Directional evolution of parasitoid strategies and contingent host shifts shape the radiation of Darwin wasps (Ichneumonidae).

Evolution of parasitoid strategies in Darwin wasps (Ichneumonidae) is strongly asymmetric across 697 species, with repeated idiobiosis→koinobiosis and ecto→endoparasitoid transitions but virtually no reversals.

• Why it matters: The study tests whether evolutionary “ratchets” constrain future trajectories, and whether interacting traits (strategy and host use) show different macroevolutionary dynamics.
• What they did: Using a phylogenomic dataset of 697 ichneumonid species spanning all subfamilies, the authors reconstructed transitions among parasitoid strategies and inferred ancestral and shifted host associations across holometabolous insect orders.
• The result: Lepidoptera were inferred as ancestral hosts, yet repeated host shifts occurred across nearly all holometabolous orders; shifts were predominantly in idiobiont lineages and became much rarer after transitions to koinobiosis.

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