Characterising the Major Histocompatibility Complex (MHC) of the critically endangered Burrunan dolphin (Tursiops australis)

Marine ecosystems are under increasing threat from anthropogenic impacts, such as pollution, over exploitation, climate change driven water temperature rise and transport for goods and recreation. The destruction of marine habitats further contributes to rise of infectious diseases, loss of genetic diversity and thus the demise of the species inhabiting them. By occupying wide geographical ranges and various trophic levels, marine mammals, including cetaceans, are at heightened risk, with a quarter of the species being on the brink of extinction. A species that is particularly impacted, is the recently described (2011) critically endangered Burrunan dolphin (Tursiops australis), that is endemic to southern Australian waters. The species has suffered from two mass mortality events as a result of Fresh Water Skin Disease (FWSD), and the remaining populations are at extreme risk of extinction. To understand the capacity of this new and threated species to respond to environmental challenges, this study investigated the genetic and structural diversity of an important marker of immune fitness, the Major Histocompatibility Complex (MHC) in Burrunan dolphins and other members of the Tursiops genus. Exons 2 and 3 from MHC Class I and DQB3 from MHC Class II revealed the presence of up to six paralogues present in the Tursiops genome, for both classes. Phylogenetic reconstructions revealed a clear pattern of paralogue-driven evolution of the gene family, with environment/pathogenic stressors most likely being the key drivers of MHC evolution in these species. Protein prediction of the MHC molecules also demonstrated that the nucleotide changes lead to variation in the protein structure, with potential impact on the antigen binding propensities of the paralogues within and between species. While it is difficult to determine the exact functional differences between the Burrunan and common bottlenose dolphin?s MHC receptors, the observed structural and biochemical changes indicate that they might differ in their pathogen recognition capacity, a pattern that might be driven by the different niches the two species occupy. This study is the first of its kind to explore the MHC in the Burrunan dolphin, and will form the basis for a follow up study on the species ability to respond to disease and pathogens, and thus contributes to the conservation management of this and other endangered cetaceans.