THE ROLE OF B-CHROMOSOMES IN THE POPULATION-DYNAMICS OF YELLOW-NECKEDWOOD MICE APODEMUS-FLAVICOLLIS (RODENTIA, MAMMALIA)

Seasonal changes in the frequency of animals with B chromosomes were S tudied during 1989 in the population of yellow-necked wood mice, Apode mus flavicollis, on the mountain Jastrebac in Yugoslavia. During the s tudy the estimated density of animals reached a very large value (378. 8 individuals/ha for the April sample), which produced stress conditio ns leading to population decline. Six samples with a total of 247 spec imens were karyotypically analyzed. Animals with Bs were present in al l samples in frequencies that differed significantly at the 5% level. This difference was produced by the reduction in frequencies of specim ens with Bs between the April and May samples (0.53 versus 0.29; chi(( 1))(2) = 9.44; p < 0.002) and appeared mostly as a result of increased elimination of B-carrying animals from the prereproductive group. You ng animals with Bs were inferior in stress conditions produced by over abundance 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 p ossible role of Bs in regulating population dynamics is discussed.