Investigating the Effects of Domestication on Crop Toxicity: Analyses in Manioc (Manihot esculenta) and Oca ( Oxalis tuberosa).

摘要

Though domestication is traditionally considered to reduce plant chemical defenses, a minority group of crops appears to have retained toxicity in domesticated forms. The most well-known example is manioc (Manihot esculenta Crantz ). Manioc varieties are divided into two distinct "use-categories", those that must be processed prior to consumption to remove toxic levels of cyanide ("bitter") and varieties that do not produce toxic quantities of cyanide and may be eaten without processing ("sweet"). Other crops, though less well-studied, appear to present a similar syndrome of domestication, with two distinct cultural use-categories separating varieties reserved for processing and those varieties that do not need to be processed prior to consumption. Oca (Oxalis tuberosa Molina), a high-altitude Andean tuber crop, is one example of a lesser-known crop that fits this description. The goal of this dissertation is to investigate what molecular bases may exist for cultural use-categories in manioc and oca. In manioc, we seek to investigate what genetic structuring may be observed in manioc populations from South America and Africa and how this genetic structuring may correlate with manioc toxicity. In oca, we seek to identify what underlying biochemistry may correlate with cultural use-categories and how this biochemistry may also relate to population genetic structuring and evolutionary history. We found evidence to suggest that although these crops may have superficially similar cultural and culinary use-categories, the evolutionary histories of the crops are quite different. Analyses in manioc demonstrate that population genetic structuring based on toxicity is entirely dependent upon strict maintenance of separation of bitter and sweet varieties from one another during cultivation. In areas of introduction of the crop in Africa, where South American traditional agricultural techniques were not employed, genetic mixing of bitter and sweet manioc is common and cyanide-related diseases are on the rise due to inconsistent classifications of toxic varieties as "bitter". In contrast, though cultural use-categories in oca appear to loosely correlate with tuber oxalic acid content, high-acid ("sour") oca cultivars form a genetically distinct lineage of ocas defined by low DNA-ploidy in the crop, suggesting a complicated evolutionary history involving multiple polyploidization events.