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

Abstract

Fifty years ago, the C4 photosynthetic pathway was first characterized. In the subsequent five decades, much has been learned about C4 plants, such that it is now possible to place nearly all C4 species into their respective evolutionary lineages. Sixty-one independent lineages of C4 photosynthesis are identified, with additional, ancillary C4 origins possible in 12 of these principal lineages. The lineages produced ∼8100 C4 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 C4 lineage is the grass lineage Chloridoideae, estimated to be near 30 million years old. Most C4 lineages are estimated to be younger than 15 million years. Older C4 lineages tend to be more speciose, while those younger than 7 million years have <43 species each. To further highlight C4 photosynthesis for a 50th anniversary snapshot, a Hall of Fame comprised of the 40 most significant C4 species is presented. Over the next 50 years, preservation of the Earth's C4 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 C4 Hall of Fame are leading threats to the natural C4 flora due to their association with human activities on landscapes where most C4 plants occur.