Parasite sex

“Indeed, nothing is more impressive in the biology of parasites than the  lengths to which they will go in order to retain amphimixis, or at least cling  to some remnant of sexuality.”

~ Graham Bell, The Masterpiece of Nature (1982), pages 379-380T

Parasites are often overlooked in favor of their charismatic hosts. Yet parasites in particular should experience strong coevolutionary selection. Indeed, selection is expected to be stronger on obligate parasites: failure to infect might mean death. Hence the reproductive strategies of parasites are an interesting, though largely unexplored, realm for the Red Queen.

The Red Queen hypothesis predicts that sexual reproduction should be more common in parasitic taxa than in their free-living relatives (Bell 1982). Jesualdo Arturo Fuentes and I tested this prediction in the Nematode phylum, which shows remarkable variation in reproductive mode and in ecology (i.e. parasitism has arisen multiple times in the Nematoda).  We use phylogenetic comparative methods to test for correlations between the evolutionary transitions in reproduction and ecology. As predicted, outcrossing is significantly correlated with parasitism of animal hosts. We find that selfing and asexuality evolve far more readily on free-living than on animal parasitic lineages. Interestingly, outcrossing is not correlated with parasitism of plant hosts. Our results argue that the Red Queen is an explanation, at least in part, for the macroevolutionary distribution of outcrossing in nematodes (Gibson and Fuentes 2015 Evolution).

Enoplus sp.: Original drawing made by W.E. Chambers for N.A. Cobb 1915: Nematodes and their Relationships, Figure 42, p. 484. Property of Nematology Investigations, USDA, Beltsville, MD. Images provided by Zafar Handoo.

Some amazing nematodes

Pratylenchus sp, female: Original drawing made by W.E. Chambers. Figure 1 from Cobb, N.A. 1917 J Ag Res. 11(1): 27-33. Originally published as Tylenchus penetrans 


  • Bell, G. 1982. The Masterpiece of Nature: the Evolution and Genetics of Sexuality. University of California Press, Berkeley.