Ablation of a maternal Cryptosporidium mRNA-binding protein results in sterile sporozoites

Infection with Cryptosporidium is a leading cause of diarrheal disease and early childhood mortality. This apicomplexan parasite undergoes asexual and sexual replication within the same host and recent studies have shown an intrinsic developmental program of obligate transition to male and female gametes and sex. While factors were identified that control male fate and development, how female gene expression is orchestrated remains largely unknown. Here we use the Cryptosporidium Single Cell Atlas to discover an RNA binding protein (F-RBP) as one of the earliest markers of female identity. Reporter parasites engineered based on this gene allowed us to calibrate transcriptional pseudotime against the real time of female development revealing a significant window of transcriptional fate ambiguity. While F-RBP is an early transcript, the protein persists throughout female development and into the zygote. Conditional ablation of the F-RBP gene showed it to be dispensable for sex determination and early female development in vitro. However, the gene is essential in vivo and its loss results in rapid cure. Cell biological experiments link this loss to the production of sterile oocysts which release sporozoites incapable of host cell invasion. F-RBP binds transcripts highly expressed in the female gamete enriched for a YBOX primary sequence motif and forms mRNA protein complexes in late females akin to processing or P bodies. We propose F-RBPs essential role to be in the regulation of long-term homeostasis of maternally inherited RNA required for sporozoite infectivity.