Comparative flower structure of three species of sundew (Drosera anglica,Drosera linearis, andDrosera rotundifolia) in relation to breeding system

Abstract

Whereas much attention has been given to the fascinating prey-trapping leaves of carnivorous plants, less research has been conducted on their flower structure and breeding systems. Accordingly, a comparative study of the floral morphology and anatomy of the three species of sundews (Droseraceae: Drosera anglica Huds., Drosera linearis Goldie, and Drosera rotundifolia L.) in Saskatchewan was performed to ascertain the presence of floral rewards for potential pollinators and to obtain pollen to ovule ratios, an indicator of breeding system. Utilizing light and scanning electron microscopy, differences between the three species were apparent in length of styles, number of placentas, anther and pollen colour, and structure of glandular trichomes on sepals. The occurrence of features unique to each species does not support the concept of D. anglica as a hybrid of the other two species. Flowers of all three species lack nectaries, although clusters of papillate cells that were reminiscent of secretory tissue were observed at the apices of anthers and at the summits of ovaries. Pollen to ovule ratios were low for all species, ranging from 9.0 to 18.7 in D. rotundifolia and D. linearis, respectively, suggesting an autogamous breeding system for each species.Résumé : Alors qu'on a accordé beaucoup d'attention aux fascinantes feuilles prédatrices des plantes carnivores, peu de recherches ont été conduites sur la structure de leurs fleurs et sur leur système de reproduction. C'est pourquoi les auteurs ont réalisé une étude comparative de la morphologie et de l'anatomie florales des trois espèces de rossolis (Droseraceae : Drosera anglica Huds., Drosera linearis Goldie et Drosera rotundifolia L.) en Saskatchewan, afin de vérifier la présence de récompense florale pour les pollinisateurs potentiels, et de déterminer les rapports pollen à ovule, un indicateur du système reproducteur. À l'aide de la microscopie photonique et électronique par balayage, on observe des différences entre les trois espèces dans la longueur du style, le nombre de placentas, la couleur des anthè-res et du pollen, ainsi que la structure du trichome glandulaire des sépales. La présence de caractères uniques à chaque espèce ne supporte pas l'hypothèse que le D. anglica serait un hybride des deux autres espèces. Les nectaires sont absents sur les fleurs des trois espèces, bien qu'on observe, à l'apex des anthères et aux sommets des ovaires, des groupes de cellules papillées qui rappellent un tissu sécréteur. Les rapports pollen à ovule sont faibles chez les trois espèces, allant de 9,0 à 18,7 chez le D. rotundifolia et le D. linearis, respectivement, ce qui suggère un système de reproduction autogame pour chacune des espèces.

The unique design of pollen tetrads inDionaeaandDrosera

“…Apertural intine protrusions are interpreted in different ways. The Dionaea/Drosera protrusions are capable to form pollen tubes as reported by Smith (1929), Jones (1964) and Murza and Davis (2003). Comparable protrusions are the 'pollen buds' in Ophiorrhiza (Rubiaceae; Weber & Igersheim, 1994) and the 'apertural chambers' in Geranium (Geraniaceae; Weber, 1996).…”

“…In Dionaea as well as in Drosera, conspicuous, so-called 'papillate protrusions' emerge from these pores as reported by several authors (Takahashi & Sohma, 1982;Halbritter, 2000;Murza & Davis, 2003). Here, the terms 'cytoplasmic/protoplasmic papillae' or 'protruding papillae', which have been used by Takahashi (1988), Murza and Davis (2003) and Rodondi et al (2004) as well as the term 'papillate protrusions' are avoided, as the term 'papilla' is occupied for a distinct morphological feature restricted to Taxodiaceae/Cupressaceae pollen (Hesse et al, 2009b).…”

Revisão taxonômica do clado tetraploide-brasileiro de Drosera L. (Droseraceae).

“…Os estudos sobre polinização de Drosera restringem-se a poucas espécies do hemisfério Norte (Juniper et al, 1989;Wilson, 1995;Murza & Davis, 2003Murza et al, 2006) e África do Sul (Anderson, 2010). Em geral, esses estudos focam no conflito polinizador-presa, o qual consiste na hipótese de que as hastes florais longas foram selecionadas em espécies de plantas carnívoras por aumentarem a distância entre as flores e as armadilhas, reduzindo, assim, as chances de os polinizadores tornarem-se presas (Juniper et al, 1989 (Murza et al, 2006;Anderson, 2010).…”

“…As flores do gênero, geralmente, não são perfumadas e carecem de nectários, sendo a oferta de pólen o principal atrativo para polinizadores (Murza & Davis, 2003 …”

“…Curiously, Drosera flowers always do not produce nectar (Murza & Davis 2003, and thus can only offer pollen to their pollinators. As a result, Drosera flowers are rarely visited and seed production would be expected to be scarce.…”