Patterns of evolution in Graomys griseoflavus (Rodentia, Muridae). IV. A case of rapid speciation

Abstract

Abstract The South American group of rodents known as Graomys griseoflavus comprises two sibling species differing only in diploid chromosomal complement: G. griseoflavus (2n = 36, 37 and 38) and G. centralis (2n=42). Reproductive barriers comprising postzygotic as well as precopulatory mechanisms prevent gene exchange between these species. We have studied genetic polymorphism in two populations of G. centralis and four of G. griseoflavus by means of gel electrophoresis of enzymes and other proteins giving information on a total of 30 loci. Values of interspecific genetic identity were similar to those obtained for conspecific populations, suggesting that fixation of Robertsonian fusions would have occurred without significant bottlenecks. It would also indicate that the cladogenetic process must be relatively recent. FIS values showed no evidence of inbreeding. Fixation indices (FST) for the ancestral species showed a tendency to form demes with very low gene flow among them, while in the derived species such tendency was not shown. However, because of the characteristics of the region they occupy, populations are of moderate size, and genic flow is low. Lack of correlation between gene flow levels and geographical distance between population pairs would indicate a recent and fast colonization of its distribution areas by the derived species. It is possible that fixation of Robertsonian fusions occurred in a marginal deme of the ancestral species, e.g. in a parapatric geographical context.