First Come, First Served: Delayed Fertilization Does Not Enhance Pollen Competition in a Wind-Pollinated Tree,Fraxinus excelsiorL. (Oleaceae)

Abstract

Under natural conditions pollen vectors act stochastically, and the order in which pollen arrives can influence seed paternity. In a number of wind-pollinated taxa, however, fertilization does not occur immediately. It has been proposed that delayed fertilization promotes pollen competition by providing a “fair start” for pollen tubes. To test this hypothesis we compared paternity after simultaneous and consecutive pollinations with multiple donors in Fraxinus excelsior. Additionally, we investigated pollen tube growth in naturally pollinated flowers. Under natural conditions pollen grains arrived on the stigma one by one, and their mean number reached 6 after 10 d of anthesis. A pollen tube from the first-arriving pollen grain entered the style after 24 h and reached the base on the second day following pollination. Fertilization by nuclei from the same first-arriving pollen tube occurred after stigma necrosis about 3 wk later. Paternity tests confirmed that pollen applied first in the experimental pollinations had an overwhelming advantage over pollen applied 24 h later. As delayed fertilization does not play a role in prezygotic selection in this species, flowering phenology might be more important for paternity success than pollen tube growth performance.