Sugarcane (Saccharum officinarum) is one of the most important crops in the world and a major source of sugar for human consumption. Despite this immense value, the circumscription of the genus Saccharum is complex, contentious, and largely unresolved. Saccharum is accepted in a broad sense by some authors or split into various genera such as Erianthus and Tripidium by others. A plastome phylogenomic analysis of sugarcane and relatives was performed in order to investigate generic delimitation, with emphasis on Tripidium (= Erianthus sect. Ripidium). Our plastome analysis demonstrates that Saccharum s.l. is polyphyletic and Tripidium (distributed in the Old World) belongs to a distinct lineage from Saccharum s.str. (Old World) and Erianthus s.str. (= Erianthus sect. Erianthus, New World). Therefore, this study confirms the recognition of Tripidium as a genus distinct from Saccharum and Erianthus, which is also supported by morphology and nuclear markers. The circumscription of Erianthus s.str. remains unclear since our results are consistent with either considering it as a distinct genus or including it in Saccharum. Better understanding of the evolutionary relationships of sugarcane and relatives may be useful for the selection of potential taxa for interspecific and intergeneric crosses in the genetic improvement of sugarcane. A taxonomic treatment of the six species of Tripidium is also presented, including descriptions, illustrations, data on geographical distribution, and three new nomenclatural combinations.
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