In vertebrate animals, genes of the major histocompatibility complex (MHC) determine the set of pathogens to which an individual's adaptive immune system can respond. MHC genes are extraordinarily polymorphic, often showing elevated nonsynonymous relative to synonymous sequence variation and sharing presumably ancient polymorphisms between lineages. These patterns likely reflect pathogen‐mediated balancing selection, for example, rare‐allele or heterozygote advantage. Such selection is often reinforced by disassortative mating at MHC. We characterized exon 2 of MHC class II, corresponding to the hypervariable peptide‐binding region, in song sparrows (Melospiza melodia). We compared nonsynonymous to synonymous sequence variation in order to identify positively selected sites; assessed evidence for trans‐species polymorphisms indicating ancient balancing selection; and compared MHC similarity of socially mated pairs to expectations under random mating. Six codons showed elevated ratios of nonsynonymous to synonymous variation, consistent with balancing selection, and we characterized several alleles similar to those occurring in at least four other avian families. Despite this evidence for historical balancing selection, mated pairs were significantly more similar at MHC than were randomly generated pairings. Nonrandom mating at MHC thus appears to partially counteract, not reinforce, pathogen‐mediated balancing selection in this system. We suggest that in systems where individual fitness does not increase monotonically with MHC diversity, assortative mating may help to avoid excessive offspring heterozygosity that could otherwise arise from long‐standing balancing selection.
Fieldwork focused on a long‐term study population of migratory song sparrows breeding near Newboro, Ontario, Canada (44.6338°N, 76.3308°W). During spring 2014 (April 14‐June 2) and 2015 (April 13‐June 6), corresponding to nesting, egg‐laying, incubating, and provisioning offspring in the study population, we captured adult song sparrows; collected blood samples for genetic analysis; and identified socially mated pairs, using trapping records and behavioral observations as detailed below.
We captured song sparrows in two‐celled, seed‐baited Potter traps, which we checked once per hour, three times a day, between 07:00 and 11:00. We captured 69 birds in 2014 (44 males, 25 females) and 87 in 2015 (49 males, 38 females). These figures include 28 birds that were captured in both study years (25 males, three females); thus, our sample comprises 128 individuals (68 males, 60 females).
From each bird, we collected ~25 μl of whole blood via brachial venipuncture the first time it was captured each year. We blotted blood onto high wet‐strength filter paper saturated with 0.5 M Na‐EDTA (pH 8.0), allowed the blot to air‐dry, then stored it at room temperature awaiting DNA extraction. We identified sex based on the presence (male) or absence (female) of a cloacal protuberance, supplemented by unflattened wing length (measured with dial calipers to the nearest 0.1 mm). If not already banded, we outfitted the bird with a numbered aluminum leg band (Environment and Climate Change Canada Scientific Banding Permit 10691B), and a unique combination of three colored plastic leg bands to permit individual identification in the field. We recorded any other song sparrows in the trap (i.e., trapped in the other cell at the same time) and released birds at their site of capture. In almost all cases, birds were later resighted and/or recaptured, implying that they were resident breeders. Animal procedures were approved by the Animal Use Subcommittee at the University of Western Ontario (protocols 2008‐054 and 2015‐047 to EAM‐S) and conducted under the required federal permits.