The tribe Sicyeae (Cucurbitaceae) is a large group of vigorously climbing vines found mostly in the New World tropics, and is composed of two subtribes, the Sicyinae and Cyclantherinae. The Sicyinae are characterized by single seeded ovules and include the important vegetable crop chayote (Sechium edule). Recent morphological studies have identified major relationships within the tribe. However, the group has not yet been subjected to molecular analysis. In this study the phylogenetic relationships of the tribe Sicyeae were investigated using sequences of the ITS nuclear ribosomal and the chloroplast trnL-F regions (spacer + intron). All genera in the tribe were sampled; the genus Cucurbita was used as the outgroup. Nucleotide sequences of each region were analyzed independently and in combination using parsimony. Sequence data from the ITS region were informative at the species level, and could distinguish between wild and domesticated Sechium edule. The trnL-F sequence data was informative principally at the generic level. Rytidostylis, and Cyclanthera were found to be monophyletic genera. The genera Sicyos and Sechium appear to be paraphyletic. Additionally, sectional relationships within Sechium do not seem to conform to earlier hypotheses based solely on morphology. Within Sechium, the Central American species form a well-defined monophyletic clade, and include members of both sections Sechium and Frantzia. The single species of section Frantzia in Mexico, Sechium mexicanum, appears to be more closely related to the genera Sicyos and Sechiopsis.

Key words: chayote, Cucurbitaceae, Sicyeae, systematics