The taxonomic distribution of C4 photosynthesis in Amaranthaceae sensu stricto

Abstract

C4 photosynthesis evolved multiple times in the Amaranthaceae s.s., but the C4 evolutionary lineages are unclear because the photosynthetic pathway is unknown for most species of the family. To clarify the distribution of C4 photosynthesis in the Amaranthaceae, we determined carbon isotope ratios of 607 species and mapped these onto a phylogeny determined from matK/trnK sequences. Approximately 28% of the Amaranthaceae species use the C4 pathway. C4 species occur in 10 genera - Aerva, Amaranthus, Blutaparon, Alternanthera, Froelichia, Lithophila, Guilleminea, Gomphrena, Gossypianthus, and Tidestromia. Aerva, Alternanthera, and Gomphrena contain both C3 and C4 species. In Aerva, 25% of the sampled species are C4. In Alternanthera, 19.5% are C4, while 89% of the Gomphrena species are C4. Integration of isotope and matK/trnK data indicated C4 photosynthesis evolved five times in the Amaranthaceae, specifically in Aerva, Alternanthera, Amaranthus, Tidestromia, and a lineage containing Froelichia, Blutaparon, Guilleminea, Gomphrena pro parte, and Lithophila. Aerva and Gomphrena are both polyphyletic with C3 and C4 species belonging to distinct clades. Alternanthera appears to be monophyletic with C4 photosynthesis originating in a terminal sublineage of procumbent herbs. Alpine C4 species were also identified in Alternanthera, Amaranthus, and Gomphrena, including one species (Gomphrena meyeniana) from 4600 m a.s.l.