Bridges and barriers to host shifts resulting from host plant genotypic variation

Abstract

Host-associated differentiation is hypothesized to be one of the primary means by which new species of herbivorous insects evolve. Divergent selection for host plant use following a host shift is a critical assumption of host-associated differentiation. Host shifts are more likely to occur between closely related host species, and the probability of a host shift and of the evolution of reproductive isolation depends on the genetic variation within both the host plant population and the herbivorous insect. We have been studying the evolution of reproductive isolation in populations of Eurosta solidaginis that form galls on Solidago altissima altissima and Solidago altissima gilvocanescens. Each of the Eurosta populations is a host race that is partially but incompletely isolated from the other host races by its adaptation to its host plants. The Solidago sp. populations show high intraspecific variation within and among populations in their resistance to attack by the host races which are adapted to them. There is also evidence that they vary in their susceptibility to host races adapted to other Solidago species. We examine the hypothesis that the intraspecific Solidago variation determines the degree of gene flow among Eurosta populations and that this can create barriers or bridges to gene flow and therefore determine the course of speciation in these insects. The intraspecific host plant variation can influence the interactions with herbivores when it is an invasive species. The restricted range of genetic variation introduced into a new region may be a good or poor match with populations of herbivores that are also introduced into the new area.