AOBPreview originally published online on December 22, 2005
Annals of Botany 2006 97(3):345-355; doi:10.1093/aob/mcj041
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Anther Cap Retention Prevents Self-pollination by Elaterid Beetles in the South African Orchid Eulophia foliosa
1 School of Biological and Conservation Sciences, University of KwaZulu-Natal, Private Bag X01, Scottsville, 3209, South Africa and 2 Department of Botany, Rhodes University, PO Box 94, Grahamstown, 6140, South Africa (Present address)
* For correspondence. E-mail c.peter{at}ru.ac.za
Received: 6 July 2005 Returned for revision: 29 September 2005 Accepted: 8 November 2005 Published electronically: 22 December 2005
• Background and Aims Pollination by insects that spend long periods visiting many flowers on a plant may impose a higher risk of facilitated self-pollination. Orchids and asclepiads are particularly at risk as their pollen is packaged as pollinia and so can be deposited on self-stigmas en masse. Many orchids and asclepiads have adaptations to limit self-deposition of pollinia, including gradual reconfiguration of pollinaria following removal. Here an unusual mechanism—anther cap retention—that appears to prevent self-pollination in the South African orchid Eulophia foliosa is examined.
• Methods Visits to inflorescences in the field were observed and pollinators collected. Visitation rates to transplanted inflorescences were compared between a site where putative pollinators were abundant and a site where they were rare. Anther cap retention times were determined for removed pollinaria and atmospheric vapour pressure deficit was recorded concurrently. Anther cap anatomy was examined using light microscopy.
• Key Results Eulophia foliosa is pollinated almost exclusively by Cardiophorus obliquemaculatus (Elateridae) beetles, which remain on the deceptive inflorescences for on average 301 s (n = 18). The anther cap that covers the pollinarium is retained for an average of 512 s (n = 24) after pollinarium removal by beetles. In all populations measured, anther cap dimensions are greater than those of the stigmatic cavity, thus precluding the deposition of self-pollinia until after the anther cap has dropped. An anatomical investigation of this mechanism suggests that differential water loss from regions of the anther cap results in opening of the anther cap flaps. This is supported by observations that as atmospheric vapour pressure deficits increased, the duration of anther cap retention was reduced.
• Conclusions Flowers of E. foliosa are specialized for pollination by elaterid beetles. Retention of anther caps for a period exceeding average visit times by beetles to inflorescences appears to prevent facilitated self-pollination in E. foliosa effectively.
Key words: Eulophia foliosa, Orchidaceae, Elateridae, pollinarium reconfiguration, pollinia, anther cap retention, geitonogamy, anti-selfing mechanism, beetle pollination
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