Changes of cell-type diversity in the polyp-to-medusa metagenesis of the scyphozoan jellyfish Aurelia coerulea (formerly sp.1).

The life cycle of most medusozoan cnidarians is marked by the metagenesis from the asexually reproducing sessile polyp and the sexually reproducing motile medusa. At present it is unknown to what extent this drastic morphological transformation is accompanied by changes in the cell type composition. Here, we provide a single cell transcriptome atlas of the cosmopolitan scyphozoan Aurelia coerulea focussing on changes in cell-type composition during the transition from polyp to medusa. Notably, this transition marked by an increase in cell type diversity, including an expansion of neural subtypes. We find that two families of neuronal lineages are specified by homologous transcription factors in the sea anemone Nematostella vectensis and Aurelia coerulea, suggesting an origin in the common ancestor of medusozoans and anthozoans about 500 Myr ago. Our analysis suggests that gene duplications might be drivers for the increase of cellular complexity during the evolution of cnidarian neuroglandular lineages. One key medusozoan-specific cell type is the striated muscle in the subumbrella. Analysis of muscle fiber anatomy and gene expression raises the possibility that the striated muscles arise from a population of smooth muscle cells during strobilation. Although smooth and striated muscles are phenotypically distinct, both have a similar contractile complex, in contrast to bilaterian smooth and striated muscles. This suggests that in Aurelia, smooth and striated muscle cells may derive from the same progenitor cells.