Genetic Variation at the MHC DQB Locus in Irrawaddy Dolphin (Orcaella brevirostris) across South-East Asia Distribution Range
Author: Sett Aung
Faculty Supervisor: Sarah Cohen
Department: Biology
Major Histocompatibility Complex (MHC) has been widely studied because of its fundamental role in the vertebrate immune system response. MHC genes are known to be the most polymorphic genes in terrestrial and aquatic mammals. The Irrawaddy dolphin (Orcaella brevirostris) is an endangered small cetacean which is widely distributed in rivers, lakes, and coastal waters across Southeast Asia habitat range. To determine the Irrawaddy dolphin variation across its distribution range we genotyped 268bp fragment of exon 2 of the MHC Class II DQB locus for 77 members of its population across South-East Asia. Up to 4 putative distinct alleles of MHC Class II DQB were identified in 35 individuals. Evidence of selection at the molecular level revealed that there is an ongoing positive selection at this exon. Phylogenetic analyses of DQB alleles from O.brevirostris and DQB alleles from other cetaceans suggested trans-specific evolution of MHC variants. Overall, this MHC study reveals that there is little variation across the entire sampled range, with few alleles and mostly homozygous individuals. Only the Gulf of Thailand, Malaysia, and Sarawak populations have heterozygous individuals. This suggests that most of the Irrawaddy dolphin populations are under conservation concern as low diversity in DQB alleles may mean a weak protection against pathogen exposure.