Duplications and positive selection drive the evolution of parasitism associated gene families in the nematode Strongyloides papillosus. Developmental transcriptome of S.papillosus

Gene duplication is one of the major mechanisms that play a role in the evolution of phenotypic complexity and in the generation of novel gene functions. By comparing parasitic and nonparasitic nematodes, a recent study found that the evolution of parasitism in Strongyloididae is associated with a large accumulation of genes in the CAP and Astacin gene families. To gain insights into the development of Strongyloides papillosus, a parasite of sheep, we have sequenced RNA-seq libraries from different developmental stages. Overall, we find that most pairwise comparisons are enriched in genes that are one-to-one orthologs of S. ratti, indicating towards conservation of developmental regulation. However, comparison of parasitic vs. non-parasitic females seems to be strongly biased towards recent and ancient gene duplications. Compatible with previous analyses in other Strongyloides species, we find CAP and Astacin domain genes to be enriched in this comparison. Phylogenetic analysis of S. papillosus CAP and Astacin genes shows that the members of same gene family have distinct expression profiles in different parasitic stages. By comparing S. papillosus with S. ratti, we found that the genes with distinct expression profiles are found in paralogous clusters with signatures of positive selection. Our findings, significant expansion favored by positive selection and stage-specific expression of Astacin and CAPs, reflects the potential role of ancient and recent duplications respectively in genus-specific and species-specific adaptations necessary to parasitize vertebrate host.