Professor University of Alberta/University of Western Ontario, Ontario, Canada
Background/Question/Methods
The recent exponential increase in the accessibility of whole-genome sequencing power brings the “genomics revolution” to wildlife conservation. Non-model species genomes can now be sequenced rapidly and inexpensively. Here we used whole-genome sequencing to study the evolutionary history of Pachyceriform sheep [bighorn (Ovis canadensis); thinhorn (Ovis dalli); snow (Ovis nivicola)]. The thinhorn species complex comprises the pure white Dall (Ovis dalli dalli) and the dark pelage Stone subspecies (Ovis dalli stonei). The speciation process of the North American thinhorn and bighorn sheep arose from glacial vicariance (i.e., geographical separation and isolation of populations during the last ice age) and ancient hybridization events. Secondary contact between species can sometimes incorporate genomic signatures/blocks, i.e., introgression signal. These two species might also have hybridized with snow sheep, although the latter is currently geographically distributed in Asia. We mapped one individual of Dall, Stone, bighorn, and snow sheep to the domestic sheep (Ovis aries) reference genome to study these patterns. These genomes were separated into 100-kb genomic blocks, and we estimated their phylogenomic relationship and introgression patterns. We compared the latter to the position of genes throughout the genomes, and the biological significance was inferred.
Results/Conclusions
Given all the datasets used, the species tree was the most common relationship observed. Other topologies were also observed, and we detected the relationship between Dall and snow sheep more often than other combinations (Stone+snow; bighorn+snow). These results were congruent with the introgression analyses, with 5.2% of these signals happening between Dall and snow sheep. When comparing these introgression signals to the position of genes, we detected the presence of interesting biological functions. One particular gene (GPR143) was present in genomic blocks with introgression signals between Dall+snow and Stone+snow. The same gene was found between Stone and bighorn sheep in a previous study (https://doi.org/10.1111/mec.16136). This gene is involved in the melanosome biogenesis and structure, maybe explaining their coat colour. We also observed a significant enrichment (p-value: 3.75E-08) for protein interactions in regions with signals between Dall and snow sheep. Many of these interactions were related to cell functions; however, we also observed genes related to behaviour (e.g., learning, memory, locomotory behaviour). This relationship and their possible biological significance were likely due to the distribution of Dall’s ancestor being closer to the Beringia, where it hybridized with the ancestor of snow sheep.
