Why fast-growing pollen tubes give rise to vigorous progeny: the test of a new mechanism

A positive correlation between the speed of pollen tube growth and the quality of the resulting progeny in several species of flowering plants has traditionally been explained as being caused by an overlap in gene expression between gametophytes and sporophytes. We experimentally manipulated the pollen tube growth rates of pollen donors, such that the genotype controlling the rate was uncoupled from the phenotype, to test the alternative hypothesis that the correlation arises because ovules fertilized early are better provisioned by the maternal plant than later-fertilized ovules. Crosses using Silene vulgaris individuals bearing distinctive genetic markers revealed a correlation between the order of fertilization by pollen grains and vigour of the resulting sporophytes, which was independent of the speed of growth of the pollen tubes. Seeds sired by donors with relatively fast-growing pollen germinated earlier and grew larger than those sired by slow-growing pollen when pollen from the two donors was applied simultaneously. Reversing the order of arrival in the ovary by placing slow-growing pollen on the styles earlier and closer to the ovary led to reverse results. These results suggest that differential provisioning by the maternal plant can lead to differences in progeny vigour following pollen competition.