Evolution of Host-Plant Manipulation by Gall Makers: Ecological and Genetic Factors in the Solidago-eurosta System

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Abstract
Plant galls are growth deformities that feed and house the larvae of specialized insect species and other organisms (nematodes, bacteria, fungi, etc.). The benefits derived by the insect from its interaction with the host plant suggest that gall-making ability is an adaptive trait of the insect subject to modification by natural selection. Our findings, that variation in gall phenotype influences insect fitness and that gall phenotypic variation is affected by genetic variation in the insect population, support this interpretation. Our study of the goldenrod gall fly, Eurosta solidaginis, shows that gall size variation results from genetic variation among flies despite selective pressures by natural enemies. Eurosta larvae that produce small galls are vulnerable to the attack of parasitoid wasps, whereas those producing large galls are more prone to attack by avian predators. In this study population, parasitoid attack was more intense; hence, a net upward selective pressure was exerted. Quantitative-genetic methods were used in a greenhouse experiment to evaluate the contribution of insect genetic variance to phenotypic variation in gall size. Significant differences in gall diameter were found among full-sib families of gall makers. Gall dimensions were genetically correlated with one another at most developmental stages. Observations of galls growing on goldenrod clones in both the field and the greenhouse suggest that plant genotype also influences gall phenotype, and thus affects gall-maker vulnerability to natural enemies.