Abstract

Schistosomes are parasitic flatworms whose infections affect millions of people worldwide. The parasite has a complex life cycle that is poorly understood at a cellular level. We have developed a cellular transcriptome atlas of the miracidium larva of Schistosoma mansoni, the first post-embryonic stage of development. The miracidium infects a snail host, inside which its stem cells generate daughter sporocysts that give rise to human-infective cercariae larvae. We use a combination of single-cell RNA sequencing, in situ hybridisation and image analysis to characterise the miracidia. Each larva comprises ∼365 cells and 19 transcriptionally distinct cell types. We show that 93% of miracidium cells are somatic (>45% neural, 19% muscle, 13% tegument, 2% parenchyma, 2% protonephridia) and the remaining 7% are stem cells. Cellular diversity within tissue types is revealed, including two transcriptionally distinct muscle clusters, corresponding to the circular and the longitudinal muscles of the body wall, as well as two stem cell populations that show different activation states and cluster further by sex. This whole-body atlas provides new insight into the organisation of the larva and lays the foundation for understanding how the somatic cells drive the behaviour and physiology necessary for infection and propagation of the stem cells inside the snail.