Pollen carryover, geitonogamy, and the evolution of deceptive pollination systems in orchids

• Published in: Ecology (Durham) Vol. 80; no. 8; pp. 2607 - 2619
• Main Authors: Johnson, S. D., Nilsson, L. A.
• Format: Journal Article
• Language: English
• Published: Washington, DC Ecological Society of America 01-12-1999
• Subjects: Analysis; Animal and plant ecology; Animal, plant and microbial ecology; Apidae; Autoecology; Bending; Biological and medical sciences; Bombus; Bumblebees; deception; Dispersal (Ecology); Evaluation; floral traits; Flowers; Flowers & plants; Fruit set; Fundamental and applied biological sciences. Psychology; geitonogamy; Inflorescences; Nectar; Orchidaceae; Orchids; Orchis; Orchis mascula; Orchis morio; Palynology; Plant reproduction; Plants; Plants and fungi; Platanthera; Platanthera chlorantha; Pollen; pollen carryover; pollen limitation; Pollinating insects; Pollination; pollinators; selection; self-pollination; traits; Sweden; United States; Nectar; Monocotyledones; Insect pollination; Insecta; Interspecific comparison; Carrying behavior; Apidae; Social insect; Experimental study; Pollination; Bombus lapidarius; Orchidaceae; Animal plant relation; Apoidea; Arthropoda; Angiospermae; Spermatophyta; Hymenoptera; Invertebrata; Aculeata; geitonogamy; Orchis mascula; Platanthera chlorantha; pollination; AGGREGATA NECTAR PRODUCTION; nectar; floral traits; deception; FLORAL NECTAR; BEHAVIORAL-RESPONSES; pollen limitation; Orchis morio; pollen carryover; GE; ADAPTIVE SIGNIFICANCE
• ISSN: 0012-9658; 1939-9170
• DOI: 10.1890/0012-9658(1999)080[2607:PCGATE]2.0.CO;2

Geitonogamy (transfer of pollen among flowers on the same plant) may lead to reduced outcrossing and interfere with sex function. Orchids with pollen packaged into pollinaria would be expected to be particularly vulnerable to the loss of cross-mating opportunities imposed by geitonogamy. We tested the hypothesis that the absence of floral rewards in many orchid species is a means of reducing geitonogamy. Experiments with the deceptive species Orchis mascula and Orchis morio showed that queen bumble bees probe more flowers and stay longer on plants when artificial nectar is added to the flowers. Overall, the data indicated that the evolution of nectar production in deceptive Orchis species would result in moderate to high levels of geitonogamy, as a consequence of the greater number of flowers probed and longer visit duration (∼ 60 s) by pollinators. However, the estimated levels of geitonogamy were less than expected, due both to a time delay before freshly withdrawn pollinaria bend into the correct position to strike a stigma and to extensive carryover of pollen. The time elapsed before a freshly withdrawn pollinarium is in the correct position to strike the stigma was found to vary 30-80 s, depending on the orchid species. Since pollinators usually spend <30 s on an inflorescence, we estimate that natural populations of the study species are highly outcrossed. The fraction of the pollen load carried over from flower to flower was found to be 0.67 in O. mascula. Selection should favor longer delays in pollinaria bending and extensive pollen carryover in nectar-producing orchids. This is corroborated by the nectariferous orchid Platanthera chlorantha, which we found to have a pollinaria bending delay of 80 s and a high pollen carryover fraction (0.87). In general, selection for traits that prevent geitonogamy should occur only when pollinators are abundant. Since fruit set of orchids is usually pollinator limited, additional explanations may have to be sought to explain deception. The most plausible complementary hypothesis is that resources in pollinator-limited orchids are invested in advertising display, rather than nectar production.