Genetic Diversity and Disease Susceptibility: MHC DQB Variation in Northwest Atlantic Harbor Seals

Undergraduate #103
Discipline: Biological Sciences
Subcategory: Genetics
Session: 1
Room: Exhibit Hall A

Olivia Hemond - University of California, Berkeley
Co-Author(s): Elizabeth Piotrowski, University of Maine, Orono; Alayna Hawkins, University of Maine, Orono; Kristina Cammen, University of Maine, Orono

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The DQB is a genetic region of the major histocompatibility complex that plays an important role in recognizing foreign pathogens and initiating an immune response. Pathogen exposure influences genetic diversity at DQB through frequency-dependent, balancing selection. Characterizing genetic diversity at DQB can therefore provide information about the evolution of immunity and capacity for disease resistance. In Eastern Atlantic gray seals (Halichoerus grypus), previous studies identified five alleles at the DQB exon 2 location and connected increased allele number to increased survival into adulthood. Sympatric harbor seals (Phoca vitulina) have greater susceptibility to phocine distemper and avian influenza infection and experience higher mortality rates than gray seals during disease outbreaks. If heterozygosity confers a survival advantage, investigating DQB variability in harbor seals could help us understand the reasons underlying their disease susceptibility. To begin this study, we sequenced the harbor seal DQB exon 2 region and tested the hypothesis that harbor seals exhibit lower DQB diversity than gray seals. Genomic DNA was extracted from tissue samples collected from bycaught Northwest Atlantic harbor seals. The DQB exon 2 region was amplified using species-specific primers and sequenced in 24 individuals. The amplified gene region from six individuals was cloned in chemically competent E. coli, and sequencing of eight clones per individual revealed at least five distinct alleles. Additional genetic variation in sequencing data suggest further research will identify additional alleles. One of the alleles found in this study was identical to a previously identified gray seal allele while the other four were unique. Evidence of positive selection acting on the peptide binding region of harbor seal DQB, the area that binds to a foreign pathogen, confirmed this region plays a role in harbor seal fitness. However, the relatively high number of alleles identified in a small number of harbor seals indicate that allelic diversity may not be the mechanism conferring viral resistance in gray seals. Alternatively, gray seals may possess alleles that structurally respond better to given viruses. Harbor seal populations in the Northwest Atlantic have suffered population declines from disease epidemics. Supplementary studies to characterize additional harbor and gray seal DQB alleles and analyses to link genetic data with disease status of individuals are needed to understand how this selective pressure impacts the genetic diversity and fitness of these seals. References: Cammen, K., Hoffman, J. I., Knapp, L. A., Harwood, J., & Amos, W. (2011). Geographic variation of the major histocompatibility complex in Eastern Atlantic gray seals (Halichoerus grypus). Molecular Ecology, 20(4), 740-752. De Assun??o-Franco, M., Hoffman, J. I., Harwood, J., & Amos, W. (2012). MHC genotype and near-deterministic mortality in grey seals. Scientific Reports, 2, 1-3.

Funder Acknowledgement(s): I would like to thank Fred Wenzel and the Northeast Fisheries Observer Program for providing the harbor seal tissue samples used in this study. Funding was provided by REU Site: Accelerating New Environmental Workskills (NSF Award #1849802).

Faculty Advisor: Kristina Cammen, kristina.cammen@maine.edu

Role: I performed all of the PCR amplifications and ran gel electrophoresis for every sample. I prepared all samples for Sanger sequencing using ExoAP purification. I cleaned up and analyzed all of the sequencing data on CodonCode software. I cloned 6 individuals using chemically competent E. coli. From the cloning results, I used the common convention for allele identification (Kennedy et al 2002) to identify 5 unique alleles. I created phylogenetic trees using the MEGA 7 computer program. I also performed statistical analyses on the peptide binding region of DQB exon 2 using dN/dS calculations.