An experiment with Brassica rapa separated the movement of pollen between plants from successful fertilization. Fewer bumblebees widened differences in access to potential mates, while competition after pollination accounted for more variation in male reproductive success. The distinction changes how sexual selection in flowering plants is measured.
Competition among pollen donors after pollination accounted for the largest share of differences in male reproductive success in Brassica rapa, a flowering plant whose individuals perform both male and female reproductive functions. Access to potential mates made a smaller contribution. The peer-reviewed experiment in PLOS Biology separated these stages by pairing observations of bumblebee movements with genetic analysis of the resulting seeds.[1]
Researchers in Montpellier, France, established 16 populations of 10 plants each and placed one, two or four Bombus terrestris bumblebees in their cages. With fewer bees, plants differed more in the number of potential mates reached through pollen transport. Yet pollinator abundance did not significantly change either genetically identified mate numbers or total reproductive success. Selection associated with mate access was stronger in the male reproductive function than in the female function.[1]
Counting mates only from offspring genetics conflated pollen delivery with fertilization and overestimated the contribution of competition before pollination. Tracking bees supplied a separate estimate of potential contacts, although individual pollen grains were not followed. That estimate depends on how much pollen is carried between successive flower visits; the team tested alternative assumptions. Caged foraging also limits how directly these results describe natural plant communities.[1]
