CHROMOSOMAL PHYLOGENY AND EVOLUTION IN THE GENUS MASTOMYS (MAMMALIA, RODENTIA)

The chromosomal phylogeny of the African genus Mastomys was undertaken to clarify their taxonomic position and to estimate the relevance of chromosomal evolution in the diversification of this genus. Four speci es of Mastomys (M. erythroleucus, M. huberti, M. natalensis, and M. co ucha) were compared to closely related African rats, Myomys daltoni an d Praomys tullbergi, and to three species of European murids. The phyl ogenetic trees generated could be grouped into two topologies in which Mastomys was either monophyletic or paraphyletic. A cladistic and phe netic analysis of available allozymic data clearly showed, however, th at Mastomys species form a closely related group and agreed with a mon ophyletic origin for this genus. Chromosomal evolution in Mastomys is characterized by seven different types of rearrangements, the most fre quent being pericentric inversions. The latter also are involved in in traspecific, chromosomal polymorphisms suggesting that pericentric inv ersions are a recurrent event in the evolution of this genus. However, pericentric inversions modify the fundamental number, but not the dip loid number, which is a criterion often used to identify the different species of Mastomys. These observations suggest the following pattern of chromosomal change in this genus. Rearrangements modifying the dip loid number occurred at the same time as major speciation events resul ting in different diploid numbers for each species whereas subsequent divergence of karyotypes proceeded mainly by accumulation of pericentr ic inversions.