Nectarless flowers with deep corolla tubes inPedicularis: does long pistil length provide an arena for male competition?

Abstract

Pollinator-mediated selection does not seem to have a direct influence on the evolution of a long corolla tube in a nectarless flower. We hypothesized that the long pistil length of the nectarless flower with a deep corolla tube provided an opportunity for male competition. Pedicularis siphonantha, a nectarless and partially self-incompatible lousewort with substantial variation in corolla tube length, was used to test the hypothesis. We tested whether and how corolla tube length affected seed production per capsule and seed germination rate with different pollination treatments. Flowers were hand-pollinated with pollen from one self donor and one outcross donor and mixed pollen grains consisting of equal amounts from the two donor types, respectively. Additionally, seeds from open-pollinated flowers with different corolla tube lengths were collected separately for measurement of germination rate. Pollination treatment and corolla tube length significantly affected number of seeds per capsule. Moreover, a significant positive relationship between seeds per capsule and corolla tube length was found when mixed hand pollination was conducted. Seeds of self hand-pollinated flowers had a lower germination rate than those from outcross-pollinated flowers. Under open pollination, seeds from flowers with longer corolla tubes tended to have higher germination rate. In P. siphonantha, outcross pollen may have a higher probability of contributing to the next generation when transferred to flowers with longer corolla tubes. The pistil length, therefore, should be seen as a female choice mechanism, which provides an arena for male-to-male competition. © 2015 The Linnean Society of London, Botanical Journal of the Linnean Society, 2015, 179, 526–532.