Macroevolutionary foundations of a recently-evolved innate immune defense (data)

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.