Transmission modes and evolution of the parasitism-mutualism continuum.

An analysis of fitness costs and benefits associated with pathogenicity suggests that modes of transmission are key determinants of evolution toward severely pathogenic, benign, or mutualistic symbioses. Specifically, this approach suggests that symbionts with mobile life history stages should evolve toward extremely severe parasitism, vector-borne symbionts should evolve toward severe parasitism in vertebrate hosts and benign parasitism in the vectors, waterborne symbionts should evolve toward severe parasitism, symbionts transmitted by predation should evolve toward severe parasitism in prey hosts and benign parasitism in predator hosts, and vertically transmitted symbionts should evolve toward benign parasitism and mutualism. Detailed reviews of the literature on human diseases support the hypothesized severity of vector-borne and waterborne transmission. Evaluation of the other associations is less detailed, but each association appears to be present. This framework draws attention to the need for detailed reviews of relationships between transmission modes and the nature of symbiotic interactions, and experimental manipulations of transmission.