Koalas may carry more genetic variation in a crucial first line of immune defence than previously recognised, according to a new study of 413 wild animals sampled across eastern Australia. Researchers at the University of Sydney analysed whole-genome resequencing data to examine toll-like receptor, or TLR, genes—molecular sensors that detect signatures of invading bacteria and viruses before the body’s more specialised immune responses begin. The results reveal that koala TLR diversity is not distributed evenly across the species’ range. Instead, it forms distinct geographic patterns that broadly resemble variation previously reported in another important immune gene family, the major histocompatibility complex. The findings offer a detailed genetic baseline for investigating why some koala populations experience heavier disease burdens than others, while also highlighting the limitations of inferring disease resistance from DNA alone.
The koala, Phascolarctos cinereus, is under intense conservation pressure. Habitat loss and fragmentation, bushfires, climate change and vehicle collisions have reduced populations, while infectious diseases remain among the most serious threats. Chlamydia can cause reproductive tract disease, urinary infections and blindness, and koala retrovirus, commonly known as KoRV, can affect immune function and contribute to other health problems. Neither infection is distributed uniformly across Australia. Chlamydial disease is often common in northern populations but appears rare or absent in some southern island populations. KoRV is widespread in Queensland and much of New South Wales, but its prevalence is lower in parts of Victoria and on several Victorian islands. These geographic differences have raised the possibility that local genetic variation may influence how koalas detect, control or tolerate infection.
TLRs are central components of innate immunity, the ancient and rapid defence system shared by animals. Each receptor is built from several functional regions. An external leucine-rich repeat, or LRR, domain forms a curved molecular surface that recognises pathogen-associated molecular patterns, such as microbial proteins, lipids or nucleic acids. A transmembrane segment anchors the receptor in the cell membrane, while an intracellular Toll/interleukin-1 receptor, or TIR, domain transmits the alarm into the cell. Once activated, TLR signalling can trigger inflammatory molecules, antiviral responses and other changes in gene activity. Koalas possess ten TLR genes, designated TLR2 through TLR10 and TLR13. Different receptors specialise in detecting different classes of threat: TLR2 and TLR4 can respond to bacterial components, while TLR3 and TLR7 to TLR9 are involved in recognising viral or microbial nucleic acids. Small genetic changes in these receptors could, in principle, alter what they recognise or how strongly they signal.
To catalogue that variation, the researchers used short-read genome sequences generated from animals collected at 50 locations extending from northern Queensland through New South Wales to Victoria. All genomes were sequenced at approximately 30-fold coverage, providing repeated reads of each position and allowing the team to identify single-nucleotide polymorphisms, or SNPs. A SNP is a one-letter difference in the DNA sequence between individuals. The researchers first located the koala TLR genes in the reference genome using annotated TLR sequences and the BLAST sequence-comparison tool. They then applied a series of quality filters to remove poorly supported or incomplete variant calls, including thresholds for allele frequency, sequencing depth, genotype quality and missing data. The remaining variants were classified according to whether they changed the encoded protein or left its amino-acid sequence unchanged.
Across the ten genes, the analysis identified 45 SNPs that produced 51 alleles. One gene, TLR1/6like, was monomorphic in the dataset, meaning that no qualifying variation was detected among the 413 koalas. The other nine genes were polymorphic, with between three and eleven SNPs per gene and between three and fifteen alleles. TLR7 was the most variable, containing eleven SNPs and fifteen alleles. Of all the variants, 28 were synonymous, causing no change in the protein sequence, while 17 were nonsynonymous and altered an amino acid. These changes were not spread randomly across the receptors. Thirteen nonsynonymous variants occurred in the LRR region, the portion most directly involved in recognising molecular features of pathogens, and twelve of those were predicted to produce changes in physicochemical properties such as charge, hydrophobicity or polarity.
Those predictions are biologically intriguing but do not, by themselves, demonstrate altered immunity. A change in an amino acid can modify how a receptor folds, how it interacts with a pathogen-derived molecule or how it communicates with signalling proteins inside the cell. However, computational predictions cannot establish whether a receptor actually binds a ligand more effectively, signals more strongly or protects an animal from disease. The study did not include infection records, clinical examinations or laboratory measurements of immune function for the sequenced koalas. As a result, the identified variants should be regarded as candidates for future functional research rather than confirmed markers of resistance or susceptibility. The authors specifically note that the animals’ Chlamydia and KoRV status was unavailable, preventing a direct test of whether any TLR allele is associated with infection or disease severity.
