Lens, Frederic , Groeninckx, Inge , Smets, Erik , Dessein, Steven .
A multidisciplinary approach to reveal the origin of woodiness within the Spermacoceae-Knoxieae alliance (Rubiaceae).
The tribe Spermacoceae is essentially an herbaceous Rubiaceae lineage, except for some species that can be described as “woody” herbs, shrubs to small trees, or lianas. Its smaller sister tribe Knoxieae contains a large number of herbaceous taxa as well, but the percentage of woody taxa is much 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 wood anatomical observations are confronted with an independent molecular phylogeny to answer this question. Therefore, wood anatomical observations 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. Our wood anatomical observations in Spermacoceae support the self generated molecular hypothesis that all woody species observed 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-on-age curves for vessel elements, the abundance of square and upright ray cells, or even the (nearly) absence of rays. These so-called paedomorphic wood features are also present in the Knoxieae genera Otiophora, Otomeria, Pentas, Pentanisia and Phyllopentas, indicating their secondarily woody origin. However, the wood structure of the other Knoxieae genera observed (Carphalea, Dirichletia, and Triainolepis) is typical of primarily woody taxa. To conclude, secondary woodiness in Spermacoceae has evolved numerous times in strikingly different habitats, whereas in Knoxieae, there is a general trend from primary woodiness towards herbaceousness and back to (secondary) woodiness.
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1 - Institute of Botany and Microbiology, Department of Botany, Laboratory of Plant Systematics, Kasteelpark Arenberg 31, Leuven, BE-3001, Belgium
2 - National Herbarium of the Netherlands, Leiden University branch, P.O.Box 9514, Leiden, NL-2300 RA, Netherlands
3 - National Botanic Garden of Belgium, Domein van Bouchout, Nieuwelaan 38, Meise, BE-1860, Belgium
Presentation Type: Oral Paper:Papers for BSA Sections
Location: Rendezvous A/Snowbird Center
Date: Tuesday, July 28th, 2009
Time: 8:00 AM