Phylogeny and photosynthesis of the grass tribe Paniceae

The grass tribe Paniceae includes important food, forage, and bioenergy crops such as switchgrass, napiergrass, various millet species, and economically important weeds. Paniceae are also valuable for answering scientific and evolutionary questions about C4 photosynthetic evolution, drought tolerance, and spikelet variation. However, the phylogeny of the tribe remains incompletely resolved.

Forty-five taxa were selected from across the tribe Paniceae and outgroups for genome survey sequencing (GSS). These data were used to build a phylogenetic tree of the Paniceae based on 102 markers (78 chloroplast, 22 mitochondrial, 2 nrDNA). Ancestral state reconstruction analyses were also performed within the Paniceae using both the traditional and two subtype classification systems to test hypotheses of C4 subtype evolution.

The phylogenetic tree resolves many areas of the Paniceae with high support and provides insight into the origin and number of C4 evolution events within the tribe. The recovered phylogeny and ancestral state reconstructions support between four and seven independent origins of C4 photosynthesis within the tribe and indicate which species are potentially the closest C3 sister taxa of each of these events.

Although the sequence of evolutionary events that produced multiple C4 subtypes within the Paniceae remains undetermined, the results presented here are consistent with only a subset of currently proposed models. The species used in this study constitute a panel of C3 and C4 grasses that are suitable for further studies on C4 photosynthesis, bioenergy, food and forage crops, and various developmental features of the Paniceae.