Ovine Toll-like Receptor 9 (TLR9) Gene Variation and Its Association with Flystrike Susceptibility

Simple Summary Flystrike is a major ectoparasitic disease of sheep and it creates both an economic impact and welfare issue for the sheep industry. Several factors control the responses of sheep to flystrike, and among these, immune response is regarded as an important factor. Toll-like receptors (TLRs) plays a crucial role in the innate immune system by recognizing pathogen-associated molecular patterns derived from various microbes. Of these receptors, TLR9 recognises unmethylated cytosine-phosphate-guanine (CpG) motifs that are known to be prevalent in bacterial genomes and are also reported in Dipteran insects, including Lucilia cuprina, one of the main species associated with flystrike in sheep. In this study, we used a polymerase chain reaction-single strand conformation polymorphism (PCR-SSCP) technique to investigate TLR9 variation in sheep with and without flystrike, and found that variation in a gene region containing the coding sequence of the putative CpG-DNA binding domain was associated with the likelihood of flystrike occurrence. This suggests that variation in ovine TLR9 may affect a sheep's response to flystrike. Toll-like receptors (TLRs) are a family of proteins that play a role in innate immune responses by recognising pathogen-associated molecular patterns derived from various microbes. Of these receptors, TLR9 recognises bacterial and viral DNA containing unmethylated cytosine-phosphate-guanine (CpG) motifs, and variation in TLR9 has been associated with resistance to various infectious diseases. Flystrike is a problem affecting the sheep industry globally and the immune response of the sheep has been suggested as one factor that influences the response to the disease. In this study, variation in ovine TLR9 from 178 sheep with flystrike and 134 sheep without flystrike was investigated using a polymerase chain reaction-single strand conformation polymorphism (PCR-SSCP) approach. These sheep were collected from both commercial and stud farms throughout New Zealand and they were of 13 different breeds, cross-breds and composites. Four alleles of TLR9 were detected, including three previously identified alleles (*01, *02 and *03) and a new allele (*04). In total six single nucleotide polymorphisms (SNPs) were found. Of the three common alleles in the sheep studied, the presence of *03 was found to be associated with a reduced likelihood of flystrike being present (OR = 0.499, p = 0.024). This suggests that variation in ovine TLR9 may affect a sheep's response to flystrike, and thus the gene may have value as a genetic marker for improving resistance to the disease.