AOBPreview originally published online on October 4, 2009
Annals of Botany 2009 104(7):1281-1291; doi:10.1093/aob/mcp238
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Adaptive significance and ontogenetic variability of the waxy zone in Nepenthes rafflesiana
1 UMR CNRS 5120 AMAP: botAnique et bioinforMatique de l'Architecture des Plantes, CIRAD–TA A51/PS2 Boulevard de la Lironde, F-34398 Montpellier cedex 5, France
2 Universiti Brunei Darussalam, Biology Department, Jalan Tungku Link, Gadong Be 1410, Brunei Darussalam
* For correspondence. E-mail laurence.gaume{at}cirad.fr
Received: 24 February 2009 Returned for revision: 5 June 2009 Accepted: 11 August 2009 Published electronically: 4 October 2009
Background and Aims: The slippery waxy zone in the upper part of pitchers has long been considered the key trapping structure of the Nepenthes carnivorous plants; however, the presence of wax is reported to be variable within and between species of this species-rich genus. This study raises the question of the adaptive significance of the waxy zone and investigates the basis for an ontogenetic cause of its variability and correlation with pitcher shape.
Methods: In Brunei (Borneo) the expression of the waxy zone throughout plant ontogeny was studied in two taxa of the Nepenthes rafflesiana complex, typica and elongata, which differ in pitcher shape and size. We also tested the adaptive significance of this zone by comparing the trapping efficiency and the number of prey captured of wax-bearing and wax-lacking plants.
Key Results: In elongata, the waxy zone is always well expanded and the elongated pitchers change little in form during plant development. Wax efficiently traps experimental ants but the number of captured prey in pitchers is low. In contrast, in typica, the waxy zone is reduced in successively produced pitchers until it is lost at the end of the plant's juvenile stage. The form of pitchers thus changes continuously throughout plant ontogeny, from elongated to ovoid. In typica, the number of captured prey is greater, but the role of wax in trapping is minor compared with that of the digestive liquid, and waxy plants do not show a higher insect retention and prey abundance as compared with non-waxy plants.
Conclusions: The waxy zone is not always a key trapping structure in Nepenthes and can be lost when supplanted by more efficient features. This study points out how pitcher structure is submitted to selection, and that evolutionary changes in developmental mechanisms could play a role in the morphological diversity of Nepenthes.
Key words: Carnivorous plant, developmental evolution, digestive liquid, epicuticular wax, insect trapping, heteroblasty, heterochrony, leaf form, morphological diversity, Nepenthes rafflesiana, ontogenetic change, pitcher plant