Resistance to pre-dispersal seed predators in a natural hybrid zone.

Plant hybrids can be more, less, or equally resistant to herbivores compared to their parental species. These patterns in resistance can be critical determinants of the fitness of plant hybrids and may also influence distribution of the herbivore. We examined resistance to a pre-dispersal seed predator by natural and experimental hybrids between Ipomopsis aggregata and I. tenuituba. These species and their hybrid offspring differed primarily in ability to avoid oviposition by Hylemya sp. (Diptera: Anthomyiidae) rather than in reducing damage to seeds by a developing larva. Plants of I. tenuituba had the lowest frequency of fly eggs and were thus the most successful at avoiding damage. Hybrids were either intermediate to or less resistant than both parental species. Because these patterns persisted in experimental arrays of interspersed potted plants, they cannot be attributed to ongoing differences in the environment between hybrid and parental sites. In experimental arrays, the frequency of fly eggs correlated positively with corolla width, a dimension of flower size that also influences the rate of pollination, suggesting seed predators can generate selection on reproductive traits of hybrids. Furthermore, in one of the arrays, oviposition on F2 hybrids exceeded the average for the F1 and the midparent. Our results underscore the need to consider genetic background of hybrids in assessing plant responses to herbivores.