Documenting genetic variance for an extended phenotype of the gallmaker Eurosta solidaginis (Fitch, 1855) (Diptera: Tephritidae): A selection experiment in the field

Plant galls are thought to evolve under natural selection as extended phenotypes of the insects that induce them. An evolutionary response to selection requires that the phenotypic variance in the selected trait be caused by underlying heritable genetic variance. We used a one-generation artificial selection experiment to verify heritable variation in the size of the gall induced by the tephritid Eurosta solidaginis (Fitch, 1855) on the stems of tall goldenrod, Solidago altissima L. (Asteraceae). Previous work under greenhouse conditions demonstrated that gall size is a heritable trait of species of Eurosta Loew, 1873, but two questions remain. Foremost, is the genetic signal that is evident in a uniform, controlled environment still detectable in heterogenous natural environments? And secondarily, since galls initiated later in the season are known to grow to a smaller size, is genetic variance in gall size attributable to genetic variance in Eurosta phenology? We imposed upward and downward selection on size by populating isolated goldenrod patches with parental flies emerging from either large diameter (> 24 mm) or small diameter (< 17 mm) or galls. Galls produced by the offspring in upward-selected patches were ∼19% larger than those in downward sites. While gall size declined with later initiation dates, there was no evidence that the upward- and downward-selected sites varied in either oviposition date or gall initiation date; thus, genetic variance in gall size is not rooted in genetic variance for phenology. Our results support the notion that while the gall is plant tissue, it develops under the influence of the insect's genotype, and thus can evolve as the gallmaker's extended phenotype.