Journey beyond the embryo: The beauty of Pristina and naidine annelids for studying regeneration and agametic reproduction

Regeneration, asexual agametic reproduction, and other forms of postembryonic development are ecologically important and widely variable across animals, yet our understanding of this important aspect of animal diversity remains limited. A significant limitation has been the dearth of adequate study systems for exploring the mechanisms and evolution of these processes. Here I describe key parts of our journey in developing naid annelids as a study system for investigating the evolution and development of regeneration and fission. Naids are small freshwater annelids that provide numerous advantages for studying postembryonic development: they are small and reproduce readily by fission, they include species with diverse regenerative abilities, and many species are easy to culture. Among the naids, Pristina leidyi is a particularly useful study species, being largely transparent and quite robust to a variety of experimental manipulations. Building on a sparse but long history of past research on these animals, we have developed this system by establishing methods and generating resources for working with them. Naids are yielding novel insights into the evolution of regeneration and fission, providing one of many examples of the value of developing new study species to enable the exploration of fundamental and understudied questions in biology. Establishing new study systems comes with challenges but is exciting and rewarding, and I provide perspectives from my own experiences with the hope of encouraging the further expansion of study systems in biology.

How the discovery of oligochaete regeneration during the Enlightenment was pivotal to the advancement of annelid research

Within a few months of the year 1741, Lyonet and Trembley in Holland, Bonnet in Geneva and Réaumur in Paris all experimented industriously with the regeneration of various invertebrates, including the still unknown microdrile oligochaetes. Some of these worms were prone to spontaneous fragmentation as a natural mode of asexual multiplication, preceded (paratomy) or followed (architomy) by regenerative processes. Bonnet and Réaumur raised the stakes and began to study regeneration in earthworms, a challenge that, especially in Italy, inspired scholars and educated people among the clergy and aristocracy. Spallanzani conducted the most rigorous experiments and was the first to obtain consecutive regenerations of the head in one and the same megadrile. He saw detached mid-body sections becoming complete with a new head and a new tail, and he determined the points of the body where regeneration does not take place and how much length to leave for each section so that it could reintegrate. Above all, Spallanzani obtained regeneration of the gonadic segments, both in terrestrial and in freshwater megadriles, the latter today identified as Criodrilus lacuum. This and other important anatomical and physiological findings by Spallanzani, largely unpublished during his lifetime and for a long time after, preceded the discoveries of later authors by almost 50 years.