CHROMOSOMAL TRANSMISSION BIAS IN LABORATORY HYBRIDS BETWEEN WILD STRAINS OF THE 2 EUROPEAN SUBSPECIES OF HOUSE-MICE

Laboratory crosses between wild strains of the two European house mous e subspecies Mus musculus domesticus (2n = 34) and M. m. musculus (2n = 40) were performed to analyze the selective processes involved in th e non-introgression of centromeric regions of Robertsonian (Rb) fusion s in the Danish hybrid zone. The chromosomal analysis of 226 backcross progeny from 22 reciprocal crosses showed that the segregation of the three Rb fusions present did not significantly differ from Mendelian expectations. However, a significant negative correlation was found be tween Rb transmission rates and the average litter sizes of the F-1 pa irs. Among the different models of selection discussed, the most likel y one supported the existence of two opposing selective factors result ing in an overall compensation of chromosomal types in the backcross p rogeny. A two-phase selective process involving embryo competition was postulated with non-Rb carriers being favored during pre-implantation but disadvantaged after implantation. Such balanced selective pressur es acting on musculus non-Rb centromeres are compatible with the steep slope and off-centered position of the chromosomal dine observed in t he Danish hybrid zone. These results suggested that these selective fa ctors may be more related to centromere origin (musculus or domesticus ) than to centromere structure (Rb or non-Rb).