Evolution, systematics, and domestication in Sechium and related genera (Sicyeae, Cucurbitaceae)

Sechium is a genus of 11 species distributed from Mexico to Panama. The genus is in subtribe Sicyinae (Sicyeae), comprising the single-seeded species of the squash family (Cucurbitaceae). There are two cultivated species in Sechium, chayote (Sechium edule ssp. edule), and tacaco (S. tacaco). The focus of this research was to use molecular sequence and marker data to place these crop species in a systematic context and within this framework explore their origin from wild relatives and patterns of crop diversification.; The first two chapters provide the evolutionary and phylogenetic framework to determine the evolutionary relationships among Sechium and related genera in tribe Sicyeae. Chapter 1 entails the tribe Sicyeae using two gene regions and Chapter 2 focuses on subtribe Sicyinae with an additional gene and morphological characters. The results reveal tribe Sicyeae to be monophyletic. However, many generic relationships suggested by previous taxonomic circumscriptions were not in accordance with these results as many genera, including Sechium, were found to be polyphyletic. Evaluation of morphological evolution in the total evidence phylogeny suggested that many of the characters used historically to define genera are homoplasious. However, the inclusion of several morphological traits improved phylogenetic resolution and some patterns of character evolution within individual clades.; Within this systematic background the origin of the crop species chayote and tacaco were explored in Chapter 3. The addition of molecular marker data resolved the relationships of the crops and their wild relatives and confirmed earlier hypotheses that chayote was domesticated in southern Mexico and tacaco in Costa Rica. The genetic markers and sequence data showed that chayote was more genetically similar to its wild relatives than tacaco is to any of its close relatives. Therefore, diversification of chayote was further explored in Chapter 4 using population and genetic analyses. The results indicated that geographic distribution and local adaptation explained the majority of genetic diversity in the crop. Results of this study provided recommendations for the conservation and maintenance of chayote diversity both ex situ and in situ.