Reduced MHC and neutral variation in the Galápagos hawk, an island endemic
1 Department of Biology, University of Missouri-St. Louis, One University Boulevard, St. Louis, MO 63121, USA
2 Department of Biological Sciences, University of Wisconsin-Milwaukee, P.O. Box 413, Milwaukee, WI 53201, USA
3 Wildlife and Ecology Unit, Veterinary Genetics Laboratory, University of California, One Shields Avenue, Davis, CA 95616, USA
4 Department of Animal Science, University of California, One Shields Avenue, Davis, CA 95616, USA
5 Department of Population Health and Reproduction, School of Veterinary Medicine, University of California, One Shields Avenue, Davis, CA 95616, USA
6 Department of Evolutionary Ecology, Estación Biológica de Doñana, Avda. Américo Vespucio, 41092 Sevilla, Spain
BMC Evolutionary Biology 2011, 11:143 doi:10.1186/1471-2148-11-143Published: 25 May 2011
Genes at the major histocompatibility complex (MHC) are known for high levels of polymorphism maintained by balancing selection. In small or bottlenecked populations, however, genetic drift may be strong enough to overwhelm the effect of balancing selection, resulting in reduced MHC variability. In this study we investigated MHC evolution in two recently diverged bird species: the endemic Galápagos hawk (Buteo galapagoensis), which occurs in small, isolated island populations, and its widespread mainland relative, the Swainson's hawk (B. swainsoni).
We amplified at least two MHC class II B gene copies in each species. We recovered only three different sequences from 32 Galápagos hawks, while we amplified 20 unique sequences in 20 Swainson's hawks. Most of the sequences clustered into two groups in a phylogenetic network, with one group likely representing pseudogenes or nonclassical loci. Neutral genetic diversity at 17 microsatellite loci was also reduced in the Galápagos hawk compared to the Swainson's hawk.
The corresponding loss in neutral diversity suggests that the reduced variability present at Galápagos hawk MHC class II B genes compared to the Swainson's hawk is primarily due to a founder event followed by ongoing genetic drift in small populations. However, purifying selection could also explain the low number of MHC alleles present. This lack of variation at genes involved in the adaptive immune response could be cause for concern should novel diseases reach the archipelago.