Evolutionary Ecology of Complex Life Cycle Parasites

Louhi, Katja-Riikka Evolutionary ecology of complex life cycle parasites from genotypes to species assemblages Jyväskylä: University of Jyväskylä, 2012, 42 p. (Jyväskylä Studies in Biological and Environmental Science ISSN 1456-9701; 240) ISBN 978-951-39-4747-7 (nid.) ISBN 978-951-39-4748-4 (PDF) Yhteenveto: Imumatoloisten evolutiivinen ekologia – genotyypeistä lajiyhteisöihin Diss. Among the main aims in evolutionary ecology is to understand the organization of biological diversity in space and time as well as to explain why organisms vary in their life-history traits. Studying these aspects of biology in parasites furthers our understanding of between-species interactions and evolution of transmission strategies. In this thesis, I examined the genetic structure and community organization of Diplostomum (Trematoda: Digenea) parasites as well as variation in their transmission traits. These parasites have a complex life cycle (CLC) that includes snail, fish and bird hosts. I found that the species D. pseudospathaceum has no population genetic structure and exhibited high levels of neutral genetic diversity and gene flow among populations of its snail hosts, suggesting effective dissemination of parasite genotypes by the final bird hosts. Multiple genotype infections (MGI) of D. pseudospathaceum were common and aggregated in snail populations and their frequencies correlated with infection prevalence. Similar pattern of genotype distribution was true also in other aquatic snail-trematode systems that I reviewed from the literature. Multiple Diplostomum-species infections were common in the eye lenses of five freshwater fish species as revealed by pyrosequencing of pooled DNA samples. The fish species also harbored significantly different Diplostomum-species assemblages. Furthermore, I found genotypic and phenotypic variation in transmission traits of clonal stages of D. pseudospathaceum indicating either an adaptive ‘bet hedging’ –strategy or phenotypic plasticity in trait expression over time. These findings suggest that populations of CLC parasites maintain high levels of neutral genetic variation and probably also genetic variation in life-history traits, which may be a benefit in unpredictable environments. The novel analytical methods described in this thesis broaden our understanding of the causes and consequences of multiple genotype and multiple species infections for the evolutionary ecology of CLC parasites.