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.