Development of galls on leaves of Alnus glutinosa and Alnus incana (Betulaceae) caused by the eriophyid mite Eriophyes laevis (Nalepa)

We studied the development of adaxial pouch galls caused by Eriophyes laevis (Nalepa 1891) (Acari: Eriophyoidea) on the leaves of two European alder species, Alnus glutinosa and Alnus incana. Standard histochemical techniques were used to analyze morphology, anatomy, developmental timing, and histochemistry. Our results demonstrated that galls on both host species were similar in development of internal structure. The adaxial pouches of all galls at maturity were composed of several undifferentiated cell layers penetrated by vascular bundles and a layer lining the interior of the cavity consisting of dark-staining, presumably nutritive tissue and papillae. The abaxial ostioles opposite the pouches were constricted by the growth of labiate ostiolar protrusions. Tannins were found in bundle sheath and bundle sheath extension derivatives. Although the galls on both host species were structurally similar, they diverged in the timing of development: gall development on A. incana lagged behind that on A. glutinosa by approximately 10 d, and tanniniferous cells appeared earlier in galls on A. incana. This difference in the timing of gall development resulted from an earlier onset of gall development in A. glutinosa; the total time course of development was similar on both hosts. We hypothesize that differences in developmental phenology could lead to temporal divergence in the completion of the mite life cycle on different hosts, therefore restricting interbreeding between mite populations and creating the potential for host-specific differentiation.