Résumé
The domesticated plant known as manioc, or cassava (Manihot esculenta ssp. esculenta Crantz, Euphorbiaceae), is grown commonly as a shrub in many parts of the tropics, including French Guiana and is an important staple for about 700 million people throughout the tropics. Cultivation involves selection and planting in soil of stem cuttings, which subsequently grow as self-supporting shrubs or treelets up to 3 m in height. Shrubs and treelets of manioc are a common sight in many parts of the tropics. Shrubs are subsequently uprooted, and their starch-bearing tuberous roots harvested and processed to make a range of food products. Recent and ongoing phylogenetic research suggests that cassava was derived from a wild species currently referred to as Manihot esculenta subsp. flabellifolia in Brazil and as Manihot tristis (considered synonymous) in French Guiana. Our ecological and biomechanical analyses of these wild taxa indicate a broad plasticity in growth form, with development of mature phenotypes as either self-supporting shrubs in open conditions or as climbing plants in forest understory. We compared growth variation, mechanical architecture and wood microfibril angle of domesticated cassava and its closest wild relatives (considered similar to their common ancestor). Our observatioins included material from a range of habitats and two regions of South America - southwestern Brazil and French Guiana. Domesticated cassava, at least in the studied areas of South America, has a tendency to grow as climbing lianoid growth forms if plantations of cultivated shrubs are abandoned. Field observations and bending tests up to stem failure indicate that stems of domesticated manioc are much more brittle than stems of the putative ancestor. The results suggest that selection during domestication did not noticeably modify the overall ontogenetic trajectory or developmental plasticity of the species, which enable them to grow as either shrub or climber depending on the ecological context, but did change the material properties of the wood of the stem, particularly in terms of failure characteristics and microfibril angle. Studies based on domestication of organisms offer a window of opportunity for studying evolutionary processes - an opportunity that Charles Darwin used to great effect. From a biomechanical point of view, the study has demonstrated a profound influence of domestication on material properties but not on ontogeny. The results have also provided evidence on a phenomenon that we have taken for granted for a number of years but have never been able to test adequately - that certain types of climbing habit must develop stem properties that are flexible and resistant to brittle fracture.