Major trends in stem anatomy and growth forms in the perianth-bearing Piperales with special focus on Aristolochia
Wagner Sarah T.; Hesse Linnea; Isnard Sandrine; Samain Marie-Stéphanie; Bolin Jay
Annals of Botany
Background and Aims 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.\n\nMethods 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.\n\nKey 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.\n\nConclusions 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.