Homoploid hybrid raciation: the beginning of speciation?

Hybrid speciation is extensive in plants and evidence is also accumulating for the process in animals. In plants hybrid speciation often involves generation of new chromosomal forms, particularly through polyploidy. New hybrid chromosomal forms may also be generated through the alternative process of homoploid hybrid speciation which does not involve a change in the genome ploidy. Here, as a first stage to hybrid speciation, we demonstrate homoploid hybrid raciation in an animal: the western European subspecies of house mouse Mus musculus domesticus. This subspecies shows extraordinary chromosomal subdivision, with at least 100 chromosomally distinctive races with diploid numbers varying from 22 to 40. These chromosomal races are characterised by sets of race-specific metacentrics formed by the centromeric fusion of ancestral acrocentric chromosomes and/or by whole- arm reciprocal translocation. There are many such races clustered around the Alps in Italy and Switzerland. In particular in Valtellina there are four distinctive chromosomal races characterised by different metacentric chromosomes: the Poschiavo (CHPO, 2n = 26), Mid-Valtellina (IMVA, 2n = 24), Upper Valtellina (IUVA, 2n = 24) and Lower Valtellina (ILVA, 2n = 22) races. Karyological and computer simulation studies suggest that IMVA and IUVA could be homoploid hybrid products of the other two races. Here we use centromeric microsatellite markers on race-specific metacentrics to confirm this process. This is the first unequivocal demonstration of hybrid raciation involving chromosomal races in animals. Once hybrid races are generated they have the potential to become reproductively isolated from other chromosomal races and there is evidence for this process in Valtellina.