The taxonomic distribution of C 4 photosynthesis in Amaranthaceae sensu stricto

C 4 photosynthesis evolved multiple times in the Amaranthaceae s.s., but the C 4 evolutionary lineages are unclear because the photosynthetic pathway is unknown for most species of the family. To clarify the distribution of C 4 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 C 4 pathway. C 4 species occur in 10 genera— Aerva , Amaranthus, Blutaparon , Alternanthera , Froelichia, Lithophila , Guilleminea , Gomphrena, Gossypianthus, and Tidestromia . Aerva, Alternanthera , and Gomphrena contain both C 3 and C 4 species. In Aerva, 25% of the sampled species are C 4 . In Alternanthera, 19.5% are C 4 , while 89% of the Gomphrena species are C 4 . Integration of isotope and matK/trnK data indicated C 4 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 C 3 and C 4 species belonging to distinct clades. Alternanthera appears to be monophyletic with C 4 photosynthesis originating in a terminal sublineage of procumbent herbs. Alpine C 4 species were also identified in Alternanthera, Amaranthus, and Gomphrena , including one species ( Gomphrena meyeniana ) from 4600 m a.s.l.