The life cycles of parasitic worms, from macroevolutionary patterns to the genetics of adaptation

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Many parasites have complex life cycles in which they infect multiple hosts in succession before reproducing. These life cycles are perplexing, because they seem risky. As the number of consecutive hosts in a life cycle increases, so does the risk of not making it to the next host and thus not completing the cycle. Additionally, such parasites must adapt to different host physiologies, immune systems, and often thermal regimes (e.g. when moving from cold- to warm-blooded hosts). What are the offsetting benefits that explain the evolution and persistence of complex life cycles in multiple parasite taxa? And what genetic changes enable such a life style? To address these questions, I propose two complementary kinds of projects: (i) cross-species comparative analyses and (ii) genomic and transcriptomic comparisons. I have compiled a comprehensive database of parasitic worm life cycles that includes nearly 1000 species. I will use this database to test hypotheses about life cycle evolution, such as whether complex life cycle parasites grow larger or infect a wider variety of hosts. These analyses will identify key differences between simple- and complex-cycle parasites. I then propose exploring how such differences are manifested in parasite genomes. I will use existing datasets to look for shared genomic features in independent lineages, like gene family expansions or increases in alternative splicing, that would be indicative of general solutions to the problems posed by complex life cycles. I also describe transcriptomic experiments assessing when and to what extent parasite stages in different hosts are genetically ‘decoupled’, i.e. whether genetic changes affecting a stage in one host have correlated effects on stages in other hosts. By combining phenotypic, genomic, and transcriptomic approaches, I hope to reveal both why (the costs and benefits) and how (the genetic mechanisms of adaptation) complex life cycles have evolved.