Major histocompatibility complex diversity in anurans

Abstract

The Major Histocompatibility Complex (MHC) is a large gene complex vital to the vertebrate immune response. The antigen-presenting molecules of the MHC class I and class II are involved in the immune surveillance of intracellular and extracellular pathogens, respectively. The regions of the MHC involved in peptide binding and presentation are often highly polymorphic and have high allelic variation within populations. High MHC diversity is theorised to provide immunogenetic competence to a population, and these genes have become a popular adaptive genetic marker in population studies, often with a conservation context. This thesis presents the characterisation of MHC diversity in two anuran species undergoing very different population histories: the cane toad (Rhinella marina), undergoing range expansions over the course of the Australian invasion; and the New Zealand endemic Hochstetter’s frog (Leiopelma hochstetteri), where populations are highly fragmented. In both cases, I investigate MHC diversity alongside neutral genetic diversity to infer the relative influences of neutral genetic forces, predominately drift, and selection in shaping allelic variation. This involved characterisation of the class I and class II in the cane toad, prior to completing a diversity study utilising genetic markers from both classes. I also characterised a class II beta gene in the Hochstetter’s frog to characterise diversity across 5 populations. The characterisation of the cane toad class I revealed a single classical locus, and an expansion of non-classical loci. This is similar to the class I organisation found in the model anuran, Xenopus laevis and across its subfamily Xenopodinae. The cane toad and X. laevis diverged around 230MYA. The characterisation of the cane toad class II revealed four class II alpha loci and three class II beta loci. It also revealed the expression of MHC class II splice isoforms at several alpha and beta genes. This is the first observation of alternative splicing in the MHC of any anuran species. The diversity of the classical class I (UA) and class II beta gene (DAB) was studied in the source of the Australian introduction (Hawaii), in a long-colonised site (Cairns) and a site on the invasion front (Timber Creek). I found that little diversity was lost as a result of the introduction and that Cairns was highly genetically representative of Hawaii. I found genetic drift acting at the invasion front, decaying genetic diversity at microsatellite markers and the UA locus in the dispersing cane toads. DAB diversity was maintained, however. Selection has retained all three DAB alleles across the toad invasion, likely mediated by pathogenic bacteria and parasites infecting the population. I found very high MHC class II DAB polymorphism in the Hochstetter’s frog across 5 sampled populations. Populations showed extreme differentiation; only two DAB alleles were shared by more than one population. Populations generally had high DAB diversity, except Otawa. The Otawa population had only two DAB alleles present. Combined with low microsatellite diversity, Otawa has likely undergone a recent decline, which has impacted genetic diversity. Low immunogenetic diversity in Otawa may predispose this population to a greater risk of extinction from emergent disease.