Patterns of chromosomal variation in southern African rodents

Mechanisms of chromosomal change, distribution patterns, extent of genotypi c, phenotypic and ecological divergence, and nature and degree of reproduct ive isolation were reviewed in 10 chromosomally variable southern African r odent species and species complexes. Genetically similar peripatric or para patric chromosome races freely interbreed (striped mice, Rhabdomys pumilio; vlei rats, Otomys irroratus), while genotypically or phenotypically diverg ent, sympatric sibling species (multimammate mice, Mastomys natalensis-M. c oucha; tree rats, Thallomys paedulcus-T. nigricauda; red veld rats, Aethomy s chrysophilus-A. ineptus) appear reproductively isolated, suggesting a cor relation between genetic distance and onset of reproductive isolation. The allopatric gerbil species pair, Tatera bratsii-T, after, shows little or no prezygotic reproductive isolation despite well-developed male-biased postz ygotic isolation (i.e., Haldane effect) that may be associated with a rearr angement on the Y chromosome. Within the actively speciating vlei rat, Otom ys irroratus, complex, despite widespread laboratory interbreeding between chromosome races, postzygotic isolation (virtual hybrid sterility) was demo nstrated due to fixation of a tandem fusion rearrangement in the high-altit ude (>1,400 m) Al race. Both post- and prezygotic isolation have developed in the absence of significant measurable genotypic divergence at either the gene or the DNA level. Acquisition of reproductive isolation and incipient speciation in the vlei rat complex appears to be mediated by both chromoso mal (postzygotic) and nonchromosomal (prezygotic) processes.