The role of B chromosomes in the population dynamics of yellow-necked wood mice Apodemus flavicollis (Rodentia, Mammalia).

Seasonal changes in the frequency of animals with B chromosomes were studied during 1989 in the population of yellow-necked wood mice, Apodemus flavicollis, on the mountain Jastrebac in Yugoslavia. During the study the estimated density of animals reached a very large value (378.8 individuals/ha for the April sample), which produced stress conditions leading to population decline. Six samples with a total of 247 specimens were karyotypically analyzed. Animals with Bs were present in all samples in frequencies that differed significantly at the 5% level. This difference was produced by the reduction in frequencies of specimens with Bs between the April and May samples (0.53 versus 0.29; chi 2(1) = 9.44; p < 0.002) and appeared mostly as a result of increased elimination of B-carrying animals from the prereproductive group. Young animals with Bs were inferior in stress conditions produced by overabundance of animals. From the first to the last sample, the frequency of animals with one B among B-containing animals constantly decreased, while the frequency of animals with more than one B increased. The possible role of Bs in regulating population dynamics is discussed.