3.6 | Potential drivers of divergence
Currently, the anthelmintics mainly have two modes of action, one is more rapid action on membrane ion channels, and the other is a relatively slow biochemical reaction. These common anthelmintics include benzimidazoles (BZs), macrolides (MLs), nicotinic acetylcholine receptor agonists (Wolf et al.) and aminoacetonitrile derivatives (AADs). We screened out the main genes that may be related to the resistance of all the above-mentioned anthelmintics that have been reported so far (Table 1). We performed selective scanning (iHS) within 50 kb of all these gene regions, and calculated the nucleotide diversity (π ) andtajima’D of the three populations with 10 kb sliding windows. The results showed that multiple resistance-related genes were strongly selected among different populations (positive or negative; Table 1). For example, we first identified three classic resistance locus (167/Phe, 198/Glu and 200/Phe) of the BZs resistance gene β-tubulin (Lake, Matthews, Kaplan, & Hodgkinson, 2009), and found that all individuals in the three populations showed non-resistant mutations through sequence alignment (Fig. S14a). We then further scanned the selection of these gene regions and found that although there were no drug-resistant mutations in these regions, and they showed significant positive selection (Fig. S14c). In addition, the selection of other resistance-related genes in the population have also been discovered (Fig. S15 - S17). For example, multidrug resistance protein pgp-3and multidrug resistance protein 1 (mrp-1 ) revealed strong positive selections in the PEc population. Glutamate-gated chloride channel alpha (glc-1 ), which was related to ivermectin resistance, showed strong positive selection in PEz and PEa populations. The selection of these regions do not have a certain rule, which may be related to the medication history in different environments. However, it was obvious that in some groups, certain genetic selections have been permanent and would continue to be passed down to the next generation.
4 | DISCUSSION
The ecological environment in which parasites inhabit is different from that of ordinary animals, and the survival of parasites is more dependent on the intestinal environment. The survival and evolution of the equine are inextricably linked to nature and human activities. Our study used a combination of explicit genetic analysis and demographic models to determine the possibly diversified mechanisms that occur in different intestinal environments. We applied δaδi 2D/3D models to roundworm populations distributed in different regions and different hosts to unveil whether the diversity of the population was caused by geographic distribution and ecological environment, or simply caused by host specificity, or was related to human domestication of its host. The ancient migration or secondary contact model, as well as the immediate size change model, were found to be effective in explaining the demographic differences and recent divergence of Parascaris spp. populations. In addition, the demographic history showed that theParascaris spp. were in the process of divergence. Selection analysis provided evidence for understanding the possible causes of the divergence, which supports the significant impact of different intestinal habitats on evolution. We summarized the main findings on the diversification of Equus roundworms, and provided a perspective for future monitoring of roundworm ecology in a timely manner to deal with possible unfavorable mutations.