Viability selection at three early life stages of the tropical tree,Platypodium elegans (Fabaceae,Papilionoideae)

Given the enormous number and high mortality of fertilized ovules in plants, it is possible that selection during the earliest stages of the life cycle plays an important role in shaping the genetic composition of plant populations. Previous research involving selection component analyses found strong evidence for viability selection in annual plant species. Yet despite this evidence, few attempts have been made to identify the magnitude and timing of viability selection as well as the mechanisms responsible for mortality among genotypes. Platypodium elegans, a Neotropical tree with high rates of early fruit mortality, represents an opportunity to study viability selection at a level of discernment not previously possible. Microsatellite markers were used to analyze the genetic composition of aborted embryos, as well as mature seeds and seedlings of the same cohort. While selection resulted in an overall decrease in self-fertilized progeny across each life stage, the greatest change in the genetic composition of progeny occurred between mature seeds and established seedlings. This suggested that inbreeding depression, and not late-acting self-incompatibility, was responsible for early selection. An investigation of the mature seed stage revealed that self-fertilized seeds weigh significantly less than outcrossed seeds. The result of this early selection conceals the mixed-mating system and high levels of inbreeding depression in Platypodium elegans, resulting in an apparently outcrossed adult population that does not differ significantly from Hardy-Weinberg expectations.