COMPLEX EVOLUTIONARY TRANSITIONS AND THE SIGNIFICANCE OF C3-C4 INTERMEDIATE FORMS OF PHOTOSYNTHESIS IN MOLLUGINACEAE

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Abstract
C(4) photosynthesis is a series of biochemical and structural modifications to C(3) photosynthesis that has evolved numerous times in flowering plants, despite requiring modification of up to hundreds of genes. To study the origin of C(4) photosynthesis, we reconstructed and dated the phylogeny of Molluginaceae, and identified C(4) taxa in the family. Two C(4) species, and three clades with traits intermediate between C(3) and C(4) plants were observed in Molluginaceae. C(3)-C(4) intermediacy evolved at least twice, and in at least one lineage was maintained for several million years. Analyses of the genes for phosphoenolpyruvate carboxylase, a key C(4) enzyme, indicate two independent origins of fully developed C(4) photosynthesis in the past 10 million years, both within what was previously classified as a single species, Mollugo cerviana. The propensity of Molluginaceae to evolve C(3)-C(4) and C(4) photosynthesis is likely due to several traits that acted as developmental enablers. Enlarged bundle sheath cells predisposed some lineages for the evolution of C(3)-C(4) intermediacy and the C(4) biochemistry emerged via co-option of photorespiratory recycling in C(3)-C(4) intermediates. These evolutionarily stable transitional stages likely increased the evolvability of C(4) photosynthesis under selection environments brought on by climate and atmospheric change in recent geological time.