Genus Megathyrsus in Family Poaceae

Genus Description

Megathyrsus (Poaceae, Panicoideae) is a small, C4 genus with approximately one widely accepted species, Megathyrsus maximus (Jacq.) B.K.Simon & S.W.L.Jacobs, the type species. Its geographic core is sub-Saharan Africa, but it has long been naturalized throughout the tropics and subtropics and is cultivated as a pasture and amenity grass. Megathyrsus poggeanus is variably treated as a separate taxon within the same lineage or as a subspecies of M. maximus, and the complex is maintained as a species aggregate in major checklists (POWO, 2024; WFO, 2024; GBIF, 2024). This widespread, warm-season grass typifies the Panicoid syndrome of single, terminal panicles bearing spikelets that disarticulate above the glumes, dorsally compressed fertile florets, and C4 photosynthesis via the NADP-ME subtype ( Watson & Dallwitz, 1992 ).

Plants are clump-forming perennials to 1.5–3 m tall, with compressed culms and short rhizomes. Blades are flat or inrolled, glabrous to pilose, and gradually narrowed to filiform tips; ligules are membranous. Panicles are large, open to contracted, and bear numerous ascending branches; spikelets are lanceolate, glabrous, and typically pedicellate. Each spikelets contains two glumes and a single fertile floret; the lower palea is well developed. The caryopsis is ovoid to ellipsoid, with a linear hilum. Vegetatively, robust, tussocky growth and coarse, often hairy leaf blades provide reliable field cues in Afrotropical rangelands ( Clayton et al., 2006 ).

Centered in eastern and southern Africa, the M. maximus complex shows strong endemism in savannas, grasslands, and open woodlands; it also occurs in forest margins and seasonally wet floodplains, with a broad altitudinal range from sea level to around 2000 m. Repeated introductions during the twentieth century established extensive naturalized populations in Australia, the Americas, and Asia, where it often persists in disturbed sites and roadsides (Clayton et al., 2006). Pollination is anemophilous; seed dispersal is primarily by wind and water, with secondary animal transport in pasture systems. Although cytological data are scattered, base chromosome numbers commonly reported for the complex are x = 8–9 (Roux & Panagos, 1980; F. W. Gould, 1975).

Megathyrsus sits within Panicoideae in the “old Panic grass” clade related to Panicum s.l. and Urochloa. Molecular and morphological analyses prompted segregation of Urochloa from Panicum, but species-level placement has remained fluid. Morphological and molecular data from the M. maximus complex support its treatment as Megathyrsus maximus, while M. poggeanus is variably treated as a distinct species or subspecies (S. L. J. Jacobs & B. K. Simon, 1996; “Urochloa clade” phylogenies, Gumede et al., 2020). Alternative taxonomic placements recircumscribing Megathyrsus and Urochloa within an expanded Panicum have been proposed by Bouchenak-Khelladi et al. (2010), illustrating ongoing sectional debate across warm-season grasses (Bouchenak-Khelladi et al., 2010).

The complex is a primary pasture species in the tropics and a valuable ornamental and soil-stabilizing grass, widely used in amenity and erosion-control plantings (Clayton et al., 2006). Some ecotypes have become naturalized and locally invasive, notably in Australia, where feral populations can dominate disturbed habitats (Jacobs & Everett, 1996). No medicinal claims are made here.

Conservation concerns are context-specific: the wild gene pool in sub-Saharan Africa remains under-documented, while invasiveness risk elsewhere warrants proactive management. Targeted phylogenetics and range-wide ecotype monitoring are needed to reconcile species limits and inform responsible use.

• Megathyrsus infestus ((Andersson) B.K.Simon & S.W.L.Jacobs)
• Megathyrsus maximus ((Jacq.) B.K.Simon & S.W.L.Jacobs)