Master of Science
MacDougall-Shackleton, Elizabeth A.
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.
Summary for Lay Audience
Throughout their evolutionary history, animals and other living things have had to defend themselves against bacteria, viruses, and other diseases. A key part of this defense is a group of genes known as the major histocompatibility complex (MHC). MHC genes make receptors that cells use to alert the immune system to infections, either inside the cell such as viruses (class 1) or outside the cell such as bacteria (class 2). The kind of infections that a receptor can recognize depends on the versions of the genes (alleles) that the host carries. Animals often prefer mates with different MHC alleles from their own, probably because this allows them to produce offspring that have several different alleles and are protected against a wider range of infections. Surprisingly, however, a previous study of song sparrows found that these birds chose mates that were similar, not different, to themselves at class 2 MHC. I asked whether this unexpected pattern also happens at class 1 MHC. I collected DNA from blood samples, and identified which birds were mated pairs by providing them with unique combinations of coloured plastic leg bands and then watching them interact in the wild. I chose to use song sparrows for this study as they can assess MHCII similarity through smell – although whether this is true for MHCI is unknown. Surprisingly, I found that song sparrows seem to pick their partners at random with respect to similarity or difference at class 1 MHC. This may suggest that song sparrows prioritize other considerations in mate choice, such as if a particular mate will be a good parent, or that unlike class 2 MHC, song sparrows simply have no way to assess similarity at class 1. Identifying whether mate choice improves animals’ ability to produce healthy offspring is important for conservation efforts, as to predict a species’ ability to evolve in response to a changing environment we must know how much genetic diversity is needed, and whether mate choice helps to maintain high levels of diversity.
Thompson, Scout R.L, "Random Mating in the Face of Balancing Selection at the Major Histocompatibility Complex Class I in Song Sparrows (Melospiza melodia)" (2023). Electronic Thesis and Dissertation Repository. 9563.