University of Missouri - Saint Louis

The Graduate School

Announcement

An oral examination in defense of the dissertation for the degree

Doctor of Philosophy

Jennifer Bollmer
M.S. in Evolution, Ecology, and Organismal Biology, August, 2000, The Ohio State University
B.S. in Zoology, May, 1997, The Ohio State University

Abstract

The major histocompatibility complex (MHC) plays an integral role in the immune system by recognizing foreign pathogens, and so the high variability usually seen at MHC loci is maintained primarily through selection for parasite resistance. However, MHC variability is also affected by neutral forces, including genetic drift. This dissertation research focused on describing variability at MHC class II genes in two bird species endemic to the Galápagos Islands and comparing their MHC diversity with that at neutral loci as well as MHC variability in their closest mainland relatives. Small island populations are predicted to have reduced genetic variability due to the effects of genetic drift; however, selection may be strong enough to prevent the loss of variability at MHC loci. The two species studied were the Galápagos hawk (Buteo galapagoensis) and the Galápagos penguin (Spheniscus mendiculus), both of which have small breeding populations on multiple islands in the archipelago. Their closest relatives are the Swainson’s hawk (B. swainsoni) and the Humboldt penguin (S. humboldti) which have much larger populations on the mainland. The results showed that both island species have reduced MHC variability compared to their more populous mainland relatives: the island species had fewer alleles and the alleles differed from each other by only one to three base pairs. This close relationship among alleles is unusual and suggests that the loci amplified were at one point fixed within each species. The MHC loci showed evidence of having been under selection; however, the low MHC diversity in the island species is lower than in the mainland species and similar to variability at neutral loci, indicating that genetic drift has had an overwhelming effect. Overall, these results, as well as analyses of the relationships among alleles from the pairs of closely related species, give us added insight into the relative strengths of the forces shaping MHC variability and more information about the evolution of MHC genes, which is still poorly understood in birds.

Defense of Dissertation Committee