Phylogeography and the origin of cassava: New insights from the northern rim of the Amazonian basin

Abstract

1. IntroductionThe origin of cassava (Manihot esculenta Crantz) is still unclear,although several recent studies have addressed this issue (Fregeneetal.,1994;Roaetal.,1997,2000;OlsenandSchaal,1999,2001;Eliaset al., 2000; Olsen, 2004). Rogers and Appan (1973) postulated thatcassava was a ‘‘compilo-species”, i.e., the result of hybridizationevents between several species, among them Manihot aesculifolia(Kunth) Pohl, a species endemic to Central America. On the otherhand,Allemhypothesizedthecroptobeissuedfromasinglespecies,Manihot esculenta, with two subspecies found only in the wild:Manihot esculenta ssp. flabellifolia (Pohl) Ciferri and Manihot escu-lenta ssp. peruviana (Muell. Arg.) Allem (Allem, 1994; Allem et al.,2001).Thesewildtaxatogetherhaveabroadecologicalrange,fromsouthwestern Amazonia to the savannas of the Guianas.Molecular studies have favored the latter scenario, generallyshowing that cassava was domesticated only once, in SouthAmerica,withnocontributionfromtheMesoamericanspeciespool,or at least not from M. aesculifolia (Roa et al., 1997, 2000; Olsen andSchaal, 1999, 2001; Olsen, 2004). Study of this and seven otherMesoamerican species (A. Duputie, unpublished data) show thatallareonlydistantlyrelatedtocassava,excludingthepossibilitythatcassava was domesticated in Central America. These studies,however,sufferseverallimitations.First,theyhaveconsideredonlya limited sample of domesticated cassava accessions. Second, theyoverlooked a part of the range of Manihot esculenta ssp. flabellifolia:whilethetaxonisdistributedonanarcpartiallycirclingtheAmazonbasin, from eastern Bolivia westwards to central Brazil eastwards,andintheGuianasandeasternVenezuelanorthwards,thesestudiesconsidered only samples from Brazil, thereby excluding any possi-bility of testing the hypothesis that cassava could have more thanone center of domestication. Yet, several other crops have beenshown to have been domesticated twice (e.g., the common bean,Gepts et al., 1986).The present study aims at filling some of these gaps. Olsen andSchaal (1999) sampled accessions of M. esculenta ssp. flabellifoliaand the closely related species M. pruinosa Pohl from Brazil and20 accessions of cultivated cassava from the CIAT core collection,a collection constituted with the goal of representing cassava’sgenetic and morphological diversity worldwide (Hershey et al.,1994). We combined their sample of wild Manihot with samplesof Manihot esculenta ssp. flabellifolia from the northern rim of theAmazonian basin, to cover most of the range of this taxon. Further-more, to test the hypothesis that cassava could have more than onecenter of domestication, we also broadened the sample of domes-ticated cassava to include landraces cultivated in the Guianas, thusfilling an acknowledged gap in earlier studies (Olsen and Schaal,2001). Even though these samples all come from the same region,