THE STRUCTURE OF CYNIPID OAK GALLS - PATTERNS IN THE EVOLUTION OF AN EXTENDED PHENOTYPE

Galls are highly specialized plant tissues whose development is induce d by another organism. The most complex and diverse galls are those in duced on oak trees by gallwasps (Hymenoptera: Cynipidae: Cynipini), ea ch species inducing a characteristic gall structure. Debate continues over the possible adaptive significance of gall structural traits; som e protect the gall inducer from attack by natural enemies, although th e adaptive significance of others remains undemonstrated. Several gall traits are shared by groups of oak gallwasp species. It remains unkno wn whether shared traits represent (i) limited divergence from a share d 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 evol ution of gall structure: (ii) closely related species generally induce galls of similar structure; (ii) despite this general pattern, closel y related species can induce markedly different galls; and (iii) sever al 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 hypothes es on the adaptive significance of gall structure.