GENETIC-VARIATION IN RESISTANCE OF HYBRID WILLOWS TO HERBIVORES

We measured herbivore abundance on two species of willows (Salix seric ea and S. eriocephala) and their interspecific hybrids in two common g arden experiments to test alternative hypotheses concerning the geneti c effects of hybridization on plant resistance. The first experiment u sed potted clones of naturally occurring parental and hybrid plants. P urity of parental plants was determined using 20 RAPD markers (pure S. eriocephala had a score of 1.0 and pure S. sericea had a score of 0). The cloned hybrid plants had hybrid scores between 0.40 and 0.60, ind icating they were intermediate hybrids and probably had recombinant (F -2-type) genotypes. The second experiment used potted one-year-old int erspecific F-1 progeny and intraspecific progeny from crosses between genetically pure parents. We counted numbers of herbivores on plants a fter exposure to attack for about 2 months. Herbivore abundance data w ere analyzed with ANOVA and a priori contrasts were used to test fit o f the data to 5 hypotheses (No Difference, Additive, Dominance, Hybrid Susceptibility, or Hybrid Resistance). We found genetic effects of hy bridization on susceptibility to some herbivores in both experiments, with the additive, dominance, and hybrid susceptibility hypotheses sup ported by different herbivore species. There was also significant gene tic variation among clones and genotypes within hybrid and parental ta xa for herbivore resistance. For several herbivore species we found no difference in the susceptibility among hybrids and parents, which ind icates genetic effects of hybridization on resistance were absent for these species. Differences between the hypotheses supported from this study and data from four years of prior field studies suggest that env ironmental variation is an important factor affecting relative hybrid and parental susceptibility in the field.