Comparative investigations of cellular dynamics in the development of medusae (Cnidaria: Medusozoa)

Medusozoans are characterized by the presence of a medusa (jellyfish) stage as part of the life cycle. Despite being a prominent trait in medusozoan evolution, the mechanisms underlying the emergence of this life cycle stage are poorly understood. Characterizing cell proliferation, cell migration and programmed cell death in several disparate species, we found that the development of the medusa displays distinct cellular mechanisms between the scyphozoan and hydrozoan lineages. Using Edu labeling, hydroxyurea treatments and cell tracing we found that in hydrozoans, the development of the medusa relies on extensive cell migration and a distinct pattern of cell proliferation. Using TUNEL assays and pan-caspase inhibitor treatments, we found that in all surveyed hydrozoans apoptosis does not play a role in the development or release of medusa. By contrast, the surveyed scyphozoan medusae undergo extensive apoptosis, and subsequent cell proliferation is required for the formation of the medusa and many of their adult structures. Our results suggest that the difference in medusa developmental trajectories between scyphozoans and hydrozoans stems from distinct cellular mechanisms. Characterization of the medusa stage at different levels of biological organization is thus required to investigate the origin of this complex life cycle stage.