Diatoms play an important role in global biogeochemical cycles and oxygen production. In addition, due to their characteristic silica cell wall, diatoms hold immense potential for biotechnological applications. This silica cell wall forces diatoms into sexual reproduction below a species-specific size threshold. Pennate diatoms have a heterothallic mating system in which sexual reproduction occurs between cells of opposite mating types. The two mating types (MT+ and MT-) find each other by producing sex-inducing pheromones (SIP+ and SIP-) that arrest the cell cycle of the opposite mating type to commit to sexual reproduction. Recently, the chemical structure of SIP+ from the diatom S. robusta has been elucidated and was speculated to be synthesized through a non-ribosomal pathway. In this project, we aim to demonstrate that SIP+ is rather derived from a protein precursor and identify its processing enzymes. This will be achieved by generating gain- and loss-of-function lines in the diatom C. closterium. In the second part of the project, we will focus on identifying upstream regulators of SIP+ expression using targeted and untargeted approaches. This will include phylogenetic footprinting, promoter deletion analysis and a forward genetic screen. The results of this project will provide valuable fundamental information on the process of sexual reproduction in diatoms and may provide the basis for controlled diatom breeding to obtain phenotypes of industrial interest.