Polyphyly of Arundinoideae (Poaceae) and awn evolution in the grasses

This study answers one of the last remaining big questions in grass taxonomy while highlighting examples of convergent evolution in an ecologically important trait, the hygroscopic, twisted geniculate awn.

The sub-family Arundinoideae represents one of the last unsolved taxonomic mysteries in the grass family (Poaceae) due to the narrow and remote distributions of many of its 19 morphologically and ecologically heterogeneous genera. Resolving the phylogenetic relationships of these genera could have substantial implications for understanding character evolution in the grasses, for example the twisted geniculate awn – a hygroscopic awn that has been shown to be important in seed germination for some grass species.

Teisher et al. conducted a phylogenetic analysis on a matrix of full-plastome sequences from 123 species in 107 genera representing all grass subfamilies, with 15 of the 19 genera in subfamily Arundinoideae. They then used parsimony and maximum likelihood mapping approaches to estimate ancestral states to explore the evolutionary history of the twisted geniculate lemma awn.

Cross-sections of species from three of the four genera misplaced in Arundinoideae
Cross-sections of species from three of the four genera misplaced in Arundinoideae s.l. Black scale bars are approx. 500 μm. (A) Phaenanthoecium koestlinii (Wood 1305) showing schlerenchyma around the vascular bundles (grey arrow) and forming a cap at the leaf margins (black arrow). (B) Alloeochaete andongensis (Gossweiler 11810) has well-defined ribs around vascular bundles that form deep pits containing stomata (black arrow) and macrohair-like prickles (grey arrow, also in Fig. 5B). (C) Nematopoa longipes (Simon 2353) appears to be C4 with small tertiary vascular bundles (black arrow) limiting the distance between bundle sheath cells.

Arundinoideae is found not to be a naturally occurring group, with members that are more closely related to species in other subfamilies. The twisted geniculate lemma awn has evolved at least seven times independently in the grasses.

Reference List

Teisher, J. K., McKain, M. R., Schaal, B. A., & Kellogg, E. A. (2017). Polyphyly of Arundinoideae (Poaceae) and evolution of the twisted geniculate lemma awn. Annals of Botany, 120(5), 725–738. https://doi.org/10.1093/aob/mcx058