Developmental Reversal May Be More Common than Thought in Lower Animals
Lower animals that are essentially tiny blobs of stem cells are in principle capable of immortality, as illustrated by hydra and the jellyfish Turritopsis dohrnii that cycles from immaturity to adulthood and back again. Even if not actually immortal, their potential life spans absent predation are too long to be experimentally confirmed in any practical way. These species are so very different from higher animals with complex nervous systems that store data that it is quite possible there is little of practical use to medicine to be learned here. A blob of stem cells can continually regrow and replace all of its component parts, or change its shape and state of maturity as needed, whereas a vertebrate is more vulnerable, more locked in to its structure and the state of that structure. We might think that aging has the look of an inevitable consequence of having a nervous system that stores data, coming to rely on the structural state of that tissue, no longer being able to easily discard cells and replace them.
To date, the capacity of one life cycle stage to transform back to the preceding stage by morphological reorganization has been regarded as a distinctive and unparallelled feature of cnidarians. This ability for reverse development known for a few cnidarian species was first reported over a century ago and gained wide renown with the discovery of the peculiar life cycle of the so-called immortal jellyfish, Turritopsis dohrnii. This hydrozoan is currently considered the only animal able to repeatedly rejuvenate after sexual reproduction, challenging our understanding of aging and suggesting a potential for biological immortality.
Ctenophores (or comb jellies) are one of the oldest extant animal lineages. Accumulated evidence supporting their phylogenetic position as the sister group to all other animals place them as a pivotal model to study unique evolutionary innovations potentially rooted within the deepest branches of the animal tree of life. Here, we demonstrate that the ctenophore Mnemiopsis leidyi is capable of reversal from mature lobate to early cydippid when fed following a period of stress. Our findings illuminate central aspects of ctenophore development, ecology, and evolution and show the high potential of M. leidyi as a unique model system to study reverse development and rejuvenation.