Comparative diversity at the major histocompatibility complex in two dolphin species

Abstract

This thesis presents investigations of diversity at three genes (class I, DQA and DQB) of the Major Histocompatibility Complex (MHC) in cetaceans. The MHC genes encode for proteins that are crucial for initiating an immune response by binding invading pathogens in vertebrates. It has been acknowledged that a high diversity at these genes results in the ability to recognise a wider range of pathogens, therefore functional diversity is important for the survival of a species. Furthermore this diversity has been created under the influence of selection, which can reveal interesting contrasts with neutral markers about the history of selection of populations and species. The diversity at two genes (DQA and DQB) in natural populations of two contrasting species of cetaceans has been investigated in more detail. The species selected included both sub-species of Hector’s dolphin, the Hector’s dolphin (Cephalorhynchus hectori hectori) and the Maui’s dolphin (Cephalorhynchus hectori maui), as well as the long-finned pilot whale (Globicephala melas). These species were chosen, because both Hector’s dolphin sub-species contrast with the pilot whale species in regards to their population size, abundance, population structure and life history. For example both sub-species of Hector’s dolphin have small population sizes and only inhabit coastal areas around New Zealand, whereas the pilot whale is an abundant, pelagic dolphin species. In Chapter 2 the expression of class II MHC genes (DQA and DQB) was demonstrated for the first time for a cetacean species, the Hector’s dolphin. Using available information from the bottlenose dolphin (Tursiops truncatus), I also designed primers to investigate class I MHC. Fragments of MHC genes were amplified from cDNA, which was derived from blood samples of two Hector’s dolphins. These dolphins were the subject of a temporary live capture, presenting a unique opportunity for blood collection. No evidence was found for duplication of both MHC class II loci, but cloning suggested a minimum of three copies of class I genes within the genomic DNA. However, the expression of all class I genes was uncertain, since only one allele could be isolated from cDNA. Functionality for all three genes (class I, DQA and DQB) was supported by the evidence for balancing selection having operated on these genes, indicated by a higher ratio of non-synonymous to synonymous substitutions. In Chapter 3, a combination of single-strand conformation polymorphism (SSCP) and direct sequencing was used to describe DQA and DQB diversity in the Hector’s and Maui’s dolphin. Genetic samples for the Hector’s dolphin were available from previously collected stranding and biopsy samples (n = 233), representing three populations from the South Coast of New Zealand and the sub-species on the West Coast of the North Island of New Zealand. For the Hector’s dolphin of the South Island, a surprisingly large number of alleles at both loci (DQA = 4, DQB = 6) were found, considering their small population size and compared to other cetacean populations with larger population sizes. The Maui’s dolphin has been classified as critically endangered with less than 100 dolphins, but showed a relatively high nucleotide diversity for DQB ( = 4.5%). This diversity was based on only three alleles that have been retained in the sub-species, representing the most divergent of all six alleles. All populations showed strong geographic differentiation at both loci (DQA: FST=0.252; DQB: FST=0.333), with the greatest differentiation between the South Island population and the North Island Maui’s dolphin. Comparison to mitochondrial and microsatellite diversity suggested influence of stochastic genetic drift, although the pressure of balancing selection acting on DQB over an evolutionary time period was also evident by a higher ratio of non-synonymous to synonymous substitutions (dN/dS=5.9) and by a pattern of trans-specific allele sharing within the family of Delphinidae. In Chapter 4 similar methods were used to describe DQA and DQB in pilot whales using genetic samples from the long-finned pilot whale that were available from five mass-strandings from around New Zealand (n = 237). A larger number of alleles than for the Hector’s dolphin were found at both loci (DQA= 8; DQB= 8), although their large population size and pelagic abundance raises the expectation of an even greater number of alleles. The overall differentiation between mass-strandings was low, but significant for both loci (DQA: FST =0.012, DQB: FST =0.014). The differentiation of all strandings was greatest for the Golden Bay mass-stranding at DQA, but deviation from Hardy-Weinberg equilibrium at DQB suggested either sub-structure within mass-strandings (Wahlund effect) or the presence of null alleles. As for the Hector’s dolphin and other mammalian species, the influence of balancing selection acting on DQB over a long evolutionary time period was evident by a higher ratio non-synonymous to synonymous substitutions (dN/dS=9.3) and by a pattern of trans-specific allele sharing within the family of Delphinidae.

Overall, diversity is surprisingly similar between these two cetacean species despite different life history characteristic, but low compared to domesticated ungulate species, such as the cow. If low MHC diversity is a general feature of cetaceans, due to the marine environment as suggested previously or rather a side effect of short-term demographic forces remains speculative. A standardised nomenclature for the increasing number of MHC alleles from cetacean is proposed in this thesis to assist with future development of this research.