Random Mating in the Face of Balancing Selection at the Major Histocompatibility Complex Class I in Song Sparrows (Melospiza melodia)
Abstract
The major histocompatibility complex (MHC) is a large group of genes encoding cell-surface proteins that recognize and bind pathogens to initiate an adaptive immune response. MHC loci experience intense pathogen-mediated selection which may be directional, where specific alleles provide the best disease protection, or balancing, where rare alleles or diverse combinations are most protective. However, balancing selection (specifically heterozygote advantage) is more common and often accompanied by disassortative mating. I sought to use genetic and behavioural information to evaluate whether balancing selection and disassortative mating occur at MHC class I (MHCI) in a population of song sparrows (Melospiza melodia). Despite evidence of balancing selection (high ratios of nonsynonymous to synonymous sequence variation and trans-species polymorphisms), the genetic distance between social mates was no different than expected under random mating. This may suggest a low impact of MHCI diversity on lifetime reproductive success, or an inability to discern MHCI genotype.