The evolutionary ecology of an insect-fungus interaction: <i>Botryosphaeria dothidea</i>, symbiotic with the goldenrod-galling midge <i>Asteromyia carbonifera</i> (Diptera: Cecidomyiidae)

Abstract

Arguably, the process of adaptive ecological speciation and radiation has been the most important mechanism in contributing to the world’s immense biological diversity. Initial theoretical treatments of adaptive speciation made special mention of species interactions, namely interspecific competition, as an important source of divergent natural selection that ultimately contributed to the formation of novel ecological and species diversity. However, accumulating evidence suggests that mutualistic associations, especially those between microbes and insects, may be important in the process of adaptive speciation, as mutualists can act as sources of important phenotypic variation. In this dissertation, I investigate aspects of the interaction between the goldenrod-galling midge, <i>Asteromyia carbonifera</i>, and its fungal symbiont, <i>Botryosphaeria dothidea</i>, and attempt to relate the findings to ongoing evolutionary diversification within the species. First, a verbal theoretical treatment of how microbial mutualists could affect the process of adaptive speciation is put forth, using insects and their diverse microbial associates as examples. Second, I show by sterol-profile analysis that <i>A. carbonifera</i> appears to feed exclusively on its fungal associate, which may be important in opening nutritional ecological opportunity. Finally, I show that while the fungal associate mediates the observed phenotypic variation in gall morphology, it is the midge itself that is directly responsible for the ecologically important phenotypic variation. Moreover, the fungal associate does not exhibit the typical evolutionary signatures of a heritable mutualism. These data suggest that, for <i>Asteromyia carbonifera</i>, the fungal symbiont may indeed be a crucial player in the ongoing evolutionary radiation of its host, but the association does not appear to be a typical host-symbiont coevolutionary interaction.