AOBPreview originally published online on December 13, 2004
Annals of Botany 2005 95(3):389-399; doi:10.1093/aob/mci040
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Annals of Botany 95/3 © Annals of Botany Company 2004; all rights reserved
Heteromorphic Incompatibility and Efficiency of Pollination in Two Distylous Pentanisia Species (Rubiaceae)
1 School of Botany and Zoology, University of Kwazulu-Natal, Private Bag X01, Scottsville, Pietermaritzburg, 3209, South Africa and 2 Department of Biological Sciences, University of Calgary, Calgary, Alberta, Canada T2N 1N4
* For correspondence. E-mail johnsonsd{at}ukzn.ac.za
Received: 7 June 2004 Returned for revision: 29 July 2004 Accepted: 12 October 2004 Published electronically: 13 December 2004
• Background and Aims Distyly has been hypothesized to promote cross-pollination by reducing intrafloral and geitonogamous self-pollination, and enhancing intermorph pollination. Distylous plants typically display both reciprocal herkogamy and a heteromorphic incompatibility system, which allows mating only between morphs. Distyly and its pollination consequences were examined in two Pentanisia species with long-tubed flowers which are pollinated almost exclusively by butterflies.
• Methods Anther and stigma heights were measured to quantify reciprocal herkogamy. The type of incompatibility system was determined by observing pollen tubes and seed production following controlled hand pollination. Pollen loads on pollinators and stigmas were also examined to assess the efficiency of intermorph pollen flow.
• Key Results Pentanisia prunelloides and P. angustifolia exhibit reciprocal herkogamy and a host of ancillary dimorphisms, including pollen colour, exine sculpturing, stigmatic papilla shape and floral-tube pubescence. Controlled hand-pollinations revealed the presence of a strong heteromorphic incompatibility system in both species. The site of incompatibility differed between the morphs; intramorph pollen tubes were blocked in the style of the short-styled morph and on the stigmatic surface of the long-styled morph. Butterflies carried pollen from the short- and long-styled morphs primarily on their head and proboscis, respectively. Natural pollination resulted in a higher proportion of pollen transfer from long- to short-styled plants than vice versa. Nevertheless, fruit set did not differ between morphs.
• Conclusions Both Pentanisia species are fully distylous. Reciprocal herkogamy results in pollen from the two morphs being carried on different locations on pollinators' bodies, which in turn promotes intermorph pollination. Intramorph pollination does not result in fertilization, because of an effective heteromorphic incompatibility system.
Key words: Butterfly pollination, distyly, pollen colour dimorphism, pollination, Pentanisia, reciprocal herkogamy, heteromorphic incompatibility, southern Africa, Rubiaceae
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