Sequence and functional variability of Toll-like receptor 9 gene in equines

Twelve toll-like receptor (TLR) genes in the family Equidae - comparative genomics, selection and evolution

Toll-like receptors (TLRs) represent an important part of the innate immune system. While human and murine TLRs have been intensively studied, little is known about TLRs in non-model species. The order Perissodactyla comprises a variety of free-living and domesticated species exposed to different pathogens in different habitats and is therefore suitable for analyzing the diversity and evolution of immunity-related genes. We analyzed TLR genes in the order Perissodactyla with a focus on the family Equidae. Twelve TLRs were identified by bioinformatic analyses of online genomic resources; their sequences were confirmed in equids by genomic DNA re-sequencing of a panel of nine species. The expression of TLR11 and TLR12 was confirmed in the domestic horse by cDNA sequencing. Phylogenetic reconstruction of the TLR gene family in Perissodactyla identified six sub-families. TLR4 clustered together with TLR5; the TLR1-6-10 subfamily showed a high degree of sequence identity. The average estimated evolutionary divergence of all twelve TLRs studied was 0.3% among the Equidae; the most divergent CDS were those of Equus caballus and Equus hemionus kulan (1.34%) in the TLR3, and Equus africanus somaliensis and Equus quagga antiquorum (2.1%) in the TLR1 protein. In each TLR gene, there were haplotypes shared between equid species, most extensively in TLR3 and TLR9 CDS, and TLR6 amino acid sequence. All twelve TLR genes were under strong negative overall selection. Signatures of diversifying selection in specific codon sites were detected in all TLRs except TLR8. Differences in the selection patterns between virus-sensing and non-viral TLRs were observed.

Molecular characterization in the toll-like receptor 9 gene of Cape Mountain Zebra (Equus zebra zebra) from three populations

Toll-like receptors (TLR) are a family of proteins that signal activation of the innate immune response through the recognition of a variety of pathogen molecular compounds. Here, we characterized the complete TLR9 gene in Cape mountain zebra (Equus zebra zebra) from three populations in South Africa and compared sequences to a variety of horse and donkey breeds. Overall, we identified six single nucleotide polymorpHisms (SNPs). A single SNP (G586S) was non-synonymous, whereas the remaining SNPs were synonymous. The G586S alteration was detected in Cape mountain zebra populations with varying frequency. In addition, adaptive diversity was found to be discordant with variation based on neutral markers. The mutation is unique to the Cape mountain zebra when compared to other equid species. The structure of TLR9 is relatively conserved and the resulting amino acid substitution was found to have minimal interaction with active sites in the protein. Future studies can explore the effects of this potentially functional mutation which will contribute to our understanding of genetic diversity within adaptive sites of the Cape mountain zebra genome.