B chromosomes and developmental homeostasis in the yellow-necked mouse, Apodemus flavicollis (Rodentia, Mammalia): effects on nonmetric traits

B chromosomes are found in almost all populations of the yellow-necked mouse, Apodemus flavicollis (Rodentia, Mammalia). Their effects on developmental homeostasis in this species were analyzed using morphological nonmetric traits (number of foramina) in a sample of 218 animals from locality Mt Jastrebac in the former Yugoslavia. Variations of the parameters of developmental homeostasis (the degree of fluctuating asymmetry--FA, the number of asymmetrical characters per individual--NA, and the total phenotypic variability--PV) were examined in three groups: in animals without Bs, with one B chromosome, and with more than one B chromosome. Significant differences in the level of FA between groups of animals were found for two characters. Carriers of one B chromosome displayed the highest level of phenotypic variability. Parameters of developmental stability (DS) were monitored in the population in which significant variations in the frequency of animals with Bs (fB) were established during the season earlier. The FA levels for four foramina out of a total of 12 examined followed the changes of frequencies of animals with Bs. Furthermore, a significant seasonal correlation between NA and fB was found. The presence of B does not cause a disturbance of homeostasis in a way that allows changes in homeostasis to be directly related to B chromosome's presence. However, carriers of B react differently to environmental changes than do noncarriers.