Possible heterotic effects of B chromosomes on body mass in a population of Apodemus flavicollis

The adaptive significance of supernumerary (B) chromosomes was studied in yellow-necked mice, Apodemus flavicollis. No significant change in the average frequencies of B chromosomes could be confirmed in a population from northern Bohemia during a period spanning 17 years. However, we found a significant regression between the frequency of B chromosomes and body mass. The best fit to a logistic function was obtained by a model that included the effects of both sex and body mass and their interaction. A significant relationship between the mean number of B chromosomes and body mass was found in males but not in females. It is suggested that a selective advantage for males possessing B chromosomes, possibly in relation to survival during winter, explains the data obtained.