Three variants were particularly notable because they were located in predicted pathogen-interaction regions. A TLR3 G305V substitution occupies a position comparable to a human TLR3 mutation that has been linked to defective signalling, although similarity in location does not prove that the koala variant has the same effect. A TLR5 Q179R substitution lies within the LRR6 region, which contributes to recognition of flagellin, the structural protein of bacterial movement appendages. A third change, TLR13 R708S, occurs near the C-terminal portion of the LRR domain, close to a region involved in recognising microbial RNA in structurally studied TLR13 proteins. The koala TLR5 and TLR13 substitutions were also predicted to cause physicochemical changes. Establishing their consequences will require experiments using koala receptor proteins, cell-based signalling assays and carefully controlled comparisons of different genetic variants.
The population analysis revealed a pronounced north-to-south pattern. Koalas from southeast Queensland and far northern New South Wales carried the largest combined set of TLR alleles, with 46 detected, followed by mid-coast New South Wales with 44, northern Queensland with 43, southern New South Wales with 34 and Victoria with 29. A principal coordinates analysis, which represents genetic differences as distances in a multidimensional plot, separated the animals into four broad TLR groups. Northern Queensland, southeast Queensland and northern New South Wales formed relatively distinct patterns, while mid-coast and southern New South Wales showed substantial overlap, and Victorian koalas formed the most separate group. A Mantel test suggested a moderate relationship between geographic and genetic distance, but the result fell short of conventional statistical significance, with a correlation of 0.6209 and a probability value of 0.0583.
The four TLR groupings differ slightly from the five genomic clusters previously identified using genome-wide markers. That contrast is important because immune genes can be shaped by forces that do not affect the rest of the genome in the same way. Population bottlenecks, isolation and genetic drift can remove variants, while pathogen exposure may favour the retention of particular immune alleles through natural selection. The similarity between the TLR patterns and recently described koala MHC diversity suggests that immune gene variation may reflect shared demographic history or common selective pressures. Yet the study cannot determine whether disease drove the pattern, whether geography and ancestry are responsible, or whether both processes contributed. The significant Tajima’s D results for TLR4, TLR5 and especially TLR9 indicate departures from a simple neutral-evolution model, but such departures can arise from selection, population history or other demographic processes.
Compared with published data from other species, koalas showed more TLR diversity than the endangered Tasmanian devil and New Zealand robin, both of which have experienced severe population reductions, but less diversity than pigs and mockingbirds. These comparisons must be interpreted cautiously because the studies used different sample sizes, sequencing technologies and methods for measuring variation. Even so, the koala results challenge the assumption that a species with relatively low genome-wide diversity necessarily has uniformly depleted immune genes. They also underline why conservation programs should preserve genetic variation across the entire koala range rather than concentrating exclusively on the largest or most genetically diverse populations. The southern groups, especially Victoria, carried fewer observed TLR alleles, but low allele counts do not automatically mean poor health or weak disease resistance; a smaller set of variants could include alleles well suited to local conditions.
The study’s most immediate value is therefore as a platform for the next stage of koala disease research. Linking these TLR variants to veterinary records, pathogen sequencing, infection status, clinical outcomes and immune-expression profiles could reveal whether particular alleles influence responses to Chlamydia or KoRV. Laboratory studies could test whether the predicted receptor changes affect ligand binding, cellular localisation or activation of inflammatory and antiviral pathways. Integrating these data with habitat quality, climate conditions, population connectivity and other immune loci may eventually help conservation managers identify populations at heightened genetic risk without reducing conservation decisions to a single gene. For now, the findings show that koala immune diversity has a complex geographic structure and that whole-genome data can uncover it efficiently. They also deliver a clear warning: genetic differences may suggest how disease resistance evolved, but only biological and clinical evidence can show how those differences affect koalas in the wild.
Subject of Research: Toll-like receptor genetic diversity and geographic population structure in wild koalas
Article Title: Spatial variation in toll-like receptor diversity in koala populations across their geographic distribution
Article References: Cui, J., Batley, K. C., Silver, L. W., McLennan, E. A., Hogg, C. J. & Belov, K. “Spatial variation in toll-like receptor diversity in koala populations across their geographic distribution.” Immunogenetics 77, Article 5 (2025). Published 30 November 2024.
Image Credits: AI Generated
DOI: https://doi.org/10.1007/s00251-024-01365-5
Keywords: Koala, toll-like receptors, TLR diversity, immune genes, single-nucleotide polymorphisms, whole-genome resequencing, Chlamydia, koala retrovirus, conservation genomics, population genetics
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