Extensive Robertsonian polymorphism in the African rodentGerbillus nigeriae: geographic aspects and meiotic data

Abstract

Robertsonian (Rb) fusions are one of the most frequent types of chromosomal rearrangements and have greatly contributed to the evolution of mammalian genome architecture. Apart from rare exceptions, investigations at the species level (i.e. polymorphism) are almost exclusively restricted to two mammalian models, namely the house mouse Mus musculus domesticus (2n=22–40) and the common shrew Sorex araneus (2n=20–33). Yet, these two species display important but usually locally restricted Rb polymorphisms. Another rodent species, Gerbillus nigeriae, has also been shown to display a wide range of diploid number variation (2n=60–74) due to Rb polymorphism. However, data about the latter species are rather scarce. We provide here a survey including recapitulation of 137 available karyotypic data that were implemented with 241 new records, thus allowing us to draw the first map of 2n variation throughout the species range. First, truly segregating centric fusions are observed in almost all localities investigated. Moreover, the geographic patterns (from 79 West African localities in total) show that local 2n variations are clearly lower than those observed at a wider scale, thus leading to some spatial structuring that may reflect phylogeographic assemblages. The meiotic study of 13 male specimens allowed us to identify several instances of double and triple Rb heterozygous individuals, and strongly suggested that heterozygosity is more the rule than the exception in the species. From there, it is tempting to speculate that the extraordinary Rb plasticity observed in G. nigeriae may be selectively maintained and confer adaptability to this species, which inhabits unstable Sahelian environments, where it is able to colonize efficiently habitats that undergo rapid human-mediated and/or climatic changes.