Phylogeny of Amaranthaceae and Chenopodiaceae and the Evolution of C₄Photosynthesis

A phylogenetic analysis of Chenopodiaceae and Amaranthaceae was carried out using sequence variation of the chloroplast gene rbcL. Our sampling included 108 species of these two families along with 29 species of Caryophyllales serving as outgroups. Phylogeny inferences with maximum parsimony and maximum likelihood indicate that the two families form a well‐supported monophyletic clade that is sister to Achatocarpaceae. Despite extensive sampling, we found that the relationship between Chenopodiaceae and Amaranthaceae remains unclear as a result of short and weakly supported basal branches. The clearly monophyletic Polycnemoideae (traditionally considered a subfamily of Chenopodiaceae) appear as sister to Amaranthaceae sensu stricto. Within Amaranthaceae, most major lineages inferred except Gomphrenoideae and Celosieae do not correspond to recognized subfamilies and tribes. Bosea and Charpentiera branch first in the Amaranthaceae. Within Chenopodiaceae, the genera of Betoideae occur in basal and largely unresolved positions. The remaining Chenopodiaceae are divided into three major clades of unclear relationship: Chenopodioideae (Atripliceae s.str., Chenopodieae I‐III); Corispermoideae (Corispermeae); and Salicornioideae (Haplopeplideae, Salicornieae), Suaedoideae (Suaedeae, Bienertieae), and Salsoloideae (Camphorosmeae, Sclerolaeneae, Salsoleae I‐II). The rbcL tree is discussed also with regard to historical classifications and morphological support for the major clades. The molecular results are used to elucidate the evolution of C4 photosynthesis in the two families. C4 photosynthesis has evolved independently at least three times in Amaranthaceae and at least 10 times in Chenopodiaceae. A survey of C4 leaf anatomy revealed 17 different leaf types that in most cases mark an independent origin of C4 photosynthesis. The application of a molecular clock indicates an age of C4 photosynthesis of 11.5–7.9 Ma in Atriplex (Chenopodioideae) and 21.6–14.5 Ma in subfamily Salsoloideae.