Genome-wide analysis of the diatom cell cycle unveils a novel type of cyclins involved in environmental signaling

By profile-based annotation of cell cycle genes, counterparts of conserved as well as new regulators were identified in T. pseudonana and P. tricornutum. In particular, the cyclin gene family was found to be expanded extensively compared to that of other eukaryotes and a novel type of cyclins was discovered, the diatom-specific cyclins. We established a synchronization method for P. tricornutum that enabled assignment of the different annotated genes to specific cell cycle phase transitions. The diatom-specific cyclins are predominantly expressed at the G1-to-S transition and some respond to phosphate availability, hinting at a role in connecting cell division to environmental stimuli.The discovery of highly conserved and new cell cycle regulators suggests the evolution of unique control mechanisms for diatom cell division, probably contributing to their ability to adapt and survive under highly fluctuating environmental conditions.Diatoms (Bacillariophyceae) are unicellular photosynthetic eukaryotes responsible for approximately 20% of the global carbon fixation [1,2]. They belong to the Stramenopile algae (chromists) that most probably arose from a secondary endosymbiotic process in which a red eukaryotic alga was engulfed by a heterotrophic eukaryotic host approximately 1.3 billion years ago [3,4]. This event led to an unusual combination of conserved features with novel metabolism and regulatory elements, as recently confirmed by whole-genome analysis of Thalassiosira pseudonana and Phaeodactylum tricornutum [5-7], which are representatives of the two major architectural diatom types, the centrics and the pennates, respectively.Besides their huge ecological importance, diatoms are interesting from a biotechnological perspective as producers of a variety of metabolites (including oils, fatty acids, and pigments) [8,9], and because of their highly structured mesoporous cell wall, made of amorphous silica [10]. Thus, understanding the basic mechanisms controlling t