A portrait of the C4 photosynthetic family on the 50th anniversary of its discovery: species number, evolutionary lineages, and Hall of Fame

The distribution of C-4 photosynthesis in higher plants is summarized by providing a list of C-4 evolutionary lineages, their affiliated genera, and species numbers. The most important C-4 species are also highlighted.Fifty years ago, the C-4 photosynthetic pathway was first characterized. In the subsequent five decades, much has been learned about C-4 plants, such that it is now possible to place nearly all C-4 species into their respective evolutionary lineages. Sixty-one independent lineages of C-4 photosynthesis are identified, with additional, ancillary C-4 origins possible in 12 of these principal lineages. The lineages produced similar to 8100 C-4 species (5044 grasses, 1322 sedges, and 1777 eudicots). Using midpoints of stem and crown node dates in their respective phylogenies, the oldest and most speciose C-4 lineage is the grass lineage Chloridoideae, estimated to be near 30 million years old. Most C-4 lineages are estimated to be younger than 15 million years. Older C-4 lineages tend to be more speciose, while those younger than 7 million years have < 43 species each. To further highlight C-4 photosynthesis for a 50th anniversary snapshot, a Hall of Fame comprised of the 40 most significant C-4 species is presented. Over the next 50 years, preservation of the Earth's C-4 diversity is a concern, largely because of habitat loss due to elevated CO2 effects, invasive species, and expanded agricultural activities. Ironically, some members of the C-4 Hall of Fame are leading threats to the natural C-4 flora due to their association with human activities on landscapes where most C-4 plants occur.