Propagule Quantity and Quality in Traditional Makushi Farming of Cassava (Manihot esculenta): A Case Study for Understanding Domestication and Evolution of Vegetatively Propagated Crops

Abstract

Modeling how crop plants evolve under domestication requires estimating among-plant variation in important parameters of the reproductive system, including fecundity -the number of propagules produced -and propagule quality. Measuring these traits poses particular problems in vegetatively propagated crop plants. Unlike seeds, vegetative propagules are not intrinsic biological entities but are prepared by farmers. Propagule number and quality are thus determined by the interaction between plant traits and how farmers prepare propagules. We conducted observations, interviews and experiments to study this interaction in cassava grown by Makushi Amerindians, examining how both sources of variation, in plant traits and in farmers' practices, combine to determine the number and quality of propagules produced. Increased stake mass, determined mostly by stem diameter, leads to increased yield and also to increased asexual 'fecundity' of the resulting plant. Farmers' practices reflect knowledge of this relationship. Diameter is the key criterion in the selection of stems for stakes. Larger diameters are preferred; when thinner stems are used, stakes are cut longer, partially compensating for reduced mass. These results suggest that conscious and unconscious selection to increase 'fecundity' and propagule quality in cassava would act to favor plants with thicker stems. Mean stem diameter is greater, and variation in stem diameter is lower, in little-branched plants. Selection for increased asexual 'fecundity' can thus have led to reduction in the degree of branching, one of the most striking differences between domesticated cassava and its wild ancestors. Measuring variation in asexual fecundity is a key step in analyzing evolution of the mixed clonal/sexual reproductive systems that characterize many vegetatively propagated crop plants.

Domestication and Diversity in Manioc (Manihot esculentaCrantz ssp.esculenta, Euphorbiaceae)

“…Although the sexual fecundity of manioc has been reduced during domestication-largely as a consequence of reduced branching and a peculiar architecture that links branching to inflorescence production (Jennings 1995;Elias, Lenoir, and McKey 2007)-most plants can still produce seeds in abundance. In fact, seed production is favored by a peculiar agronomic advantage of manioc, which Amerindian cultivators fully exploit, that is, the plant's capacity for "live storage."…”

“…The abundance of clone mates and the fact that 9. For the Makushi, see Elias (2000); ; Elias, Panaud, and Robert (2000); Elias, Rival, and McKey (2000); Elias et al (2001aElias et al ( , 2001bElias et al ( , 2004; Elias, Lenoir, and McKey (2007); and McKey et al (2001). For the Palikur, see Pujol (2004); Pujol and McKey (2006); Pujol et al (2002Pujol et al ( , 2005bPujol et al ( , 2007Pujol et al ( , 2008; and Pujol, David, and McKey (2005a).…”

Wild Manihot species: botanical aspects, geographic distribution and economic value

“…Conscious and unconscious farmers' selection for increasing the number and quality of cassava cuttings would act to favor plants with thicker stems (Nassar, 2002f;Elias et al, 2007). Thus, this selection for increased asexual propagation may have led to reduction in the degree of branching, which is regarded as one of the most striking differences between the cultigen and its wild ancestors (Nassar, 2002b).…”

The Evolutionary Fate of Phenotypic Plasticity and Functional Traits under Domestication in Manioc: Changes in Stem Biomechanics and the Appearance of Stem Brittleness

“…The plant is propagated by stem cuttings (‘stakes’). Stakes are cut from mature stems of moderate to large diameter (usually at least 15–20 mm diameter, although exceptionally as small as 8 mm) and of varying length (usually at least 30 cm) [29]. Stakes are planted in the ground, usually at an angle, with about half of the stake protruding from the soil, but cultivators sometimes completely bury them, more or less horizontally, in the soil.…”

“…Degree of branching varies greatly among landraces [29]–[31].Time to harvest, and thus also the size of the plant, also vary greatly among landraces, some typically being harvested after periods as short as three to six months, others remaining in the ground for up to two years. At harvest, plants are usually uprooted, although plants are occasionally abandoned in fallows.…”