Host preference as the primary cause of divergence
We have roundworm samples from various hosts from multiple locations around northern China, including grassland pastures, zoos, and family farms. Our clustering analysis revealed that even if a sampling site of the horse roundworm (PEc) was closer to the zebra roundworm (PEz), the horse roundworm still clustered into one clade in terms of genetic relationship. This specificity deserves our attention. It suggests that the single-origin Equus roundworm may have differentiated, and this divergence was probably related to the parasitic host. In addition, clustering results also showed that even if they were geographically closer, the host’s impact on roundworms was obviously more critical. The host provides habitat and nutrition for the parasites. Apparently, the parasites’ local adaptation to the host causes the parasites of the same host to evolve in the same direction (Kaltz & Shykoff, 1998). Studies have shown that a parasite population has higher mean performance in local hosts compared to foreign host populations(Lively, 1996; Schulte, Makus, Hasert, Michiels, & Schulenburg, 2011). When combined with population structure, it is clear that the association between geographical cause and Parascaris spp. evolution was not strong.