SEED PREDATION, PATHOGEN INFECTION AND LIFE-HISTORY TRAITS IN BRASSICA-RAPA

Herbivory and disease can shape the evolution of plant populations, bu t their joint effects are rarely investigated. Families of plants of B rassica rapa (Brassicaceae) were grown from seeds collected in two nat uralized populations in an experimental garden. We examined leaf infec tion by the fungus Alternaria, seed predation by a gall midge (Cecidom yiidae) and plant life-history traits. Plants from one population had heavier seeds, were more likely to flower, had less fungal infection, had more seed predation and were more fecund. Fungal infection score a nd seed predation rate increased with plant size, but large plants sti ll had the greatest number of undamaged fruits. Spatial heterogeneity in the experimental garden was significant; seed predation rate and fe cundity varied among blocks. An apparent tradeoff existed between susc eptibility to disease and seed predation: plants with the highest fung al infection score had the lowest seed predation rate. Alternaria infe ction varied between populations, but the disease had no effect on fec undity. Seed predation did reduce fecundity. Damaged fruits had 31.4% fewer intact seeds. However, evidence for additive genetic variation i n resistance to seed predation was weak. Therefore, neither disease no r seed predation was likely to be a strong agent of genetically based fecundity selection.