Comparative Anatomy of the Floral Elaiophore in Representatives Of
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Annals of Botany 112: 839–854, 2013 doi:10.1093/aob/mct149, available online at www.aob.oxfordjournals.org Comparative anatomy of the floral elaiophore in representatives of the newly re-circumscribed Gomesa and Oncidium clades (Orchidaceae: Oncidiinae) Małgorzata Stpiczyn´ska1, Kevin L. Davies2,*, Agata Pacek-Bieniek1 and Magdalena Kamin´ska1 1University of Life Sciences, Akademicka 15, 20-950 Lublin, Poland and 2School of Earth and Ocean Sciences, Cardiff University, Main Building, Park Place, Cardiff CF10 3AT, UK * For correspondence. E-mail [email protected] Downloaded from https://academic.oup.com/aob/article/112/5/839/139696 by guest on 29 September 2021 Received: 19 March 2013 Returned for revision: 8 April 2013 Accepted: 14 May 2013 Published electronically: 24 July 2013 † Background and Aims Recently, molecular approaches have been used to investigate the phylogeny of Oncidiinae. This has resulted in the transfer of taxa previously considered to be species of Oncidium Sw. into Gomesa R. Br. and the re-circumscription of both genera. In this study, the structure of the floral elaiophore (oil gland) is described and compared for Gomesa echinata (Barb. Rodr.) M.W. Chase & N.H. Williams, G. ranifera (Lindl.) M.W. Chase & N.H. Williams, Oncidium amazonicum (Schltr.) M.W. Chase & N.H. Williams and O. oxyceras (Ko¨niger & J.G. Weinm.) M.W. Chase & N.H. Williams in order to determine whether phylogenetic revision is supported by differences in its anatomy. † Methods The floral elaiophore structure was examined and compared at three developmental stages (closed bud, first day of anthesis and final stage of anthesis) for all four species using light microscopy, fluorescence microscopy, scanning electron microscopy, transmission electron microscopy and histochemistry. † Key results In all species investigated, the floral elaiophore occurs on the labellar callus and is of the epithelial type, comprising cuboidal to palisade-like, secretory epidermal cells and a layer of sub-epidermal cells, both tissues en- closing ground parenchyma supplied with collateral vascular bundles and containing idioblasts, often with raphides or phenolic contents. A bi-layered cuticle comprising an outer, lamellate and an inner, reticulate layer is present, and sub-cuticular accumulation of secreted material results in distension of the cuticle. Secretion-filled cavities are present at anthesis in the elaiophore cell walls and, in most species, the outer, tangential walls of the elaiophore have small, peg-like projections that protrude into the cytoplasm. In all taxa examined, the elaiophore organelle com- plement, especially the smooth endoplasmic reticulum (SER), is typical of lipid-secreting cells. † Conclusions In terms of location, morphology, anatomy and ultrastructure, the floral elaiophores of both Gomesa and Oncidium species examined are very similar, and distinction between these genera is not possible based on elaio- phore features alone. Furthermore, manyof these elaiophore characters are shared with representatives of otherclades of Oncidiinae, including the Ornithocephalus clade. Consequently, elaiophores are considered homoplasious and of limited value in investigating the phylogeny of this subtribe. Key words: Anatomy, elaiophore, lipid secretion, micromorphology, oil glands, Oncidiinae, ultrastructure. INTRODUCTION studies of elaiophore structure did not commence until relatively The flowers of Oncidiinae (sensu Chase et al., 2003; Neubig et al., recently (Singer and Cocucci, 1999; Pacek and Stpiczyn´ska, 2012) display the greatest diversity of pollination systems known 2007; Stpiczyn´ska et al., 2007; Stpiczyn´ska and Davies, 2008; for Orchidaceae (Neubig et al.,2012). Most are rewardless and Aliscioni et al., 2009; Davies and Stpiczyn´ska, 2009; Pansarin attract insect pollinators by deceit (Chase, 2009; Neubig et al., and Pansarin, 2011; Pacek et al.,2012; Gomiz et al.,2013), 2012). However, of those species that offer food rewards to polli- and were mainly restricted to the epithelial elaiophores of nators, a significant number produce floral oil (Chase et al.,2003; genera then assigned to the Oncidium clade, such as Gomesa Chase, 2005; Neubig et al.,2012) in special epidermal glands or R. Br., Oncidium Sw. and Ornithophora Barb. Rodr., as well glandular hairs called epithelial and trichomal elaiophores, re- as Trichocentrum Poepp. & Endl. (Singer and Cocucci, 1999; spectively (Vogel, 1974). Furthermore, these flowers are thought Pacek and Stpiczyn´ska, 2007; Stpiczyn´ska et al., 2007; to mimic those of oil-secreting Malpighiaceae or sympatric Stpiczyn´ska and Davies, 2008; Aliscioni et al.,2009). These species of Calceolaria L. (Calceolariaceae; Neubig et al., 2012, elaiophores were found to be morphologically very diverse and and references therein). A complex mix of Batesian and did not always closely reflect the phylogeny accepted at that Mu¨llerian mimicry probably best explains the floral morphology time. However, their oil was characterized by the presence of dia- of Oncidiinae (Roy and Widmer, 