Meeting Abstract
Life-history evolution results from innovation in developmental trajectories enabling lineages to explore and adapt to available ecological niches. Annelids show a wide array of embryonic and post-embryonic developmental strategies to survive and thrive in land and sea. Mapping these strategies to current phylogenies of the phylum suggests that some novel developmental trajectories have evolved independently many times in several clades, while completely absent in others. Agametic reproduction by fission is an example of such a novel trajectory. Annelids have evolved fission numerous times, yet morphogenetic analyses reveal surprising similarities across independent gains. Why does fission present such remarkable developmental convergence? As a case study, I present data from observations of morphogenetic events during regeneration and fission in several species of clitellate annelids spanning at least three independent gains of fission. Post-amputation morphogenesis is mostly similar in all species capable of regeneration, and is characterized by wound healing, local gut de-differentiation, blastema formation, restoration of anterior central nervous system neuropil and longitudinal muscle by elongation of pre-existing tissue, and de novo formation of brain, ventral ganglia and circular muscle. Among fission trajectories, elements shared with regeneration are similar, but novel processes show differences that correlate with separate origins. These data strongly support that fission has evolved by co-opting most of regeneration’s developmental trajectory. Furthermore, they suggest that evolution of this novel trajectory is contingent on the pre-existence of good regenerative abilities, and explain the absence of agametic asexual reproduction in annelid lineages with poor regeneration.
