Diverse pyrenoids in Chloromonas lineage (Volvocales, Chlorophyceae)

Abstract

Pyrenoids found in chloroplasts of various algae and hornworts are suborganelles composed mainly of RubisCO. Pyrenoids contribute to the CO₂-concentrating mechanism that allows efficient photosynthesis; however, pyrenoids have been independently lost in various lineages of algae and in the origin of land plants. Recently, it was suggested that the hydrophobicity of two helices on the RubisCO small subunit (RBCS) surface is important for pyrenoid formation in the green alga, Chlamydomonas reinhardtii. Furthermore, several molecules were reported to be important for pyrenoid formation, such as EPYC1 which mediates RubisCO aggregates. However, details of molecular basis of formation and evolutionary loss of pyrenoids remain unclear. In this article, I introduce Chloromonas lineage, a potential model lineage for studying the evolution of pyrenoids. The Chloromonas lineage belonging to the order Volvocales with Chlamydomonas reinhardtii, consists of several genera/clades that differ in pyrenoid characteristics, such as the Reticulata group, which contains both pyrenoid-possessing and pyrenoid-lacking species, and the genus Gloeomonas, which, unlike Chlamydomonas reinhardtii, has several pyrenoids without starch sheaths. In addition, I also introduce our recent study which compared RBCS sequences of three pyrenoid-containing and two pyrenoid-lacking species within the Reticulata group.