The unstriped grass rat, Arvicanthis Lesson 1842, is one of the most common genera of murid rodents in African savannas. However, from a systematic viewpoint, very little is known about this group. Following recent investigations which showed karyotypic variability within the species A. niloticus, the present study attempts to clarify the nature and distribution of these chromosomal variants, as well as to determine their taxonomic rank.
The chromosomes of 15 individuals from different West African localities were prepared from fibroblast cultures, and R- and C-banded karyotypes were constructed. In addition, the levels of genetic divergence (DNA/DNA hybridization) and reproductive isolation (attempted crossbreeding in captivity) were examined. The results confirm the existence of two differentiated karyomorphs, differing by numerous chromosomal rearrangements such as pericentric inversions and translocations, as well as differences in the quantity of constitutive heterochromatin. These karyomorphs appear to be genetically and reproductively isolated and are parapatrically distributed; their areas of distribution correspond to the sahelian and sudano-guinean domains, respectively. The distinctness of these karyomorphs, the absence of hybrids in laboratory crosses, and the pronounced genetic divergence provide good evidence for the recognition of two distinct sibling species. We propose to keep the designation A. niloticus for the northern sahelian form and discuss the naming alternatives for the other.