A geographic mosaic of coevolution between Eurosta solidaginis (Fitch) and its host plant tall goldenrod Solidago altissima (L.)

A geographic mosaic of coevolution has produced local reciprocal
adaptation in tall goldenrod, Solidago altissima (L.), and the goldenrod
ball gall fly, Eurosta solidaginis (Fitch 1855). The fly is selected to
induce gall diameters that minimize mortality from natural enemies, and
the plant is selected to limit gall growth that reduces plant fitness. We
conducted a double reciprocal transplant experiment where S. altissima and
E. solidaginis from three sites were grown in gardens at each site to
partition the gall morphology variation into fly genotype, plant genotype,
and the environment components. The host plant gall diameter induced by
each E. solidaginis population was adapted to inhibit local natural
enemies from ovipositing on or consuming enclosed larvae. Reciprocally,
increasing the gall size induced by the local fly population increased the
resistance of the local plant host population to gall growth. Differences
among sites in natural enemies produced a mosaic of hotspots of
coevolutionary arms races between flies selecting for greater gall
diameter and plants for smaller diameters, and coldspots where there is no
selection on plant or fly for a change in gall diameter. In contrast, the
geographic variations of gall length and gall shape were not due to
coevolutionary interactions.