Wing shape as a taxonomic trait: separating genetic variation from host-induced plasticity in aphid parasitoids

Abstract

Separating environmental effects on an organism's phenotype from genetic effects is not only of evolutionary interest, but also important in morphological taxonomy. Ideally, morphological traits to distinguish species are robust to environmental variation. In taxonomically difficult groups of insects such as aphid parasitoids (Hymenoptera: Braconidae: Aphidiinae), wing shape quantified by geometric morphometrics is increasingly used to distinguish cryptic species that are difficult to separate with other morphological characters. However, aphid parasitoids may use multiple host species; the validity of this approach thus hinges on wing shape being a genetically hard-wired trait that is not strongly moulded by the host in which the parasitoid develops. The occurrence of asexual lines in the aphid parasitoid Lysiphlebus fabarum (Marshall) provides an opportunity to test this assumption by rearing genetically identical individuals in different environments. We reared five asexual lines of L. fabarum in four different aphid species to quantify the relative importance of these factors on parasitoid wing shape. We found that the parasitoid genotype explains a much larger proportion of the observed variation in wing shape than the host species. The host also significantly affected wing shape, but to a lesser extent, and largely via an effect on size (allometry), which can be corrected for. Our study shows that even at the level of lineages within a single species, the influence of the host environment is small relative to the genetic determination of wing shape, thus validating the use of this trait for taxonomic purposes.