Solanum sisymbriifolium is a member of the large plant genus Solanum, which has about 1,500 species distributed worldwide.
It belongs to the group in the genus that is called the spiny solanums (Leptostemonum) - so-called for the abundant prickles that appear on the stems and often on leaves.
Within the Leptostemonum group it has no close relatives that have been identified using DNA data, but we find out more about the relationships of these plants all the time.
It is morphologically most similar to another Chaco species, Solanum euacanthum, which grows only in Paraguay and Argentina. Solanum euacanthum however, has dry fruits that do not turn red at maturity.
The broad species definition we currently use for Solanum sisymbriifolium may need to be revised as we collect more in the Chaco and other habitats in South America - there may be hidden diversity we just don’t recognise yet.