Antagonistic interactions between hosts and parasites may drive the evolution of novel host defenses, or new parasite strategies. Host immunity is therefore one of the fastest evolving traits. But where do the novel immune traits come from? Here, we test for phylogenetic conservation in a rapidly evolving immune trait – peritoneal fibrosis. Peritoneal fibrosis is a costly defense against a specialist tapeworm, Schistocephalus solidus (Cestoda), expressed in some freshwater populations of threespine stickleback fish (Gasterosteus aculeatus, Perciformes). We asked whether stickleback fibrosis is a derived species-specific trait or an ancestral immune response that was widely distributed across ray-finned fish (Actinopterygii) only to be employed by threespine stickleback against the specialist parasite. We combined literature review on peritoneal fibrosis with a comparative experiment using either parasite-specific, or non-specific, immune challenge in deliberately selected species across fish tree of life. We show that ray-finned fish are broadly, but not universally, able to induce peritoneal fibrosis when challenged with a generic stimulus (Alum adjuvant). The experimental species were, however, largely indifferent to the tapeworm antigen homogenate. Peritoneal fibrosis, thus, appears to be a common and deeply conserved fish immune response that was co-opted by stickleback to adapt to a new selective challenge.

There are two datasets used to generate the results of our article, the first is a database of published articles that form the basis of the literature review, and then the other file contains data from an experimental study of phylogenetic conservation of peritoneal fibrosis in 17 species of ray-finned fish. A detailed description of the methodology can be found in the article (https://doi.org/10.1101/2020.07.08.191601). The accompanying code is available at FigShare (https://doi.org/10.6084/m9.figshare.12619367) and Zenodo (https://doi.org/10.5281/zenodo.5137001).

All the necessary information can be found in the README.txt file in the data folder.