Single-cell analysis of Schistosoma mansoni reveals a conserved genetic program controlling germline stem cell fate

Schistosomes are parasitic flatworms causing one of the most prevalent infectious diseases from which millions of people are currently suffering. Their germline outputs many fertilized eggs, which are both the transmissible agents and the cause of the infection-associated pathology. Given its significance, the schistosome germline has been a research focus for more than a century. Nonetheless, little is known about the molecular mechanisms that regulate its development. Here, we construct a transcriptomic cell type atlas of juvenile schistosomes. This allows us to capture germline stem cells (GSCs) during de novo gonadal development. We identify a genetic program that balances the fate of GSC between proliferation and differentiation. This program is controlled by onecut, a homeobox transcription factor, and boule, an mRNA binding protein. Evaluating this genetic program in schistosomes free-living evolutionary cousin, the planarian, shows that this germline-specific regulatory program is conserved but its function has changed significantly during evolution.