Within-host competitive interactions as a mechanism for the maintenance of parasite diversity

Abstract

Variation among parasite strains can affect the progression of disease or the effectiveness of treatment. What maintains parasite diversity? Here I argue that competition among parasites within the host is a major cause of variation among parasites. The competitive environment within the host can vary depending on the parasite genotypes present. For example, parasite strategies that target specific competitors, such as bacteriocins, are dependent on the presence and susceptibility of those competitors for success. Accordingly, which parasite traits are favoured by within-host selection can vary from host to host. Given the fluctuating fitness landscape across hosts, genotype by genotype (G×G) interactions among parasites should be prevalent. Moreover, selection should vary in a frequency-dependent manner, as attacking genotypes select for resistance and genotypes producing public goods select for cheaters. I review competitive coexistence theory with regard to parasites and highlight a few key examples where within-host competition promotes diversity. Finally, I discuss how within-host competition affects host health and our ability to successfully treat infectious diseases.

Keywords: G×G interactions; bacteriocins; coexistence; parasite diversity; public goods; within-host competition.

Figure 1.

Mechanisms of within-host competition. Each large circle represents a host individual either infected with parasite A (smaller grey circles) alone, parasite B (smaller white circles) alone or simultaneously infected with both parasites. The relative growth rate of each parasite is indicated by the number of parasites present within each host individual. (a) When parasites compete through faster or more efficient exploitation of host resources, within-host selection favours parasites with superior exploitation (here, parasite A). (b) If parasites achieve greater host exploitation through the release of public goods (smallest black circles), then within-host selection favours parasites that produce less of the public good (here, parasite B). (c) When parasites compete over access to host infection sites, the parasite that occupies a site first (here, parasite A) can interfere with the attachment of later arriving parasites. (d) When parasites compete through the production of alleopathic agents (black lightning bolts), within-host selection favours parasites that produce costly competitive toxins (here, parasite A). (e) When parasites compete via the host immune systems, parasites who benefit when infecting alone by being resistant to the host immune response they elicit (white lightning bolts) may also benefit via within-host selection by eliminating immune-sensitive competitors.