House Finches, parasites and the evolution of genetic polymorphism for disease resistance

House Finches (Carpodacus mexicanus) are a common songbird found in both the western U.S., where they are native, and in the eastern U. S. and Hawaii, where they have been introduced since the late 1940s and 1960s respectively.  Because they are easy to maintain in captivity and to conduct behavioral experiments with, House Finches have become a model species in studies of mate choice, sexual selection, plumage variation and secondary sexual signals.  Much of the recent work on House Finches has been conducted by Geoff Hill, a professor at Auburn University in Alabama and a collaborator with us in our efforts to uncover the genetic bases of mate choice and disease resistance in birds.

For several years the lab has been characterizing major histocompatibility complex (Mhc)genes in House Finches.  Most recently this work has been spearheaded by graduate student Chris Hess.  Mhc genes are the most polymorphic genes in vertebrates and are thought to be subject to diversity-enhancing types of natural selection such as balancing selection -- i.e., heterozygote advantage or rare-allele advantage.  Mhc genes are under such selection and are so diverse presumably because the parasites that are targets of Mhc genes are constantly changing, trying to escape   detection by the host immune system -- a true molecular coevolutionary arms race.  In 1995, Edwards cloned the first coding sequences of Mhc genes in House Finches, and more recently Chris Hess and others in the lab have sequenced a large (32 kb) segment of genomic DNA containing Mhc sequences from a house finch. 

House finches are of interest in the context of Mhc genes because this species has strong evidence for geographical variation in blood parasite loads, for mate choice based on honest signals of male quality, and has recently been colonized by an obligate bacterial parasite, Mycoplasma gallisepticum, that escaped from its usual host, chickens.  In collaboration with Drs. Hill and Susan Roberts, a microbiologist also at Auburn University, we hope to find out whether or not Mhc genes play a role in differential resistance of finches to this parasite, and whether or not the demonstrable preferences for particular males by females have a basis in Mhc variation, as has been suggested for mice and humans.