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AOBPreview published online on December 22, 2005
Annals of Botany, doi:10.1093/aob/mcj041
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© The Author 2005. Published by Oxford University Press on behalf of the Annals of Botany Company. All rights reserved. For Permissions, please email: journals.permissions@oxfordjournals.org
Received July 6, 2005
Revised September 29, 2005
Accepted November 8, 2005

Article

Anther Cap Retention Prevents Self-pollination by Elaterid Beetles in the South African Orchid Eulophia foliosa

CRAIG I. PETER 1 * and STEVEN D. JOHNSON 2

1 School of Biological and Conservation Sciences, University of KwaZulu-Natal, Private Bag X01, Scottsville, 3209, South Africa; Present address: Department of Botany, Rhodes University, PO Box 94, Grahamstown, 6140, South Africa
2 School of Biological and Conservation Sciences, University of Kwazulu-Natal, Private Bag X01, Scottsville, 3209, South Africa

* To whom correspondence should be addressed.
CRAIG I. PETER, E-mail: c.peter{at}ru.ac.za


  Abstract

• 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.

Keywords: Eulophia foliosa, Orchidaceae, Elateridae, pollinarium reconfiguration, pollinia, anther cap retention, geitonogamy, anti-selfing mechanism, beetle pollination.
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