Host sexual dimorphism and parasite adaptation

Abstract

In species with separate sexes, parasite prevalence and disease expression is often different between males and females. This effect has mainly been attributed to sex differences in host traits, such as immune response. Here, we make the case for how properties of the parasites themselves can also matter. Specifically, we suggest that differences between host sexes in many different traits, such as morphology and hormone levels, can impose selection on parasites. This selection can eventually lead to parasite adaptations specific to the host sex more commonly encountered, or to differential expression of parasite traits depending on which host sex they find themselves in. Parasites adapted to the sex of the host in this way can contribute to differences between males and females in disease prevalence and expression. Considering those possibilities can help shed light on host-parasite interactions, and impact epidemiological and medical science.

Figure 1. Possible outcomes of experimental tests with parasites sampled and tested in male and female hosts.

Figure 2. Parasite evolution in relation to host sexual dimorphism and likelihood of encountering the other host sex.

In red and blue are parameter combinations, which lead to monomorphic or dimorphic parasite populations, respectively. The higher the degree of host sexual dimorphism and the lower the probability of encountering the same host sex, the higher the likelihood is that a parasite will adapt specifically to its common host sex (A). When one host is different from the other, and so rare that a parasite cannot persist in it (e.g., males in a facultative sexual species like many rotifers, cladocerans, and aphids), then the parasite species may specialize entirely on the common sex (B). When one host is very different from the other in a trait important for the parasite (e.g., a primary sexual trait), then, disregarding the rate at which the opposite sex is encountered, the parasite may specialize entirely on the more suitable host (C). When males and females are very different from the parasite's point of view and the parasite encounters both sexes equally often (D), the parasite might evolve phenotypic plasticity (e.g., Wolbachia).

Figure 3. Photos of the ectoparasitic mite Spinturnix andegavinus (B) and of its host bat Myotis daubentoni (A) to which the parasite is sex specifically adapted.

Image credit: Manuel Ruedi and Philippe Christe.