Proximal Causes of Multiple Paternity in Wild Radish, Raphanus sativus

Abstract

We compared the frequencies of multiply and singly sired fruits produced from simultaneous and sequential depositions of different pollen types in order to examine the mechanisms by which high frequencies of multiple paternity are produced in wild radish, Raphanus sativus. We found that most multiply sired fruits resulted from simultaneous pollen deposition (pollen carryover) and that the first pollen donor in a series of sequential pollen applications sires most seeds. Frequent sequential pollinations could produce multiply sired fruits, but insect visits to individual flowers in the field occur only at long intervals. We conclude, therefore, that multiple paternity in the field is a result of pollen carryover. Additional data on the sequence of seeds by father within field-collected fruits and on the number of fathers per fruit confirm this conclusion. Given the high frequency of multiply sired fruits in the field, most insects must carry pollen from several pollen sources. Our results suggest that there are frequent opportunities for competition among the pollen tubes from different pollen donors. Moreover, since the first pollen load appears to fertilize most seeds, maternal choice among potential mates is likely to occur primarily as post-pollination discrimination among pollen grains, zygotes, or fruits.