Molecular fingerprints of cell size sensing and mating type differentiation in pennate diatoms

A unique cell size sensing mechanism is at the heart of the life cycle of diatoms. During population growth, cell size decreases until a Sexual Size Threshold (SST) is reached, below which cells become sexually competent. In most pennate diatoms, two genetically determined mating types undergo biochemical and behavioral differentiation below the SST, although the molecular pathways underlying their size-dependent maturation remain unknown. Here, we developed a method to shorten the generation time of Cylindrotheca closterium through experimental cell size manipulation, allowing the comparison of genetically identical large and undifferentiated cells with small, sexually competent cells. Transcriptome profiling of six different genotypes revealed a set of 112 size- and mating type-responsive genes. We identified 21 genes upregulated in small cells regardless of the mating type, revealing how cells undergo significant transcriptional reprogramming when passing the SST. Among genes with a mating type specific response, about a third was only expressed below the SST, including a cluster with three sex inducing pheromone-responsive genes. In addition, comparative transcriptomics with the pennate diatom Seminavis robusta revealed a shared mating type specificity of Mating-type Related Minus 2 (MRM2) homologs. A combination of phylogenetics, protein structure prediction and transcriptomic profiling during reproduction suggests that these leucine-rich repeat receptor-like proteins are part of a conserved partner-recognition mechanism in pennate diatoms. In conclusion, this study illustrates how different mating types acquire sexual competence in a strictly size-dependent manner, revealing part of the complex machinery underlying size-dependent maturation, mating behavior and the origin of heterothally in pennate diatoms.