An exception to Darwin's syndrome: floral position, protogyny, and insect visitation in Besseya bullii (Scrophulariaceae)

• Published in: Oecologia 1995, Vol.101 (1), p.68-74
• Main Authors: McKone, M.J. (Carleton Coll., Northfield, MN (USA). Dept. of Biology), Ostertag, R, Rauscher, J.T, Heiser, D.A, Russell, F.L
• Format: Article
• Language: English
• Subjects: ALOGAMIA; ANATOMIA DE LA PLANTA; ANATOMIE VEGETALE; Animal and plant ecology; Animal, plant and microbial ecology; Anthers; APIDAE; Autoecology; Biological and medical sciences; Bumblebees; BUSQUEDA DE ALIMENTO; COMPORTEMENT ALIMENTAIRE; Flower stigma; Flowers; Fundamental and applied biological sciences. Psychology; General aspects; HABITOS ALIMENTARIOS; Halictidae; Hymenoptera; INFLORESCENCE; Inflorescences; INFLORESCENCIAS; Insect behavior; Insect pollination; OUTBREEDING; Plant reproduction; Plants; POLINIZACION; POLINIZADORES; Pollen; POLLINISATEUR; POLLINISATION; RECHERCHE DE NOURRITURE; SCROPHULARIACEAE
• ISSN: 0029-8549; 1432-1939
• DOI: 10.1007/BF00328902

Darwin pointed out that plants with vertical inflorescences are likely to be outcrossed if the inflorescence is acropetalous (flowers from the bottom up), the flowers are protandrous (pollen is dispersed before stigmas are receptive), and pollinators move upward on the inflorescence. This syndrome is common in species pollinated by bees and flies, and very few exceptions are known. Flowering phenology and pollinator behavior was investigated in Besseya bullii (Scrophulariaceae) and found that it did not fit Darwin's syndrome. The vertical inflorescence was acropetalous but the flowers were distinctly protogynous, so flowers with newly receptive stigmas appeared on the inflorescence above those with dehiscing anthers. A number of small insects visited B. bullii; bees of the family Halictidae (Augochlorella striata and Dialictus spp.) were most common. When insects moved between gender phases within inflorescences, they moved up more often than down (61%, versus 39% of observations, respectively) but this difference was only marginally significant. Most visits were to male-phase flowers only, and this preference was more pronounced for pollen-foraging insects than for nectar-foraging insects. B. bullii was self-compatible, so its flowering characteristics potentially could result in considerable self-pollination. However, an average of 38% of the lowermost flowers opened before any pollen was available on the same inflorescence; these "solo females" had a high probability of outcrossing (though fruit set was relatively low in the bottom portion of the inflorescence).