In the broad sense
Machaeranthera Nees in the broad sense is a North American generic complex of about 30 species with chromosome base numbers of x = 5, 4, 3 and 2. Plants are tap rooted, have spinulose tipped leaves and fruits with a multiseriate pappus. The evolution of the various groups within the genus is complex. Hartman (1990) provided a conspectus of the complex, following the break up of the old Haplopappus sensu Hall (1928). Morgan (1997) examined the cpDNA of the complex and related genera; he showed that Psilactis was a distinct group of species, which are part of the Symphyotrichum s.l. aster clade. Morgan and Hartman (2003; Sida 20: 1387-1416.) divided the complex into a number of monophyletic genera leaving Machaeranthera sensu strictu limited to just a few species. The single-species genus Xanthisma was expanded to include sect. Blepharodon, sect. Havardii and, sect. Sideranthus. In doing so, both North American species of Astereae with well-studied B-chromosomes were placed in a single genus: X. texanum and X. gracile (syn:Haplopappus gracilis; 2n=4, the plant with the lowest known chromosome number). Other species of the broadly defined Macheranthera were placed in the segregate genera Arida and Dieteria.
In the narrow sense
Morgan and Hartman (2006 Flora North America) recognized just two violet-rayed, aster-like species:
- Machaeranthera tanacetifolia
- Machaeranthera tagetina