contrasting epidemic histories reveal pathogen-mediated balancing selection on class ii mhc diversity in a wild songbird对比鲜明的流行历史揭示pathogen-mediated平衡选择在野生songbird二类mhc的多样性.pdfVIP

Contrasting Epidemic Histories Reveal Pathogen- Mediated Balancing Selection on Class II MHC Diversity in a Wild Songbird ¤ Dana M. Hawley* , Robert C. Fleischer Center for Conservation and Evolutionary Genetics, Smithsonian Institution, Washington D.C., United States of America Abstract The extent to which pathogens maintain the extraordinary polymorphism at vertebrate Major Histocompatibility Complex (MHC) genes via balancing selection has intrigued evolutionary biologists for over half a century, but direct tests remain challenging. Here we examine whether a well-characterized epidemic of Mycoplasmal conjunctivitis resulted in balancing selection on class II MHC in a wild songbird host, the house finch (Carpodacus mexicanus). First, we confirmed the potential for pathogen-mediated balancing selection by experimentally demonstrating that house finches with intermediate to high multi-locus MHC diversity are more resistant to challenge with Mycoplasma gallisepticum. Second, we documented sequence and diversity-based signatures of pathogen-mediated balancing selection at class II MHC in exposed host populations that were absent in unexposed, control populations across an equivalent time period. Multi-locus MHC diversity significantly increased in exposed host populations following the epidemic despite initial compromised diversity levels from a recent introduction bottleneck in the exposed host range. We did not observe equivalent changes in allelic diversity or heterozygosity across eight neutral microsatellite loci, suggesting that the observations reflect selection rather than neutral demographic processes. Our results indicate that a virulent pathogen can exert sufficient balancing selection on class II MHC to rescue compromised levels of genetic