Major Histocompatibility Complex variation in the genus Ctenomys (tuco-tucos): demography, history and selection

CUTRERA, AP

Albuquerque, New Mexico, USA

Congreso; 87th American Society of Mammalogists Meeting; 2007

American Society of Mammalogists

Genes of the Major Histocompatibility Complex (MHC) provide a particularly appropriate opportunity to examine the impact of selection on genetic structure; high levels of polymorphism characteristic of these loci are thought to be maintained by selective pressures arising due to pathogen exposure and inbreeding depression, both of which may be influenced by demography. To explore the interactions among current and historical demography, natural selection and genetic drift, I assessed patterns of MHC variability in the genus of subterranean rodents Ctenomys (tucotucos). Specifically, I examined the effects of (a) intraspecific differences in current demography in two populations of C. talarum: Mar de Cobo (MC) and Necochea (NC), (b) interspecific differences in historical demography between two sympatric species: C. talarum and C. australis, and (c) balancing selection on patterns of MHC variation for 18 species of Ctenomys. For each of these analyses, variability at exons 2 of MHC class II DRB and DQA loci was quantified. In C. talarum, allelism and heterozygosity were greater among animals from MC, the population with the highest risk of parasite transmission. In contrast, variation at microsatellite and DQA intron 2 was greater for NC, suggesting that the forces maintaining variation at MHC exons differ from those acting on neutral markers. Instead, in C. australis, both exons and the intron showed patterns of variability consistent with a scenario of recent demographic expansion. However, a molecular signature of positive selection was evident, suggesting that selection has contributed substantially to shape MHC variation in this species. Finally, extensive evidence of trans-species polymorphism was detected for MHC exons among the 18 tuco-tuco species examined, a pattern consistent with the proposed action of balancing selection maintaining favorable alleles for extended periods. This work contributes to understand the complexity of the factors shaping genetic variation in natural populations of vertebrates.