The American Naturalist 133(6): 751-765 (June 1989)
Regulation of Mate Number in Fruits of Wild Radish
Diane L. Marshall and Norman C. Ellstrand

Since the number of mates that sire a plant's seeds may have consequences for offspring success, we asked whether mate number is random or regulated in wild radish. Previous experiments show that fruits with one mate are more likely to abort and get fewer resources than fruits with multiple mates; mate number is thus regulated above one. To examine directly the relation between donor number per pollination and father number per fruit, we performed a new experiment in which donor number per pollination was increased from two to six. As donor number increased, father number per fruit leveled off. Thus, mate number per fruit was not a simple consequence of donor number. In fact, actual father number per fruit was lower than that predicted by three different estimates of expected father number per fruit. All estimates accounted for the actual distributions of seeds per fruit, and the second and third accounted for differences in the abilities of donors to sire seeds. Four- to six-donor crosses produced even fewer fathers per fruit than predicted numbers for crosses having one to three fewer donors than the actual number. Data from previous experiments and from the current one, taken together, imply that mate number in wild radish is nonrandom and that it is regulated at some intermediate number, perhaps two. Two is the modal mate number per fruit in both the field and the greenhouse. The physiological mechanisms that might produce these patterns are not understood. From an evolutionary perspective, an intermediate mate number might allow sampling an optimal number of mates or produce an advantageous level of genetic diversity among offspring.