Abstracts (first author)
Are Robertsonian fusions barriers to introgression in a house mouse hybrid zone?
Genomic regions of low recombination are expected to contribute to the accumulation of incompatibilities allowing speciation with gene flow to occur. While chromosomal inversions are well documented in such processes, the role of Robertsonian (Rb) fusions (that join two acrocentric chromosomes by their centromere to form a metacentric one) is less clear. The house mouse is an emblematic model of Rb variation since one subspecies, M. m. domesticus, displays >80 Rb races. Previous cytogenetic studies demonstrated a decrease in recombination rates along the proximal 50% of Rb chromosomes in both homozygotes and heterozygotes. This study explores the role of Rb fusions in reducing gene flow by investigating their effect on genome admixture between two subspecies of the house mouse. In Denmark where the two taxa form a tension zone, M. m. domesticus carries 3 Rb fusions while M. m. musculus has an all-acrocentric karyotype. Genome admixture was assessed using 127 SNPs diagnostic of the two subspecies and covering the proximal 25% of 3 chromosomal arms involved in Rb fusions and 2 acrocentrics. We compared patterns of genomic admixture in the proximal regions to those at 86 diagnostic loci randomly distributed in the genome by fitting a genomic cline model with 3 parameters (centre and slope in each subspecific context). In the domesticus genomic context, Rb clines were much steeper than that of the average reference loci. Their centre was displaced in favour of musculus, however, due to introgression of large chromosomal blocks, and both of these patterns were stronger for the most proximal loci. This is compatible with the predicted effects of reduced recombination in Rb fusions coupled with incompatibilities in these genomic regions. In addition, clines for acrocentrics were smoother than average in the musculus genomic context, suggesting that other undescribed effects are acting, such as a possible advantage of domesticus acrocentric centromeres.