Microevolutionary Sequences in House Mouse Chromosomal Speciation

Abstract

For the past approximately 6,000 years, chromosomal fusions have been occurring in populations of Mus domesticus in the mountainous regions of central and northern Italy and southern Switzerland. From these areas, 13 different karyotypically transformed races of communal house mice have been described with chromosome numbers reduced to 22, 24, 26, and 28 from the original 40 still possessed by the known surrounding populations of house mice. It is likely that, once fused, chromosomes do not separate again in subsequent generations. The establishment of a race with a newly fused chromosome pair requires several complicated steps, so that nearby races with a common fusion very likely share that common fusion event in their evolutionary history. Each fusion, identified by G-banding patterns, has been used as a cladistic character in compatibility analysis to construct plausible hypotheses of phylogenetic relationships among these races of Robertsonian mice. Those fusion events that seem most likely to be incompatible with history are identified and discussed; both parallel evolution and hybridization are suggested as possible explanations.