating from sex chromosome pericentromeric regions. B-specific chromatin spatially resembled pericentromeric sex chromosomes, suggesting a similar mechanism to bypass meiotic checkpoints. Current data support a heterotic model in A. flavicollis. Bs likely contribute to species adaptability by increasing genetic variability, potentially expanding their distribution.