Chromosomal and ecological processes in the cladogenesis of two sibling species of Graomys (Rodentia, Cricetidae)

NOEMI GARDENAL; JUAN JOSÉ MARTÍNEZ; GERARDO RAÚL THEILER; RAÚL ENRIQUE GONZÁLEZ ITTIG

Mendoza, Argentina

Simposio; International Mammalogical Congress 10; 2009

IFM, Sarem

The sibling species Graomys centralis (2n=42) and Graomys griseoflavus (2n=34-38) would have initiated their divergence by chromosomal rearrangements involving Robertsonian fusions from the ancestral karyotype (2n=42). Individuals of the complex 2n=34-38 preferably inhabit the arid temperate Monte and Patagonic steppe eco-regions; G. centralis occurs in the area of xerophytic woodlands called Chaco and Espinal. Although asymmetric post-zygotic isolation of the two forms was demonstrated in laboratory tests, interspecific hybrids have not been detected in nature. Pre-zygotic isolation mechanisms involving olfactory discrimination in females were also described. In G. centralis, this discrimination ability is significantly higher in a population close to the area of contact with that of the other species, suggesting stronger selective pressures to promote reinforcement mechanisms. Population genetic studies using allozymic data revealed high levels of heterozygosity and allelic richness in both the ancestral and the derived species, indicating that the cladogenesis would have been a rapid and relatively recent process, with absence of severe genetic bottlenecks. In order to evaluate this proposal using a phylogeographic approach, we investigated the relationships among D-loop mitochondrial haplotypes of individuals belonging to G. centralis and G. griseoflavus from central-western Argentina. The average nucleotide divergence between the species was relatively low, with a marked overlap with the ranges of intraspecific genetic distances. The phylogenetic network showed two well-defined clusters, one for each species, connected by two haplotypes from relatively nearby localities, one belonging to the Monte eco-region and the other, to the Chaco. The mismatch distribution analyses and the lack of an isolation by distance pattern supported the idea of a recent and fast expansion of G. griseoflavus in the Monte. The colonization of the new environment could have favored a rapid speciation as a by-product of adaptation to divergent selective pressures.