Interplay between selection, drift, and history: MHC polymorphism in two South American subterranean rodents (Ctenomys australis and C. talarum)

CUTRERA, A.P, E.A. LACEY & M. MORA

Amherst, Massachusetts, USA

Congreso; 86th American Society of Mammalogists Meeting; 2006

American Society of Mammalogists

The genes of the Major Histocompatibility Complex (MHC) are among the most variable functional loci known in vertebrates. Variation at these genes, which are involved in detection of and response to pathogens, is thought to be maintained primarily by balancing selection. At the same time, MHC genes are subject to the same processes that affect variation at neutral loci, raising intriguing questions regarding the interplay between selection and other determinants of genetic structure at MHC loci. To explore the effects of current selection versus historical demography, we compared patterns of MHC variability in two species of subterranean rodents: Ctenomys australis and C. talarum. Although these species co-occur in the same geographic area, they have undergone different historical regimes; while mtDNA analyses of C. talarum reveal a stable pattern of differentiation by genetic drift and limited gene flow, C. australis has experienced a recent demographic expansion following Quaternary sea level changes. Comparisons of two MHC Class II genes, DRB and DQA, revealed that allelic variability is lower but heterozygosity is higher for C. talarum. Significant population-level evidence of selection was found for DQA in C. talarum. Significant molecular-level evidence of selection was obtained for DRB from C. talarum and DQA from C. australis. For C. australis, estimates of Tajima’s D were significantly negative for both loci, as expected following population expansion. For C. talarum, these estimates were significantly positive, as expected under balancing selection. These results are discussed in light of the interplay between balancing selection and drift, as mediated by the different historical regimes experienced by populations of these species.