Geographic patterns of inversion polymorphisms in a wild African rodent, Mastomys erythroleucus.

By suppressing recombination and reducing gene flow, chromosome inversions favor the capture and protection of advantageous allelic combinations, leading to adaptive polymorphisms. However, studies in non-model species remain scarce. Here we investigate the distribution of inversion polymorphisms in the multimammate rat Mastomys erythroleucus in West Africa. More than 270 individuals from 52 localities were karyotyped using G-bands and showed widespread polymorphisms involving four chromosome pairs. No significant deviations from Hardy-Weinberg equilibrium were observed either through space or time, nor were differences retrieved in viability or sex contribution between cytotypes. The distribution of chromosomal variation, however, showed perfect congruence with that of mtDNA-based phylogeographic clades. Thus, inversion diversity patterns in M. erythroleucus appeared more related to historical and/or demographic processes than to climate-based adaptive features. Using cross-species chromosome painting and G-banding analyses to identify homologous chromosomes in related out-group species, we proposed a phylogenetic scenario that involves ancestral-shared polymorphisms and subsequent lineage sorting during expansion/contraction of West African savannas. Our data suggest that long-standing inversion polymorphisms may act as regions in which adaptation genes may accumulate (nucleation model).