Selective pressures on fruit seediness: differential predation of fly larvae on the fruits of Berberis hispanica

Developing Berberis hispanica Boiss & Reuter berries are heavily attacked by a single species of fly larvae which feed on seed contents. There is a steady increase in fruit attack incidence with increasing number of seeds in the fruit, and individual developing seeds have a greater average probability of escaping predation when single in fruits (0.688) than when two (0.379) or more (0.120) are together in the same berry. With increasing fruit seediness, larval food increases and larval mortality decreases. Differential predation on differently-seeded fruits apparently results from higher larval survival in multi-seed fruits, which in turn has selected for differential oviposition among fruit classes by female flies. The proportions of differently-seeded fruits are linearly correlated with the estimated number of surviving seeds per mass unit of pulp expenditure, suggesting that B. hispanica plants are probably adjusting the distribution of seeds among fruit classes to maximize absolute number of surviving seeds through accounting simultaneously for predation risk and pulp costs. Mean number of seeds per fruit was found to be greater in a population which did not suffer fly attack, but no difference existed with regard to average pre-abortion seed number per fruit. Fly predation on fruits and seeds of B. hispanica seems therefore to influence fruit seediness through seed abortion-mediated plant responses.