Patterns of Adaptive Radiation and Diversification in Cecidogenous Insects

Selection drives speciation but adaptive radiation triggers that selection by developing 'new' behaviours in organisms, which get established in their genetic constructs over time; consequently, a new species eventuates. Within the broad context of insect phytophagy, the capability to induce galls is a highly specialized and sophisticated behaviour evident only in particular taxa within specific groups of the Insecta. The most striking behaviour among gall-inducing insects is that a majority of them are specialists displaying a high level of fidelity to their host plants. Adaptive radiation among gall-inducing insects through host shifts resulting in speciation is directed to a large extent through host-plant traits and competitive interactions of the gall inducer in selecting the 'best' host resources, and to a lesser extent through windows of escape from natural enemies. Gall-inducing insects demonstrate a high degree of orientation to specific trophic niches. Close to 90% of them are host specific, displaying a high level of fidelity to particular species of plants. More importantly, gall-inducing insects are specific to particular plant organs, i.e., a leaf-gall inducing insect will induce galls only on leaves. In this article, this concept has been discussed within the overall context of patterns of adaptive radiation and biodiversification among gall-inducing insects of the world, and the apparent patterns in the gall-inducing insects of the Indian subcontinent, with a specific case analysis of the Cecidomyiidae that infest species of Indian Anacardiaceae. The case in point, viz., the Anacardiaceae-infesting Indian Cecidomyiidae, show weak patterns of adaptive radiation. The possible absence of resistance-breaking genes in Indian Cecidomyiidae explains why these organisms have not radiated and diversified as aggressively as the gall-inducing Eulophidae (Hymenoptera) have on different taxa of Eucalyptus introduced for purposes of commercialplantations into several exotic localities, from the Middle-east to India. Alternatively, the host-plant populations are restricting the gene flow between specific gall-midge populations, through their secondary chemistry because, the host-plant mediated impediments on the breeding behaviours impact on the radiation of gall-inducing insects. In conclusion, a majority of gall-inducing insects of the Indian subcontinent are implicated to show features of conservative diversification; the little known, established examples among the Indian gall-inducing taxa indicate that radiation is strongly mediated by the radiation and diversification that occur in their host plants.