THE EFFECT OF STAMINODE REMOVAL OF FEMALE REPRODUCTIVE SUCCESS IN A WYOMING POPULATION OF THE ENDANGERED BLOWOUT PENSTEMON, PENSTEMON HAYDENII S. WATS. (SCROPHULARIACEAE)

Authors: HAWK, JESSICA - UNAFFLIATED,N.ORLEANS,LA; Tepedino, Vincent

Submitted to: Madrono
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/1/2007
Publication Date: 7/3/2007
Citation: Hawk, J., Tepedino, V.J. 2007. The effect of staminode removal of female reproductive success in a wyoming population of the endangered blowout penstemon, penstemon haydenii s. wats. (scrophulariaceae). Madrono. 2007. 54(1) :22-26

Interpretive Summary: We examined the effect of removing the staminode of the rare beardtongue, Blowout penstemon, P. haydenii, on female reproductive success in a south-central Wyoming population. We found no significant difference in fruit set or in seeds per fruit between flowers with and without staminodes. The most frequent flower visitor, the megachilid bee, Osmia brevis, modified its’ behavior when visiting flowers without staminodes. In intact flowers, O. brevis collected pollen by straddling the staminode and rubbing its’ head and thorax across the anthers. In flowers without staminodes, O. brevis, appeared to compensate for the increased distance to the anthers from the corolla floor by simply stretching its legs to make contact with the anthers. The smaller, native generalist sweat bee Dialictus pruinosus, which commonly collected pollen in intact flowers, appeared disoriented in flowers without staminodes, and usually left without collecting pollen. Other taxa (e.g., bumblebees, Bombus spp., the masarid wasp Pseudomasaris vespoides), appeared unaffected and plied the flowers without incident. We speculate that the staminode of Blowout Penstemon is intermittently vestigial: where O. brevis, bumblebees and P. vespoides are common, the staminode is of little consequence; where D. pruinosus is the primary pollinator, the staminode is essential to sexual reproduction by the plant.

Technical Abstract: One of the defining characteristics of beardtongues (the plant genus Penstemon) is the possession of a sterile stamen called a staminode on the floor of the flower. The purpose of this structure has been the subject of much speculation but little experimentation for many years. The main general hypothsis for the presence of the staminode is that it increases reproductive success of the flowers. We tested this by experimentally excising the staminode from a set of flowers and comparing this with a control set of flowers with staminodes present. We found no significant difference in fruit set or in seeds per fruit between flowers with and without staminodes. The most frequent flower visitor, the native bee, Osmia brevis, modified its’ behavior when visiting flowers without staminodes. In intact flowers, O. brevis collected pollen by straddling the staminode and rubbing its’ head and thorax across the anthers. In flowers without staminodes, O. brevis, appeared to compensate for the increased distance to the anthers from the corolla floor by simply stretching its legs to make contact with the anthers. The smaller, native generalist sweat bee Dialictus pruinosus, which commonly collected pollen in intact flowers, appeared disoriented in flowers without staminodes, and usually left without collecting pollen. Other taxa (e.g., bumblebees, Bombus spp., the masarid wasp Pseudomasaris vespoides), appeared unaffected and plied the flowers without incident. We speculate that the staminode of Blowout Penstemon is intermittently vestigial: where O. brevis, bumblebees and P. vespoides are common, the staminode is of little consequence; where D. pruinosus is the primary pollinator, the staminode is essential to sexual reproduction by the plant.