Nonviable seed set enhances plant fitness: the sacrificial sibling hypothesis

Many tree species produce far more fruits than eventually mature, with a large proportion of developing fruits being aborted midway through the development process. Whether this is a maternally controlled late-acting self-incompatibility mechanism, or an expression of inbreeding depression, is difficult to determine. In either case, however, selection is expected to favor early abortion of inbred or incompatible zygotes to minimize loss of resources. In many species, this does not occur, suggesting the possibility of adaptive reasons for retaining selfed or inbred seeds that are aborted at relatively late developmental stages. We propose that such seeds serve an important function in diluting the impact of pre-dispersal seed predators by acting as seed predator sinks and thereby increasing the survival probabilities of outcrossed and fully viable seeds. We suggest that selfed seeds retained and developed through the periods of seed predator attack are effectively offered and sacrificed for the benefit of outcrossed seeds.