Effects of Plant Aggregation, Spatial Genetic Structure, and Pollen Origin on Pollination Success of a Nursery-Pollinated Palm (Chamaerops humilis)

Premise of research. Plant aggregation and pollen origin can profoundly influence pollination success, as measured by fruit initiation and development. The effect of these variables should be more significant in plants with highly specialized pollination systems, as the pool of potential pollinators is assumed to be smaller than in plants with generalized pollination systems, although this possibility has seldom been investigated. Methodology. During two consecutive seasons, we used field observations and hand-pollination experiments in the nursery-pollinated palm Chamaerops humilis to evaluate the effects of pollen origin and the level of plant aggregation on fruit initiation (pollination success) and fruit development. Through nuclear microsatellite markers, we also examined the occurrence of C. humilis spatial genetic structure (SGS) within and between the two populations and its potential effect on pollination success. Pivotal results. Our hand-pollination experiments showed no effect of pollen origin on pollination success and fruit development, which was consistent with the detected lack of C. humilis genetic spatial structure within and between populations. Pollen addition slightly increased fruit initiation, independent of pollen origin. We found differences in fruit initiation/development at the population level. Conclusions. Our results demonstrate that populations in our study were not pollen-quality limited and that plant aggregation did not affect fruit initiation and development. Within our populations, the low fruit initiation was not correlated with the quality of pollen received by the inflorescences and may instead depend on resource allocation to the developing ovaries and competition between siblings. Density-dependent processes influence pollination success, presumably through plants' competition for resources. Long-distance pollen dispersal could compensate for low population density and limit genetic drift, thus maintaining high genetic diversity within populations.