Bilcke, G.; Cleyman, A.; Rijsdijk, N.; Forment, T.; Eekhout, T.; Belisova, D.; Chaerle, P.; Grones, C.; Audoor, S.; Van Bel, M.; Porters, J.; Manosalva Perez, N.; Mylle, E.; Van Damme, D.; De Rybel, B.; De Veylder, L.; Vyverman, W.; Vandepoele, K.

Diatoms are the foundation of aquatic food webs and contribute about 40% of the total marine primary productivity. Yet, the regulation of their complex size-dependent life cycles remains obscure. Here, we leveraged single-cell transcriptomics and transgenic reporter lines to uncover the molecular mechanisms behind partner recognition, nuclear fusion, and the remarkable 15-fold size expansion of auxospores. Gene regulatory network inference revealed that the irreversible commitment to differentiate into gametes is controlled by Myb transcription factors, whose specific activity in the global ocean underscores their significance for ploidy transitions across diatom clades. These findings reinforce microalgae as powerful models to study cell fate transitions and provide a mechanistic framework for the life cycle dynamics that underpin the functioning of aquatic systems worldwide.