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AOBPreview originally published online on March 11, 2009
Annals of Botany 2009 103(7):1049-1064; doi:10.1093/aob/mcp048
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© The Author 2009. Published by Oxford University Press on behalf of the Annals of Botany Company. All rights reserved. For Permissions, please email: journals.permissions@oxfordjournals.org

Woodiness within the Spermacoceae–Knoxieae alliance (Rubiaceae): retention of the basal woody condition in Rubiaceae or recent innovation?

Frederic Lens1,*, Inge Groeninckx1, Erik Smets1,2 and Steven Dessein3

1 Laboratory of Plant Systematics, Institute of Botany and Microbiology, K.U. Leuven, Kasteelpark Arenberg 31 Box 2437, BE-3001 Leuven, Belgium
2 Nationaal Herbarium Nederland – Leiden University Branch, P.O. Box 9514, NL-2300 RA Leiden, The Netherlands
3 National Botanic Garden of Belgium, Domein van Bouchout, Nieuwelaan 38, BE-1860 Meise, Belgium

* For correspondence. E-mail frederic.lens{at}bio.kuleuven.be

Received: 12 November 2008    Returned for revision: 11 December 2008    Accepted: 16 January 2009    Published electronically: 11 March 2009

Background and Aims: The tribe Spermacoceae is essentially a herbaceous Rubiaceae lineage, except for some species that can be described as ‘woody’ herbs, small shrubs to treelets, or lianas. Its sister tribe Knoxieae contains a large number of herbaceous taxa, but the number of woody taxa is higher compared to Spermacoceae. The occurrence of herbaceous and woody species within the same group raises the question whether the woody taxa are derived from herbaceous taxa (i.e. secondary woodiness), or whether woodiness represents the ancestral state (i.e. primary woodiness). Microscopic observations of wood anatomy are combined with an independent molecular phylogeny to answer this question.

Methods: Observations of wood anatomy of 21 woody Spermacoceae and eight woody Knoxieae species, most of them included in a multi-gene molecular phylogeny, are carried out using light microscopy.

Key Results: Observations of wood anatomy in Spermacoceae support the molecular hypothesis that all the woody species examined are secondary derived. Well-known wood anatomical characters that demonstrate this shift from the herbaceous to the woody habit are the typically flat or decreasing length vs. age curves for vessel elements, the abundance of square and upright ray cells, or even the (near-) absence of rays. These so-called paedomorphic wood features are also present in the Knoxieae genera Otiophora, Otomeria, Pentas, Pentanisia and Phyllopentas. However, the wood structure of the other Knoxieae genera observed (Carphalea, Dirichletia and Triainolepis) is typical of primarily woody taxa.

Conclusions: In Spermacoceae, secondary woodiness has evolved numerous times in strikingly different habitats. In Knoxieae, there is a general trend from primary woodiness towards herbaceousness and back to (secondary) woodiness.

Key words: Knoxieae, LM, primary woodiness, Rubiaceae, Rubioideae, secondary woodiness, Spermacoceae, wood anatomy


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