Systematic review of triploidy among parasitic worms

Parasitic worms have significant medical, veterinary, and economic importance. Numerous studies have therefore addressed various aspects of parasitic worms’ biology. In contrast, the ploidy of parasitic worms remains comparatively understudied, despite a few known triploid species. Polyploidy is known to have phenotypic and genetic effects in animals, which can lead to changes at the evolutionary scale. The evolutionary consequences of polyploidy have been addressed in host-parasite systems; however, most studies have focused on the host perspective, with relatively few studies examining the parasitic side of the story. In this review, we provide available information on the distribution of triploidy among parasitic worms, along with information on such aspects as reproduction, origin of triploidy, and life history. Across 100 selected papers (out of 416 screened), triploidy was reported for 15 parasitic Platyhelminthes and 13 parasitic Nematoda. Most triploid species of parasitic worms (24 out of 28) were documented at the adult stage. Triploid lineages reported for the adult stage reproduce predominantly via parthenogenesis. Finally, we discuss the potential effects of ploidy variation for host-parasite dynamics and adaptation rates of parasitic worms. In particular, we discuss how parasitic worms adapt to their hosts despite potential constraints of asexuality. As well as whether triploid individuals outcompete diploid conspecifics when both ploidy levels coinfect the host, as could be expected if triploids have higher virulence.