@ARTICLE{TreeBASE2Ref21403, author = {Juan Carlos Villarreal and Susanne S Renner}, title = {Hornwort pyrenoids: carbon-concentrating mechanisms evolved and were lost at least five times during the last 100 million years}, year = {2013}, keywords = {hornwort, pyrenoid, carbon concentrating mechanisms, CO2}, doi = {}, url = {http://}, pmid = {}, journal = {Proceedings of the National Academy of Sciences U.S.A.}, volume = {}, number = {}, pages = {}, abstract = {RuBisCO has a crucial role in carbon fixation but a slow catalytic rate, a problem overcome in some plant lineages by physiological and anatomical traits that elevate carbon concentrations around the enzyme. Such carbon-concentrating mechanisms (CCMs) are hypothesized to have evolved during periods of low atmospheric CO2. Hornworts, the sister to vascular plants, have a CCM that relies on pyrenoids, proteinaceous bodies mostly consisting of RuBisCO. We generated a phylogeny based on mitochondrial and plastid sequences for 36% of the c. 200 hornwort species to infer the history of gains and losses of pyrenoids in this clade; we also used fossils and multiple dating approaches to generate a chronogram for the hornworts. The results imply 5-6 origins and an equal number of subsequent losses of pyrenoids in hornworts, with the oldest pyrenoid gained ca. 100 mya, and most others at <35 mya. The non-synchronous appearance of pyrenoid-containing clades, the successful diversification of pyrenoid-lacking clades during periods with low CO2, and the maintenance of pyrenoids during episodes of high [CO2] all argue against the previously proposed relationship between pyrenoid origin and low [CO2]. The selective advantages, and costs, of hornwort pyrenoids thus must relate to additional factors besides atmospheric CO2. } }

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