Krameria tomentosa oil flowers and their pollinators: Bees specialized on trichome elaiophores exploit its epithelial oil glands

Abstract

Oil flowers are spread over eleven angiosperm families and exploited by bees specialized on oil gathering. This reward is produced by glandular surfaces of specific floral whorls called elaiophores. Unlike trichome elaiophores, such as those of Plantaginaceae, Krameria species bear lateral petals modified into epithelial elaiophore, convergently similar to oil-secreting Malpighiaceae calyx. Krameriaceae is an oil-offering monotypic group of eighteen species with little information on their pollination biology and interactions with oil-collecting bees. In this study, we evaluate the breeding system and describe the floral biology of Krameria tomentosa, a widespread species that occurs mainly in open vegetation habitats of northeastern Brazil. We listed its flower visitors, surveyed throughout four years, and recorded their visits by season. The pollination treatments showed that K. tomentosa is self-compatible; however, its flowers depend on pollinators to set fruits. Twenty-one Apidae visited the flowers, including the oil-bee Centris hyptidis specialized on trichome elaiophores and stingless-bees which foraged for pollen. Centris nitens and Trigona fulviventris were the most frequent visitors. K. tomentosa flowers received relatively few oil visits throughout the day and on some days no flower visitors were seen on its flowers. Nevertheless, this did not reflect on a low natural fruit set. C. hyptidis has been associated with Angelonia (Plantaginaceae) trichome elaiophores mainly due to its long forelegs and their specialized oil-collecting apparatus. Nonetheless, it exploited the K. tomentosa epithelial elaiophores in an area with no Plantaginaceae species. We presume that the association between the morphology of the oil-collecting apparatus and the elaiophore type is not enough to predict relationships in oil-systems. Additionally, we conclude that the diversity of bee visitors suggests that pollen-collecting bees also play an important role in the maintenance of K. tomentosa pollination.