Crop wild relatives are important but often poorly known. This is the case for subtribe Saccharinae (Poaceae: Andropogoneae), which includes sugarcane (Saccharum) and sorghum (Sorghum). We present a phylogenetic and taxonomic account of the Malagasy endemic genus Lasiorhachis, previously included in Saccharum. New plastome and nuclear sequences were assembled from genome skimming data. Phylogenetic analyses of whole plastomes place Lasiorhachis as sister to Sorghum and only distantly related to the interbreeding Saccharum complex. This is also supported by analyses of nuclear ITS and partial assemblies for eight low-copy regions. Some nuclear regions suggest that Lasiorhachis is closer to Sorghum section Sorghum than other Sorghum spp. sometimes segregated as Vacoparis. We present a full taxonomic treatment of Lasiorhachis with an identification key, synonymy, typification, descriptions, drawings and a distribution map of the three species, all endemic to the central highlands of Madagascar. Lasiorhachis species occupy contrasting ecological niches and at least one of them, L. viguieri, has significance for ecosystem function. As crop wild relatives of sorghum, Lasiorhachis species should be included in germplasm collections.
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