Hornwort pyrenoids, carbon-concentrating structures, evolved and were lost at least five times during the last 100 million years

Abstract

Ribulose-1,5-Biphosphate-carboxylase-oxygenase (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 are hypothesized to have evolved during periods of low atmospheric CO 2 . Hornworts, the sister to vascular plants, have a carbon-concentrating mechanism that relies on pyrenoids, proteinaceous bodies mostly consisting of RuBisCO. We generated a phylogeny based on mitochondrial and plastid sequences for 36% of the approximately 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 five to six 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 nonsynchronous appearance of pyrenoid-containing clades, the successful diversification of pyrenoid-lacking clades during periods with low [CO 2 ], and the maintenance of pyrenoids during episodes of high [CO 2 ] all argue against the previously proposed relationship between pyrenoid origin and low [CO 2 ]. The selective advantages, and costs, of hornwort pyrenoids thus must relate to additional factors besides atmospheric CO 2 . independent origins T he enzyme most important in photosynthesis is Ribulose-1,5-Biphosphate-carboxylase-oxygenase, or RuBisCO. This enzyme has a low CO 2 attraction and catalytic rate, which plants compensate for by increasing their CO 2 affinity, synthesizing large quantities of the enzyme, or increasing the CO 2 concentration near it (1-3). Among green organisms (algae and land plants), CO 2 -concentrating mechanisms (CCMs, also referred to as carbonconcentrating mechanisms) have evolved in numerous lineages to increase the CO 2 :O 2 ratio inside their photosynthetic cells. These mechanisms are C 4 photosynthesis, Crassulacean acid metabolism, and the pyrenoids found in algae and hornworts (4, 5).

Horizontal transfer of an adaptive chimeric photoreceptor from bryophytes to ferns

“…S5). This date is far more recent than published divergence estimates between ferns and hornworts (at least 400 Mya) (12) but is congruent with the date estimates for the stem branch leading to Phymatoceros + Nothoceros + Megaceros (85-244 Mya) (13). The disparity in divergence times rejects the hypothesis invoking multiple neochrome origins or losses and reinforces the HGT scenario.…”

“…As recommended by PartitionFinder, the phototropin dataset was partitioned by codon position, each with the GTR+Γ+I substitution model. A total of 15 calibration priors were used (see SI Appendix, Table S2 for details) (8,13,(70)(71)(72)(73)(74)(75)(76)(77), and a birth-death speciation prior was used as the tree prior. We used the uncorrelated relaxed-clock model with rates drawn from a lognormal distribution.…”

Extant diversity of bryophytes emerged from successive post-Mesozoic diversification bursts

“…Sphagnum was employed as an outgroup. For hornworts, sequences from two plastid loci (matK and rbcL) and one mitochondrial locus (nad5) were obtained from two published studies for the 12 genera recognized to date 36,37 . Leiosporoceros was used as an outgroup 36,37 .…”

Spore germination and young gametophyte development of the endemic Brazilian hornwort Notothylas vitalii Udar & Singh (Notothyladaceae - Anthocerotophyta), with insights into sporeling evolution

“…2D); although its gametophytes have pyrenoids (Villarreal & Renner 2012), none were detected in the early stages of sporeling development. The cells continue dividing into several groups of four cells, forming a loose mass, and, subsequently, the thallus.…”

“…To examine sporeling-type evolution among hornwort genera, we optimized this trait using the parsimony method, employing WinClada version 1.00.08 software (Nixon 2002) and a simplified phylogenetic tree from Villarreal & Renner (2012;2013). Five character states were considered: 1) the Leiosporoceros-type, represented by a club-shaped mass of cells; 2) the Anthoceros-type, represented by a columnar mass of cells preceded by a germ tube; 3) the NotothylasPhaeoceros-type, represented by a cylindrical/globose mass of cells; 4) the Megaceros-Nothoceros-type, with a large globose Spore germination and young gametophyte development of the endemic Brazilian hornwort Notothylas vitalii Udar & Singh (Notothyladaceae -Anthocerotophyta), with insights into sporeling evolution mass of cells; and, 5) the Nothoceros-Dendroceros-type, which shows endosporic development (Nehira 1983;Menéndez 1988;Herguido & Ron 1989;Villarreal & Renzaglia 2006;JC Villarreal unpubl.…”