Sexual Selection in Raphanus sativus: Experimental Data on Nonrandom Fertilization, Maternal Choice, and Consequences of Multiple Paternity

Abstract

We determined the paternity of seeds in the multiseeded fruits of Raphanus sativus in order to measure the potential mechanisms of sexual selection after experimental pollination with mixed pollen loads. We also compared singly and multiply sired fruits. We found effects of pollen-donor identity on the number of ovules fertilized, the position of ovules fertilized, the fruit set, seeds per fruit, and seed weight. One pollen donor was inferior by all measures and had the slowest pollen germination. The effect of pollen-donor identity may be attributed to male-male competition because it is uniform over the maternal plants, or the effect may be caused by all females "choosing" in the same way. Another effect, that multiply sired fruits have a greater total weight of seeds per fruit than do fruits sired singly by any of the pollen donors, appears to be a case of maternal choice. Comparison of the seed sizes of two of the pollen donors suggests that competition between seeds for resources may be more intense in multiply sired fruits; however, seeds sired by another pollen donor were larger in multiply sired fruits than in singly sired fruits. Microgametophytic competition was not evident when the amount of pollen from a single donor was varied. Thus, in R. sativus, the consequences of microgametophytic competition among several pollen donors may be far more profound than competition among the male gametophytes produced by one pollen donor.