The structure of cynipid oak galls: patterns in the evolution of an extended phenotype

Galls are highly specialized plant tissues whose development is induced by another organism. The most complex and diverse galls are those induced on oak trees by gallwasps (Hymenoptera: Cynipidae: Cynipini), each species inducing a characteristic gall structure. Debate continues over the possible adaptive signi¢cance of gall structural traits; some protect the gall inducer from attack by natural enemies, although the adaptive signi¢cance of others remains undemonstrated. Several gall traits are shared by groups of oak gallwasp species. It remains unknown whether shared traits represent (i) limited divergence from a shared ancestral gall form, or (ii) multiple cases of independent evolution. Here we map gall character states onto a molecular phylogeny of the oak cynipid genus Andricus, and demonstrate three features of the evolution of gall structure: (i) closely related species generally induce galls of similar structure; (ii) despite this general pattern, closely related species can induce markedly di¡erent galls; and (iii) several gall traits (the presence of many larval chambers in a single gall structure, surface resins, surface spines and internal air spaces) of demonstrated or suggested adaptive value to the gallwasp have evolved repeatedly.We discuss these results in the light of existing hypotheses on the adaptive signi¢cance of gall structure.