Post-zygotic reproductive isolating mechanisms between two chromosomally distinct, allopatric Otomys irroratus (Brants, 1827) populations (Kamberg and Karkloof) were studied in the laboratory by means of breeding trials, and by comparing the postnatal development of young of intrapopulation and interpopulation pairs. In contrast to the 100% breeding success of intrapopulation pairs, fewer than half of the interpopulation pairs produced young, and overall reproductive performance of these pairs was reduced. Furthermore, fitness of hybrids was dramatically impaired, as indicated by increased pre-weaning mortality, and inhibited growth in respect of body mass (postnatal development), as well as almost complete sterility (backcross and hybrid-cross matings) of surviving young. The results indicate the existence of chromosomal and/or genetic incompatibility between the two populations. The presence of a tandem fusion in the Kamberg karyotype is thought to be particularly significant in causing hybrid sterility, and the Kamberg population may be regarded as an incipient sibling species.
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