title: Evolution under domestication: contrasting functional morphology of seedlings in domesticated cassava and its closest wild relatives.
abstract: Although cassava (Manihot esculenta ssp. esculenta) is asexually propagated, farmers incorporate plants from seedlings into planting stocks. These products of sex are exposed to selection, which in agricultural environments should favour rapid growth. To examine whether seedling morphology has evolved under domestication, we compared domesticated cassava, its wild progenitor (M. esculenta ssp. flabellifolia) and their sister species (M. pruinosa) under controlled conditions. Field observations complemented laboratory study. In both wild taxa, the hypocotyl did not elongate (hypogeal germination) and cotyledons remained enclosed in the testa. In domesticated cassava, the hypocotyl elongated (epigeal germination), and cotyledons emerged and became foliaceous. The difference in hypocotyl elongation was fixed, whereas cotyledon morphology varied with environmental conditions in M. pruinosa. Comparative analysis suggests that epigeal germination is primitive in Manihot, that the lineage including wild ancestors of cassava evolved hypogeal germination--which confers greater tolerance to risks in their savanna environment--and that with domestication, there was a reversion to epigeal germination and photosynthetic cotyledons, traits conferring high initial growth rates in agricultural habitats.
authors.affiliation: Department of Population Biology, Centre for Functional and Evolutionary Ecology (CEFE, UMR 5175 CNRS), 1919, route de Mende, 34293 Montpellier Cedex 05, France. benoit.pujol@cefe.cnrs.fr
authors.forename: Benoît
authors.initials: B
authors.lastname: Pujol
authors.forename: Gilda
authors.initials: G
authors.lastname: Mühlen
authors.forename: Nancy
authors.initials: N
authors.lastname: Garwood
authors.forename: Yael
authors.initials: Y
authors.lastname: Horoszowski
authors.forename: Emmanuel J P
authors.initials: EJ
authors.lastname: Douzery
authors.forename: Doyle
authors.initials: D
authors.lastname: McKey
mesh_terms: D000383:Agriculture; D005075:Biological Evolution; D019665:Euphorbiaceae / Q000033:anatomy & histology; D002365:Manihot / Q000033:anatomy & histology* / Q000235:genetics / Q000254:growth & development; D018517:Plant Roots / Q000254:growth & development; D018520:Plant Shoots / Q000254:growth & development; D036226:Seedlings / Q000033:anatomy & histology*; D012639:Seeds / Q000502:physiology