Weak sinks could cradle mutualistic symbioses – strong sources should harbour parasitic symbioses

Abstract

Using a population model of selection on an obligate symbiont and its host, we evaluate how demographic differences across geographical landscapes can produce selection mosaics in interacting species. The model assumes that the host populations vary geographically from demographic sources to sinks in the absence of effects by the symbionts, and that a virulent and a relatively avirulent form of the symbiont compete with one another across all habitats. Our results indicate that productivity gradients can create selection mosaics across habitats, resulting in complex fitness landscapes over which evolution occurs. We find that relatively virulent symbionts only persist if they have an advantage over avirulent strains or species in terms of interference (i.e. competition, and/or cross‐transmission) interactions. When such a trade‐off exists, we predict that the more virulent symbiont is most likely to be found in habitats where host population growth is highest, whereas the more avirulent symbiont should tend to persist in more marginal habitats or even habitat sinks for symbiont‐free hosts. Demographic sinks may be the habitats most likely to favour the origin of new mutualisms. Very productive mutualisms can be exploited by hyperparasites or cheaters. We discuss our findings in terms of geographical scenarios for the emergence of mutualisms, and the long‐standing debate about geographical patterns in the maintenance of sex.

Diet–dependent effects of gut bacteria on their insect host: the symbiosis ofErwiniasp. and western flower thrips

“…Thus, the symbionts seem to benefit their host most especially in poor environments. This is exactly what is predicted from theoretical models of the evolution of mutualism and parasitism on a gradient of habitat qualities (Hochberg et al 2000;Hochberg & Holt 2002).…”

Evolutionary signals of symbiotic persistence in the legume–rhizobia mutualism

“…However, we were able to extract three soil properties for our global legume occurrences to test the idea that stable fixing is associated with lower quality soils. We expected to find increased symbiotic persistence when environmental nitrogen availability, a function of soil pH, clay content, and nitrogen content (71), was lower because hosts theoretically derive more benefit from symbiosis in low-quality environments (72)(73)(74). Surprisingly, we found no significant relationship between soil variables and symbiotic persistence.…”

Haplodiploidy as an Outcome of Coevolution Between Male-Killing Cytoplasmic Elements and Their Hosts

“…However, nutritionally marginal habitats such as woody plant stems may impose particularly stringent selection for efficiency of mutualism (Hochberg et al 2000). Therefore, it should not be too surprising if this environment has given rise on multiple occasions to a peculiarly benign mode of male-killing: a killing method (destruction of the paternal genome) that minimally harms (and can ultimately benefit) the matriline upon whose fitness the fitness of the endosymbiont most intimately depends.…”