Taxonomy of Sarga, Sorghum and Vacoparis (Poaceae: Andropogoneae)

Abstract

Sorghum taxa are evaluated in light of recent molecular and morphological evidence. The data suggest that three distinct lineages exist, but relationships among these lineages are unresolved. Each lineage is recognised here as a distinct genus in the context of overall variation in tribe Andropogoneae. The type species for the name Sorghum is S. bicolor, the cultivar. S. halepense and S. nitidum are also retained in Sorghum. The name Sarga is resurrected to encompass the set of species formerly making up the bulk of subgenera Parasorghum and Stiposorghum. A new genus, Vacoparis, is defined to include the cytologically and morphologically distinct Australasian taxa, V. macrospermum and V. laxiflorum. The taxonomy proposed is contrasted with a rankless alternative to illustrate the smaller number of name changes that can be accomplished when rank constraints are not enforced. Uncertain relationships among the three lineages as well as among a large number of taxa comprising subtribe Saccharinae exemplifies the difficulties in assigning ranks to taxa that may have future dramatic name changes with new data. As discussions progress concerning the validity and utility of rankless classifications, concrete examples, such as the revision presented here, can provide insights into specific cases where strengths and weaknesses can be evaluated. Species boundaries in Sarga are different from those formerly defined. Continuous variation across specimens in characters used to distinguish taxa in the past led to the decision to broaden species limits so that fewer and morphologically variable species are recognised.