Immune gene variation in the group living lizard Egernia stokesii

Author: Sarah Pearson

Pearson, Sarah, 2017 Immune gene variation in the group living lizard Egernia stokesii, Flinders University, School of Biological Sciences

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Immune gene variation has consequences for individual fitness and species persistence. Because of their role in disease resistance, mate choice, and kin recognition, the genes of the major histocompatibility complex (MHC) are important for species adaptability. Yet lizard MHC structure is unknown and few lizards have had their MHC characterised. Skinks are a basal lizard lineage. Their inclusion in MHC studies should provide valuable insights into the evolutionary ecology of the MHC.

The first aim of this thesis was to characterise MHC variation in one skink species, E. stokesii, among three discrete sampling sites. This work derived a reliable method of obtaining lizard DNA from their scats (Chapter 2) and comprehensive methodology for deriving MHC alleles and genotypes from next-generation DNA sequencing (Chapter 3). This represents the most comprehensive characterisation of skink MHC to date, facilitating reliable population level MHC variation inferences and comparisons.

The second aim was to analyse processes and mechanisms that generated the observed E. stokesii MHC variation. Although selection is purported to explain MHC variation, selection on skink MHC has been untested. Non-mammalian MHC amino acid sites of selection are usually inferred from human MHC despite often lacking concordance. This work represents the first analysis of selection on skink MHC and demonstrates positively selected skink MHC amino acid sites do not correspond to those in human MHC (Chapter 4), strengthening calls for investigations of reptilian MHC structure.

Genetic drift and gene flow influence MHC variation, not selection alone. These processes influence MHC variation on small spatial scales, yet are usually investigated at broad scales. This work provides evidence that selection on the MHC is stronger than genetic drift and constrained gene flow at a fine spatial scale (Chapter 5). Thus, adaptive and neutral genetic variation do not always align. Both should be accounted for in species genetic diversity assessments.

Social structure can influence MHC variation, yet studies of social structure effects on MHC variation are rare. Space use is an important aspect of social structure. In this work, almost 50 lizards sampled from three discrete sites had been captured nearly a decade before and 65% of lizards were recaptured in the same space they previously occupied (Chapter 6). Site fidelity was more likely in larger lizards.

Parasite mediated balancing selection and MHC based mate choice drive MHC variation. Comparative studies of mate choice across taxa representing a diversity of social structures should improve our currently limited understanding of social structure effects on MHC variation. Results from this study indicated mate choice was predicted by both adaptive and neutral genetic variation of potential mates. Group membership together with genetic variables predicted E. stokesii reproductive pairings (Chapter 7), suggesting social structure influences genetic variation in this group living species.

Future work will benefit from the characterisation of MHC variation and selection in additional skinks and other lizard taxa. The members of the Egernia group of lizards, which represent a diversity of social structures, are suitable candidates for future investigations of the influence of group living on the MHC.

Keywords: Major histocompatibility complex, MHC, Immune gene variation, Squamata, Scincidae, Egernia stokesii, lizard

Subject: Biological Sciences thesis

Thesis type: Doctor of Philosophy
Completed: 2017
School: School of Biological Sciences
Supervisor: Michael Gardner