GENOMIC INCOMPATIBILITIES IN THE HYBRID ZONE BETWEEN HOUSE MICE IN DENMARK - EVIDENCE FROM STEEP AND NONCOINCIDENT CHROMOSOMAL CLINES FOR ROBERTSONIAN FUSIONS

The pattern of chromosomal variation is investigated in house mice fro m the Danish hybrid zone between the translocation-prone us musculus d omesticus and the chromosomally conservative M. m. musculus. The cytog enetic analysis confirmed the non-introgression of three pairs of Robe rtsonian (Rb) fusions from M. m. domesticus into the M. m. musculus ge nome, The geographic distribution of two of these Rb fusions was shown to follow staggered chromosomal dines which increased in steepness th e closer they were to the centre of the hybrid zone as defined by allo zymes. Analysis of alternate hypotheses suggests that chromosomal diff erentiation of the Danish domesticus occurred after contact was establ ished with musculus. The staggering of the dines would reflect the ord er of arrival of the Rb fusions into the hybrid zone. Several models w ith different processes of underdominance of the chromosomal heterozyg otes are discussed to account for the difference in width between clin es, A selective model with increasing levels of genomic underdominance due to interaction with a progressively enriched musculus genome prov ides the best fit for the observed pattern. Selection against Rb fusio ns with little effect on the recombination of linked allozyme markers supports the view that no reduction in gene flow due to chromosomal he terozygosity is yet apparent through the hybrid zone and that only the centromeric segments of the Rb fusions are incompatible with the musc ulus genome.