EVOLUTIONARY HISTORY OF BLEPHARIS (ACANTHACEAE) AND THE ORIGIN OF C4 PHOTOSYNTHESIS IN SECTION ACANTHODIUM.

Premise of research. Plants with C4 photosynthesis are able to produce carbohydrates more efficiently than plants with C3 photosynthesis in warm climates when levels of atmospheric CO2 are reduced. The C4 pathway has evolved multiple times in distantly related lineages, but it is not known whether the same physiological transitions occurred in all lineages. Species with intermediate C3-C4 physiology and anatomy offer the opportunity to study how plants transition from C3 to C4. It is thus vital to characterize phylogenetic relationships and photosynthetic pathways in groups with C3-C4 intermediate species, as well as C3 and C4 species. Methodology. We assessed photosynthetic pathway evolution in the Afro-Asian genus Blepharis (Acanthaceae) by sampling 99 species for carbon isotope ratios, 18 species for leaf anatomy, and 36 species for phylogenetic analysis. We estimated when Blepharis clades diverged using a BEAST molecular dating analysis, and we estimated ancestral distributions using BioGeoBEARS. We also estimated ancestral photosynthetic pathways in Blepharis, along with the rate of transitions between C3, C3-C4 intermediates, and C4 photosynthesis. Finally, we analyzed the climatic niches of 93 Blepharis taxa to better understand the current distribution patterns of species with different photosynthetic pathways. Pivotal results. Of the 99 species of Blepharis sampled for carbon isotope ratios, 13 are C4, 2 are C4-like, and 84 have values that indicate that they use a C3 cycle. Nine species are putative C3-C4 intermediate species based on leaf anatomy. All of the C4 and C3-C4 intermediate species are in section Acanthodium and are closely related. Our estimates suggest that C4 photosynthesis evolved two or three times in southern Africa and Asia between 1 and 5 million years ago. Conclusions. We present a phylogenetic framework of Blepharis and hypotheses of where and when C4 photosynthesis evolved in the genus. There are more than 40 species in section Acanthodium that are not C4, some of which may be C3-C4 intermediate species. Blepharis thus contains many candidate C3-C4 intermediate species and provides an opportunity for detailed comparative analyses of the evolution of photosynthetic pathways. [ABSTRACT FROM AUTHOR]