Natural History of CXCL16 in Horses: Evidence for Caballine, Non-Caballine Hybridization

Two alleles were identified for the scavenger receptor protein gene, CXCL16. One (susceptibility or S) is dominant and serves as a receptor for equine arteritis virus (EAV) infection of equine cells while the other (resistane or R) does not. The two alleles differed by 4 non-synonymous mutations in the first exon of the gene with susceptibility dominant to resistance. DNA sequences were compared for the 2kb region surrounding CXCL16 exon 1 for 9 horses (Equus caballus) (2 R/R, 2 R/S and 5 S/S) and 4 non-caballines [2 asses (E. africanus asinus, E. africanus somaliensis), 1 kiang (E. kiang) and 1 Grevy zebra (E. grevyi) ; all S/S]. High homology was observed for sequences within a 947 bp region for 7 susceptible horses (R/S & S/S) and 4 non-caballine equids. Within this region we observed 13 SNPs common to the susceptible haplotype of horses and all 4 non-caballine equids. This same region also contained 2 SNPs only common to the non-caballine equids and 1 SNP common only to the susceptible horses. These results suggest a hypothesis of cross-hybridization of caballine and non-caballine species between 400,000 and 600,000 years ago. This would be a remarkable hybridization event considering that the divergence of caballine and non-caballine equids is thought to have taken place over 4.5 million years ago. This suggests cross-hybridization within a genus is a novel way for species to acquire new genes and further suggests that the co-existence of related species is an important resource for evolution.