PHYLOGENETIC EVIDENCE THAT APHIDS, RATHER THAN PLANTS, DETERMINE GALLMORPHOLOGY

Many diverse taxa have evolved independently the habit of living in pl ant galls. For all but some viral galls, it is unknown whether plants produce galls as a specialized plant reaction to certain types of herb ivory, or whether herbivores direct gall development. Here I present a phylogenetic analysis of gall-forming cerataphidine aphids which demo nstrates that gall morphology is extremely conservative with respect t o aphid phylogeny, but variable with respect to plant taxonomy. In add ition, the phylogeny reveals at least three host plant switches where the aphids produce galls most similar to the galls of their closest re latives, rather than galls similar to the galls of aphids already pres ent on the host plant. These results suggest that aphids determine the details of gall morphology essentially extending their phenotype to i nclude plant material. Based on this and other evidence, I suggest tha t the aphids and other galling insects manipulate latent plant develop mental programmes to produce modified atavistic plant morphologies rat her than create new forms de novo.