Major histocompatibility complex class II sequence variation in cetaceans: DQβ and DRβ variation in beluga (Delphinapterus leucas) and DQβ variation in North Atlantic right whales (Eubalaena glacialis)
Date of Award
Doctor of Philosophy (PhD)
Allelic variation at the Major Histocompatibility Complex (Mhc) class II loci DQβ and DRβ was assessed in a large sample (~300) of beluga (Delphinapterus leucas) and a sample (~12) of narwhal (Monodon monoceros) in order to study the evolutionary significance of the Mhc in cetaceans and to compare levels of variation among beluga populations. Mhc class II DQβ allelic variation was also analysed in a preliminary survey of right (Eubalaena glacialis) and bowhead (Balaena mysticetus) whales. In each survey variation was assessed by analysis of class II loci Bchain exon 2 nucleotide sequences. Exon 2 encodes for the peptide binding region of the class II molecules. These sequences were amplified via the polymerase chain reaction, followed by either cloning and DNA sequencing or single-stranded conformation polymorphism analysis.
A low amount of variation was observed at the beluga DQβ locus while a low to moderate amount of variation was observed at the beluga DQβ1 locus and the right whale DQβ loci when compared with terrestrial mammals. Two DRβ loci in beluga and two DQβ loci in right and bowhead whales were detected. Comparison among beluga DQβ, among beluga DRβ1, and among right whale DQβ sequences show, at each locus, a high ratio of nonsynonymous to synonymous substitutions per site. Further, in each case, the majority of substitutions did not maintain the physio-chemical properties of the residue and were found at sites implicated as being important in the selective binding of foreign antigen. This evidence of positive selection is similar to that found at functional Mhc loci and is consistent with the functional significance of Mhc class II loci in the immune response of cetaceans. Comparison of beluga and right whale DQβ sequences shows a reduced rate of synonymous substitutions in cetaceans. This may explain the low to moderate levels of variation found in the order.
No significant differences in Mhc allelic or genotypic frequencies were observed among beluga summering populations which are believed to share a common overwintering area in either the Hudson Strait or the Bering Sea Significant differences (p ≤ 0.0001) were found among sampling locations of the High Arctic/Baffin Bay beluga population. This suggests a recurrent role of Arctic polynyas as overwintering locations for some groups of High Arctic beluga, i.e. beluga sampled from Cunningham Inlet. DRβ1 allele and genotype frequencies were also significantly different (p ≤ 0.0001) among the overwintering locations, Hudson Strait, Bering Sea, Baffin Bay, Cunningham Inlet, and the St. Lawrence. DQβ allele frequencies were only significantly different (p ≤ 0.005) between the High Arctic/Baffin Bay beluga and all other sampling locations.
Comparison of DQβ and DRβ1 allele and genotype frequencies within the St. Lawrence population between dead beached beluga and live biopsy darted whales shows no evidence of a sample collection bias, however, a larger sample size is needed to detect non-trivial small effects. No reduction in the number of DQβ alleles was found in the St. Lawrence beluga population. A slight reduction in the number of DRβ1 alleles was observed (six vs. eight), however, these five alleles each represent one of the five genetically distinct allelic lineages found in beluga This indicates that the evolutionary potential of this population at these class II molecules remains, despite a recent population bottleneck. The effects of the historic bottleneck are observed upon examination of DRβ1-DQβ haplotypes. Evidence for linkage disequilibrium between these loci is noted in the St. Lawrence. Comparison of possible haplotypes shows the St. Lawrence has about half the haplotypes found in Arctic populations. The short term effects of this reduction are unknown and further investigation of Mhc variation within this population is warranted.
Murray, Brent William, "Major histocompatibility complex class II sequence variation in cetaceans: DQβ and DRβ variation in beluga (Delphinapterus leucas) and DQβ variation in North Atlantic right whales (Eubalaena glacialis)" (1997). Open Access Dissertations and Theses. Paper 3415.