COS 35-7
Sexual Interference and floral trait evolution in a self-compatible flowering plant
The remarkable diversity of floral form is often attributed to how floral form affects plant success in siring and mothering seeds. For some floral characters, no single trait value maximizes both fitness components, leading to an evolutionary tug-of-war between male and female function. Trait values should then represent an evolutionary compromise between the sexual functions, since neither is free to evolve to its fitness peak. This sexual interference is thought to be widespread in hermaphroditic plants, and traits influencing separation of male and female function may be adaptations to reduce the conflict. A trait that is especially likely to influence sexual interference is timing of stigma closure. Stigma closure may prevent deposition of additional pollen, at substantial cost to female function. However rapid closure may at the same time increase siring success because it removes a physical barrier that may prevent pollen removal from anthers, or may scrape pollen off the pollinator as it exits the flower. We studied stigma closure, and its effects on both male and female pollination success in the bee-pollinated herb Mimulus ringens.
Results/Conclusions
We found that stigma closure response varied markedly and heritably among individuals (time from pollination to closure ranged from 2 minutes to >2 hours). We also found strong support for the hypothesis that the timing of stigma closure has different consequences for male and female fitness. Specifically, stigmas that stayed open longer exported less fluorescent dye to subsequently visited flowers (an index of success in siring seeds). In contrast, plants with slow-closing stigmas mothered fewer seeds per fruit than did plants with fast-closing stigmas. These results document a strong tradeoff between success through the two sexual functions, and may therefore help explain the existence of so much heritable variation in closure speeds within populations.