School of BioSciences - Theses
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ItemEvolution and ecology of major histocompatibility complex (MHC) genes in south-eastern Australian passerines( 2015)Understanding how variation in adaptive genes arises and is maintained is a central focus of evolutionary genetics. A growing number of studies are focussing on the structure, evolution and ecology of major histocompatibility complex (MHC) genes in a range of vertebrate taxa, including fish, amphibians, reptiles, birds and mammals. Comprising the most polymorphic genes in the vertebrate genome, the MHC presents an ideal system for understanding the mechanisms maintaining genetic variation in natural populations. The passerine MHC is typified by a large structure with complex organisation, yet little is known about the origin of this complexity. Most studies have focussed on species within the infraorder Passerida, which represents the bulk of passerine diversity, whereas there is very little information on the MHC in basal passerine lineages. In this study, I focussed on seven Australian passerine species (Brown Treecreeper Climacteris picumnus, Superb Fairy-wren Malurus cyaneus¸ Spotted Pardalote Pardalotus punctatus, Striated Pardalote P. striatus, White-plumed Honeyeater Lichenostomus penicillatus, Fuscous Honeyeater L. fuscus, Yellow-tufted Honeyeater L. melanops) from four families (Climacteridae, Maluridae, Pardalotidae, Meliphagidae) forming a prominent component of the basal passerine lineages. I estimated levels of MHC class II polymorphism and allelic diversity in these species and reconstructed phylogenetic relationships among basal and derived passerine species. I found high levels of allelic diversity and polymorphism in the seven species and evidence of a monophyletic grouping of basal and derived passerine species, suggesting an early evolutionary origin of a complex MHC structure in passerines. The high levels of polymorphism and allelic diversity characteristic of MHC genes are thought to be generated and maintained by a combination of different evolutionary processes. I characterised the MHC class II β exon 2 genes in two congeneric study species, the spotted and striated pardalote, and tested for signatures of recombination, gene conversion and balancing selection at the species level. I found evidence of multiple gene loci and putative pseudogenes, as well as evidence of MHC variation having been shaped by historical balancing selection, recombination and gene conversion in both species. MHC variation at the population level may be affected by both selective and neutral processes. I examined how habitat configuration and dispersal behaviour influence MHC class II β exon 2 variation in the brown treecreeper. Dispersal behaviour in the brown treecreeper is sex-biased and dispersal ability is strongly linked to the availability of connecting habitat. I performed pairwise comparisons of population genetic differentiation at MHC and microsatellite markers to determine if the relative influence of selective and neutral processes varied between a) the dispersive (female) and philopatric (male) sexes, and b) habitats with dispersed and aggregated configurations of tree cover. MHC alleles were also clustered into functional supertypes to test for signatures of balancing selection across populations. I found differences in extent of genetic differentiation based on sex and habitat type, reflecting males’ philopatric behaviour and females’ reliance on suitable habitat for dispersal. Most MHC supertypes were maintained across populations, indicating the effects of balancing selection. The MHC plays a crucial role in the vertebrate immune response to pathogens and parasites. Understanding the evolutionary implications of host-parasite interactions first requires an understanding of the key pathogens and parasites affecting passerine species. Avian haematozoa are ideal models for studying host-parasite interactions because they infect a large number of host species and are widely distributed. I assessed the prevalence and diversity of avian haematozoa in three host species, the brown treecreeper, spotted pardalote and striated pardalote using microscopy and molecular techniques. I identified four lineages of Haemoproteus infection in total, of which three are previously unreported, as well as present the first record of haematozoan infection in these species of passerine. This study highlights the complexity of the MHC in basal passerine species and demonstrates the influence of multiple evolutionary processes in shaping MHC variation at the species and population level. The research presented in this thesis adds to a growing body of knowledge on the MHC in non-model vertebrate species and provides a number of avenues for further research into the evolution of the passerine MHC.