Histological study of galls induced by aphids on leaves of Ulmus minor: Tetraneura ulmi induces globose galls and Eriosoma ulmi induces pseudogalls

The histological study of galls may provide information on the evolution of the organisms that induce them. The walls of two aphid-induced galls on leaves of Ulmus minor were therefore studied histologically: a globose gall induced by Tetraneura ulmi and a pseudogall induced by Eriosoma ulmi. Galls are regarded as extended phenotypes of aphids, and therefore, they can be used as tools for phylogenetic studies. The walls of the galls induced by T. ulmi are not reminiscent of the ungalled leaf structure of U. minor in any area, showing both cellular hypertrophy and hyperplasia. Galls induced by E. ulmi resemble the leaf structure of U. minor in certain aspects, but in most aspects they do not, showing only cellular hypertrophy. Processes of growth and differentiation are observed in both galls. Based on the findings of this study and other recent publications, we propose to categorize the galls induced by aphids into four types: (1) pseudogalls; (2) closed galls with a “door”; (3) closed galls determined by active processes of growth and differentiation of the lamina of the leaf; and (4) closed galls determined by active processes of growth and differentiation of the midvein.