PERFECTION SUBVERTED? A CONTRIVANCE FOR OUTCROSSING IN A RARE ORCHID IS INFLUENCED BY POLLINATOR ABUNDANCE

Authors: SIPES, SEDONIA - SO. ILL. UNIV; Tepedino, Vincent

Submitted to: The Journal of the Torrey Botanical Society
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/24/2006
Publication Date: 11/1/2006
Citation: Sipes, S.D., Tepedino, V.J. 2006. Perfection subverted? A contrivance for outcrossing in a rare orchid is influenced by pollinator abundance. The Journal of the Torrey Botanical Society. 133(3):412-420.

Interpretive Summary: Flowers of most plants are bisexual, i.e., both male and female. Many flowering plants have specialized mechanisms to promote cross-pollination, i.e., to separate the time of the maturation of male (pollen presentation) and female (stigma receptivity) function within a flower. Such separation in the time of potency of the sexes decreases the likelihood of self-pollination. Our subject in this report is Ute ladies' tresses, a rare orchid listed as threatened under the Endangered Species Act. Ute ladies' tresses is a typical orchid in that it is self-compatible, i.e., self pollen can fertilize ovules when they reach the stigma. It is usually prevented from doing so by the morphology of the flower and by the foraging behavior of bees which move in a direction (upward on the flowering stalk) which renders self-pollination unlikely. In Ute ladies' tresses, self-pollination becomes possible when pollinator visitation rate is low because pollen remains viable for long periods of time, and morphological developments of the flower make it possible for that pollen to be picked up and deposited on other female flowers higher up on the flower stalk. This is the first demonstration of selfing which is mediated by the number of pollinators. It is particularly important for this species because of its rarity and because selfing has the potential to erode genetic variability and adaptibility of plants through inbreeding.

Technical Abstract: The combination of protandrous flowers and acropetal inflorescence development in bee-pollinated species is thought to maximize cross-pollination because bees visit vertical inflorescences from the bottom up. However, incomplete protandry may allow bees to carry out geitonogamous pollinations. We examined the overlap in male and female phases in the rare orchid Spiranthes diluvialis, a plant with the above combination of characteristics. We found that unvisited male phase flowers proceed to a hermaphroditic phase, not a female phase, because each flower's single pollinarium remains viable and may be removed by pollinators throughout anthesis. Pollinator visitation rates, as estimated by pollinaria removal rates, varied among five populations in Utah and Colorado, USA. More hermaphroditic phase flowers accrued on inflorescences in populations with low visitation rates than in those with higher visitation rates. We conclude that the cross-pollination mechanism of S. diluvialis requires some minimum threshold of bee visits in order to work optimally. When bees are plentiful, male and female functions remain temporally separated and crosspollination is maximized. In contrast, the potential for geitonogamy in this self-compatible species is much higher in populations with low visitation rates. We suggest that pollinator abundance may affect the mating system of other protandrous, acropetal, bee-pollinated plants if protandry depends upon the timely removal of long-lived pollen.