Phylogenetic analysis of tribe Salsoleae (Chenopodiaceae) based on ribosomal ITS sequences: Implications for the evolution of photosynthesis types

Diversity in photosynthetic pathways in the angiosperm family Chenopodiaceae is expressed in both biochemical and anatomical characters. To understand the evolution of photosynthetic diversity, we reconstructed the phylogeny of representative species of tribe Salsoleae of subfamily Salsoloideae, a group that exhibits in microcosm the patterns of photosynthetic variation present in the family as a whole, and examined the distribution of photosynthetic characters on the resulting phylogenetic tree. Phylogenetic relationships were inferred from parsimony analysis of nucleotide sequences of the internal transcribed spacer regions (ITS) of the 18S-26S nuclear ribosomal DNA of 34 species of Salsola and related genera (Halothamnus, Climacoptera, Girgensohnia, Halocharis, and Haloxylon) and representative outgroups from tribes Camphorosmeae (Camphorosma lessingii, Kochia prostrata, and K. scoparia) and Atripliceae (Atriplex spongiosa). A highly resolved strict consensus tree largely agrees with photosynthetic type and anatomy of leaves and cotyledons. The sequence data provide strong support for the origin and evolution of two main lineages of plants in tribe Salsoleae, with NAD-ME and NADP-ME C-4 photosynthesis, respectively. These groups have different C-4 photosynthetic types in leaves and different structural and photosynthetic characteristics in cotyledons. Phylogenetic relationships inferred from ITS sequences generally agree with classifications based on morphological data, but deviations from the existing taxonomy were also observed. The NAD-ME C-4 lineage contains species classified in sections Caroxylon, Malpigipila, Cardiandra, Belanthera, and Coccosalsola, and the NADP-ME lineage comprises species from sections Coccosalsola and Salsola. Reconstruction of photosynthetic characters on the ITS phylogeny indicates separate NAD-ME and NADP-ME lineages and suggests two reversions to C-3 photosynthesis. Reconstruction of geographic distributions suggests Salsoleae originated and diversified in central Asia and subsequently dispersed to Africa, Europe, and Mongolia. Inferred patterns and processes of photosynthetic evolution in Salsoleae should further our understanding of biochemical and anatomical evolution in Chenopodiaceae as a whole.