Variation of the Toll-like receptors in two marsupial species, Tasmanian devils, Sarcophilus harrisii, and Koalas, Phascolarctos cinereus

Abstract

Genetic diversity plays an important role in wildlife conservation, particularly genetic diversity in immune genes, as some genetic variants are able to contribute to disease resistance and susceptibility. Low genetic diversity often occurs in endangered species and low genetic diversity in key immune genes can lead to higher disease susceptibility. To maximise the chance of protecting endangered species, it is possible to study and then manage genetic diversity in immune genes. One of the key gene families involved in adaptive immunity is the Major Histocompatibility Complex (MHC) and numerous studies have described a relationship between MHC alleles and susceptibility to certain diseases. Another family involved in innate immunity are the Toll-like receptors (TLRs) and in this thesis I have measured genetic diversity at TLRs in two iconic Australian marsupials, the Tasmanian devil and the koala, in order to better inform management practices. The Tasmanian devil is endangered due to the emergence of a new transmissible cancer called Devil Facial Tumour Disease (DFTD). Low genetic diversity has been found across the devil genome, including at neutral markers and the MHC genes. In order to protect the devil from extinction, an insurance population has been established and the primary purpose of this programme is to capture all genetic diversity in devils and to protect this diversity over time. In the first section of this thesis I characterise the TLR genes in the devil and then measure the TLR diversity in wild devil populations. Ten TLR genes were identified in the devil transcriptome sequence database and low levels of diversity were found in 25 devils from across Tasmania. TLR2, TLR3 and TLR1/6like have two alleles and the other seven TLRs are monomorphic. A study of the polymorphic TLR genes in another 50 devils from the insurance populations was conducted and the same variants found. This indicates that the insurance population has captured all known variants at TLRs. The TLRs are important genes for pathogen resistance and previous studies have described a correlation between TLR variants and Chlamydia susceptibility in humans. Chlamydia is having significant impacts on koala populations and our knowledge of koala innate immunity is still limited. The second part of the thesis focuses on the characterisation of TLRs in the koala in order to provide a measure of geneticdiversity at TLRs. We found nine TLRs in the koala immune tissue transcriptome and one TLR from a draft sequence of the koala genome. We surveyed genetic diversity in 20 koalas from New South Wales, Australia and found that TLR10 is monomorphic and the other nine TLRs have between two and twelve alleles. Measuring TLR diversity in these two species provides a springboard to future studies on innate immunity in the marsupials.