Major trends in stem anatomy and growth forms in the perianth-bearing Piperales, with special focus on Aristolochia.
OBJECTIVE
The order Piperales has the highest diversity of growth forms among the earliest angiosperm lineages, including trees, shrubs, climbers and herbs. However, within the perianth-bearing Piperales (Asarum, Saruma, Lactoris, Hydnora, Prosopanche, Thottea and Aristolochia), climbing species only occur in the most species-rich genus Aristolochia. This study traces anatomical and morphological traits among these lineages, to detect trends in growth form evolution and developmental processes.


METHODS
Transverse stem sections of different developmental stages of representatives of Asarum, Saruma, Lactoris, Hydnora, Thottea and Aristolochia were compared and anatomical traits were linked to growth form evolution. Biomechanical properties of representative climbers were determined in three-point bending tests and are discussed based on the anatomical observations. Growth form evolution of the perianth-bearing Piperales was reconstructed by ancestral character state reconstruction using Mesquite.


RESULTS
While species of Asarum and Saruma are exclusively herbaceous, species of the remaining genera show a higher diversity of growth habit and anatomy. This growth form diversity is accompanied by a more complex stem anatomy and appropriate biomechanical properties. The ancestral growth form of the perianth-bearing Piperales is reconstructed with either a shrub-like or herbaceous character state, while the following three backbone nodes in the reconstruction show a shrub-like character state. Accordingly, the climbing habit most probably evolved in the ancestor of Aristolochia.


CONCLUSIONS
Since the ancestor of the perianth-bearing Piperales has been reconstructed with a herb- or shrub-like habit, it is proposed that the climbing habit is a derived growth form, which evolved with the diversification of Aristolochia, and might have been a key feature for its diversification. Observed anatomical synapomorphies, such as the perivascular fibres in Lactoris, Thottea and Aristolochia, support the phylogenetic relationship of several lineages within the perianth-bearing Piperales. In addition, the hypothesis that the vegetative organs of the holoparasitic Hydnoraceae are most probably rhizomes is confirmed.
Institut für Botanik, Technische Universität Dresden, D-01062 Dresden, Germany sarah.wagner@tu-dresden.de.
Sarah T
ST
Wagner
Institut für Botanik, Technische Universität Dresden, D-01062 Dresden, Germany Institut für Spezielle Botanik, Johannes Gutenberg-Universität Mainz, D-55112 Mainz, Germany.
Linnea
L
Hesse
IRD, UMR-AMAP, Laboratoire de Botanique et d'Écologie Végétale Appliquées, BPA5, 98800, Nouméa, New Caledonia.
Sandrine
S
Isnard
Instituto de Ecología, A.C., Centro Regional del Bajío, 61600 Pátzcuaro, Michoacán, Mexico.
Marie-Stéphanie
MS
Samain
Department of Biology, Catawba College, Salisbury, NC 28144, USA.
Jay
J
Bolin
Department of Biological Sciences, University of Namibia, Windhoek, Namibia.
Erika
E
Maass
Institut für Botanik, Technische Universität Dresden, D-01062 Dresden, Germany.
Christoph
C
Neinhuis
Université Montpellier 2, UMR-AMAP, Montpellier, F-34000 France; CNRS, UMR AMAP, Montpellier, F-34000, France.
Nick P
NP
Rowe
Institut für Botanik, Technische Universität Dresden, D-01062 Dresden, Germany.
Stefan
S
Wanke
D029065:Aristolochiaceae / Q000033:anatomy & histology* / Q000254:growth & development*; D005075:Biological Evolution; D001696:Biomechanical Phenomena; D018547:Plant Stems / Q000033:anatomy & histology* / Q000254:growth & development*
Aristolochia; Asarum; Hydnora; Lactoris; Saruma; Thottea; anatomy; biomechanics; growth form; heterochrony; perianth-bearing Piperales; secondary woodiness