Nick Rowe, L'ea M'enard, Gilda M"uhlen & Doyle McKey

The domesticated plant known as cassava (Manihot esculenta ssp. esculenta Crantz, Euphorbiaceae) is grown commonly as shrubs in many parts of the tropics, including French Guiana, and is an important staple. Cultivation involves selection and planting of stem cuttings in the soil and then subsequent growth as self-supporting shrubs or treelets up to 3 m in height. Such growth forms are a common sight in many parts of the tropics. Shrubs are subsequently harvested and the starch-bearing tuberous root processed for a range of food products. Recent and ongoing phylogenies suggest that cassava was derived from a wild species currently referred to as Manihot esculenta subsp. flabellifolia in Brazil and Manihot tristis in French Guiana. Our ecological and biomechanical analyses of these wild type species 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 and mechanical architectures of the ancestral species and domesticated cassava in a range of habitats and in two regions of South America - 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 domesticated climbing stems are much more brittle than putative ancestral climbing stems. The results suggest that selection during domestication did not noticeably modify the overall ontogenetic trajectory and developmental plasticity of the species to grow as either shrub or climber depending on the ecological context but did drastically change the material properties of the wood of the stem.
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 biomechanics 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 never been able to test adequately - that certain types of climbing habit must develop stem properties that are flexible and resistant to brittle fracture.