Reduced MHC and neutral variation in the Galápagos hawk, an island endemic.

Jennifer L Bollmer,Joshua M Hull,José H Sarasola,Patricia G Parker,Holly B Ernest

doi:10.1186/1471-2148-11-143

Abstract

BackgroundGenes 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).ResultsWe 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.ConclusionsThe 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.

Highlights

Genes at the major histocompatibility complex (MHC) are known for high levels of polymorphism maintained by balancing selection
We investigated MHC and neutral genetic variation in an island species, the Galápagos hawk (Buteo galapagoensis), and its closest mainland relative, the Swainson’s hawk (B. swainsoni; [20])
MHC diversity Sequencing of exon 2 from MHC class II B genes revealed that Galápagos hawks had lower MHC diversity than

Summary

Introduction

Genes at the major histocompatibility complex (MHC) are known for high levels of polymorphism maintained by balancing selection. In small or bottlenecked populations, genetic drift may be strong enough to overwhelm the effect of balancing selection, resulting in reduced MHC variability. Genes at the major histocompatibility complex (MHC) are known for their high levels of polymorphism [1], and for their importance in initiating the adaptive vertebrate immune response by binding to foreign peptides and presenting them to T cells [2]. Most studies of small island [15,16] and mainland [17,18] populations that have undergone severe bottlenecks have documented reduced MHC diversity and concluded that genetic drift had overwhelmed selection (reviewed in [19])

Methods

Results

Discussion

Conclusion