Adaptive radiation of gall-inducing insects

Abstract

Summary The adaptive radiation of four taxa of gall-inducing insects is compared: sawflies (Hymenoptera: Tenthredinidae), oak gall wasps (Hymenoptera: Cynipidae), aphids (Homoptera: Aphididae), and gall midges (Diptera: Cecidomyiidae). The sawflies, with only three genera but many species per genus, colonized many adaptive zones. Many sibling species have been recently recognized. Such characteristics suggest a dynamic evolutionary state of radiation. Innovations in the substrates used for oviposition by sawflies, probably involving ovipositional accidents, have enabled evolution from free-feeding ancestors into diverse gall types and galling sites – from leaf edge to leaf lamina, leaf midrib, petiole, bud, and stem. Host shifting among species of willows (Salix) has been extensive and opportunistic. The oak gall wasps, like the sawflies, colonized many adaptive zones, in terms of galling sites on the plant and diverse gall types. The gall wasps include large genera, and use a geographically widespread host-plant genus rich in species, with largely opportunistic colonization of new hosts. The gall-inducing aphids on Pistacia in Israel show radiation into many adaptive zones, with different gall types and galling sites on the host plant, even though the radiation is much less extensive than in sawflies and gall wasps. A major limiting factor in this smaller radiation appears to be the low richness of Pistacia species and the host specificity of the galling aphids to this plant genus. The gall midges are similar to the sawflies and gall wasps with many large genera. Their radiation is much greater than in the related fungus-feeding midges, indicating the importance of a rich host flora for insect radiation. The study of adaptive radiation in gall-inducing insect taxa offers many opportunities for comparative studies in evolutionary biology.