AOBPreview originally published online on February 7, 2008
Annals of Botany 2008 101(4):509-520; doi:10.1093/aob/mcm328
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Root Character Evolution and Systematics in Cranichidinae, Prescottiinae and Spiranthinae (Orchidaceae, Cranichideae)
1 Programa de Botánica, Colegio de Postgraduados, Km. 36.5 Carretera México-Texcoco, Montecillo, 56230, Estado de México, Mexico
2 Departamento de Botánica, Instituto de Biología, Universidad Nacional Autónoma de México, Avenida Universidad 3000, Ciudad Universitaria, Apartado Postal 70-367, 04510 México DF, Mexico
* For correspondence. E-mail gasc{at}servidor.unam.mx
Received: 6 July 2007 Returned for revision: 19 September 2007 Accepted: 29 November 2007 Published electronically: 7 February 2008
Background and Aims: Previous studies have suggested that velamen characteristics are useful as taxonomic markers in Orchidaceae. Members of tribe Cranichideae have been assigned to two velamen types constructed based on combinations of characters such as the presence of secondary cell-wall thickenings and pores. However, such characters have not been analysed on an individual basis in explicit cladistic analyses.
Methods: The micromorphology of roots of 26 species of Cranichideae was examined through scanning electron microscopy and light microscopy, scoring the variation and distribution of four characters: number of velamen cell layers, velamen cell-wall thickenings, presence and type of tilosomes, and supraendodermal spaces. The last three characters were analysed cladistically in combination with DNA sequence data of plastid trnK/matK and nuclear ribosomal internal transcribed spacer (ITS) regions and optimized on the resulting phylogenetic tree.
Key Results: Thickenings of velamen cell walls group Prescottiinae with Spiranthinae, whereas tilosomes, documented here for the first time in Cranichideae, provide an unambiguous synapomorphy for subtribe Spiranthinae. Supraendodermal spaces occur mostly in species dwelling in seasonally dry habitats and appear to have evolved three times.
Conclusions: Three of the four structural characters assessed are phylogenetically informative, marking monophyletic groups recovered in the combined molecular–morphological analysis. This study highlights the need for conducting character-based structural studies to overcome analytical shortcomings of the typological approach.
Key words: Cranichideae, Cranichidinae, ITS, Orchidaceae, phylogeny, Prescottiinae, trnK/matK, root anatomy, Spiranthinae, tilosomes, velamen
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