1999). Floral elaiophores cylglycerols in which the acetyl group was invariably located at occur in members of several clades of Oncidiinae, where they position 1 of the glycerol moiety, and the fatty acid at position 2, are thought to have arisen at least seven times (Renner and with the long-chain fatty acid having either hydroxyl or acetoxy Schaefer, 2010). Although there are records of their involvement groups at position 3 and 7 (Reis et al.,2000, 2003; Silvera, in the attraction and rewarding of pollinators (van der Pijl and 2002; Singer et al.,2006). The major component of the floral oil Dodson, 1969; Dressler, 1990; van der Cingel, 2001; Davies and of Gomesa radicans (Rchb.f.) M.W. Chase & N.H. Williams Stpiczyn´ska, 2008; Davies, 2009, and referencestherein), detailed [as Ornithophora radicans (Rchb.f.) Garay & Pabst] is # The Author 2013. Published by Oxford University Press on behalf of the Annals of Botany Company. All rights reserved. For Permissions, please email: [email protected] 840 Stpiczyn´ska et al. — Floral elaiophore structure in the Gomesa and Oncidium clades (2S,3’R,7’R)-1-acetyl-2-(3’,7’-diacetoxy-eicosanoyl)-glycerol N.H. Williams and G. ranifera (Lindl.) M.W. Chase & N.H. (Reis et al., 2003), and acylglycerols are thought also to occur in Williams (formerly Baptistonia echinata Barb. Rodr. and the floral oils of Gomesa loefgrenii (Cogn.) M.W. Chase & N.H. Oncidium raniferum Lindl., respectively), and two species of Williams (as Oncidium loefgrenii Cogn.) and other species Oncidium, namely O. amazonicum (Schltr.) M.W. Chase & of Gomesa (R. B. Singer, pers. comm., 2006), including N.H. Williams and O. oxyceras (Ko¨niger & J.G. Weinm.) Gomesa praetexta (Rchb.f.) M.W. Chase & N.H. Williams (as M.W. Chase & N.H. Williams (formerly Sigmatostalix amazo- Oncidium enderianum auct.) and related taxa. These orchids nica Schltr. and S. oxyceras Ko¨niger & J.G Weinm. bis, respect- are pollinated by oil-gathering Centris, Paratetrapedia and ively) in order to investigate whether the recent phylogenetic Tetrapedia bees, and the oil is fed to larvae or used as waterproof- revision is supported by elaiophore anatomy. ing for larval cells (Buchmann, 1987; Dressler, 1990; Singer and Cocucci, 1999; van der Cingel, 2001; Torretta et al., 2011; MATERIALS AND METHODS Neubig et al., 2012, and references therein). Downloaded from https://academic.oup.com/aob/article/112/5/839/139696 by guest on 29 September 2021 The recent increased use of molecular approaches has permitted Plants used in this study were grown at the Botanic Garden of the resolution of Oncidiinae into a number of strongly supported the Maria Curie-Skłodowska University, Lublin, Poland. They clades, with the concomitant realignment of genera (Williams include Gomesa echinata (Barb. Rodr.) M.W. Chase & N.H. et al., 2001; Chase et al., 2009; Neubig et al., 2012). Thus, based Williams (accession no. 103/07), G. ranifera (Lindl.) M.W. mainly on the generic concepts of Chase (2009) and the molecular Chase & N.H. Williams (accession no. 28/06), Oncidium amazoni- work of Neubig and co-workers (2012), Oncidiinae currently cum (Schltr.) M.W. Chase & N.H. Williams (accession no. 65/10) includes 61 genera containing approx. 1600 species. Consequently, and O. oxyceras (Ko¨niger & J.G. Weinm.) M.W. Chase & N.H. of those Oncidium species for which elaiophore data were avail- Williams (accession nos 101/07 and 153/09). Spirit-preserved ma- able, most (with some exceptions, such as O. cheirophorum terial was deposited at the herbarium of Warsaw Botanic Garden, Rchb.f. and Oncidium sotoanum R. Jime´nez & Ha´gsater [syn. Poland under the accession numbers WABG002573 (G. echinata), O. ornithorhynchum Kunth], which remain in Oncidium), as WABG002574 (G. ranifera), WABG002575 (O. amazonica)and well as Ornithophora radicans, were transferred to the newly WABG002576 (O. oxyceras). Abbreviations for authors of plant re-circumscribed genus Gomesa to join G. recurva R. Br. names follow Brummitt and Powell (1992) throughout. Other taxa for which elaiophore structure has been examined, The position of elaiophores in intact, fresh flowers of all species but which are currently assigned to other clades, include investigated was determinedbymeansof a Nikon SMZ100 stereo- Trichocentrum cavendishianum (Bateman) M.W. Chase & microscope. Elaiophore tissue was subsequently examined using N.H. Williams, T. pumilum (Lindl.) M.W. Chase & N.H. light microscopy (LM), scanning electron microscopy (SEM) Williams, and the genus Lockhartia Hook., the last two taxa pos- and transmission electron microscopy (TEM) for three develop- sessing trichomal elaiophores (Singer and Cocucci, 1999; Pacek mental stages of the flower: closed bud, first day of anthesis and and Stpiczyn´ska, 2007; Stpiczyn´ska et al., 2007; Stpiczyn´ska final stage of anthesis. Floral oil secretion was monitored for the and Davies, 2008; Aliscioni et al., 2009; Davies and duration of anthesis, from the closed bud stage (approx. 3–4 Stpiczyn´ska, 2009; Pansarin and Pansarin, 2011; Neubig et al., weeks prior to anthesis – the earliest stage at which it was possible 2012; M. A. Blanco, K. L. Davies, M. Stpiczyn´ska, B. S.