The little-studied tribe Melanthieae is a group of temperate "petaloid lilioid monocots"—so named for their small lily-like flowers with petal-like tepals. Defining genera has been more problematic for these superficially similar plants than for most, if not all, other monocots. Until our current analyses, the circumscriptions of the constituent core genera of the Melanthieae (Amianthium, Schoenocaulon, Stenanthium, Veratrum-Melanthium, Zigadenus s.l.) had not been the subjects of rigorous phylogenetic character analyses, and their intergeneric relationships were also unresolved. The circumscription and relationships of these genera (29 representative taxa) were evaluated using parsimony analyses of ITS (nuclear ribosomal) and trnL-F (plastid) DNA sequence data. Based on the molecular cladograms, Stenanthium is biphyletic, and the traditional Zigadenus s.l. is polyphyletic. Amianthium and Schoenocaulon are distinct entities; the Veratrum complex is conservatively treated as one large monophyletic genus (including Melanthium). Although some intergeneric relationships are not highly resolved, the analyses provide strong support for Zigadenus glaberrimus as sister to the rest of the tribe, and Amianthium muscitoxicum as closely related to Veratrum s.l. As a result of this study, seven genera (some with novel circumscription) are recognized within the tribe Melanthieae: Amianthium, Anticlea, Schoenocaulon, Stenanthium, Toxicoscordion, Veratrum, and Zigadenus. These generic circumscriptions, strongly supported by the tree statistics and topologies in all analyses, are correlated with potential morphological synapomorphies (at the proper level of universality). The study includes practical recommendations, such as documentation of diagnostic characters to be ultimately used in keys and descriptions. Such features include rootstock type, bulb shape, inflorescence structure, indumentum type, tepal shape, nectary morphology, and ovary position.

Key words: ITS, Melanthiaceae, Schoenocaulon, trnL-F, Veratrum, Zigadenus