Induced responses in willow determine feeding success of a gall-forming insect

In the willow Salix viminalis L. (Salicaceae), growth and defence are key responses determining feeding success of the gall midge Dasineura marginemtorquens Bremi (Diptera: Cecidomyiidae). The interaction includes induced growth (gall formation), and induced defence (hypersensitive response, HR). Salix viminalis exhibits large genotypic variation in resistance (larval mortality) against D. marginemtorquens. This thesis focuses on testing hypotheses developed from field observations of interactions between the willow and the gall midge. A specific aim was to gain insight into the mechanism(s) of resistance. Combined greenhouse and field experiments were used to test whether abiotic factors induce susceptibility in otherwise resistant genotypes at the time of gall initiation. The results suggest that plasticity in plant resistance is linked to ambient conditions, probably light. Resistance was associated with HR, but to different degrees among willow genotypes. Hydrogen peroxide, a marker of HR, was induced in genotypes expressing HR but not in resistant genotypes not showing symptoms. The genetic architecture of plant traits that determine larval mortality and HR was investigated by means of a QTL analysis. The presence of one particular allele at one single locus was enough to determine life or death of the gall midge. The defence hypothesis was not supported when examining the homologous region in the poplar genome. However, an auxin gene was located within the QTL area, lending some support for the starvation and anti-manipulation hypothesis. The place of birth (leaf position) was found to be important for insect fitness, and the quality of a gall seemed to be associated with the duration of the site as a resource sink. In conclusion, data suggest that the mechanism of resistance involves anti-manipulation of growth rather than defence. Such, nonreactivity genes may have pleiotropic effects, and the frequency of genes insensitive to manipulation might be restricted in nature due to selection compared with the gall stimulus phenotype