Decoupling of fission and regenerative capabilities in an asexual oligochaete

Agametic reproduction by fission has evolved numerous times in the Annelida. In this group, as well as others, the developmental processes of fission appear to be based on those of regeneration. Thus far, all fissiparous annelids whose regenerative abilities have been investigated fully are capable of regenerating both anteriorly and posteriorly. However, in this paper, I describe a unique exception to this association between fission and regenerative abilities. Regeneration experiments demonstrate that the fissiparous annelid Paranais litoralis (Oligochaeta: Naididae) has lost the capacity for full anterior regeneration. The four anterior segments of P. litoralis are produced routinely during each round of fission, yet this species is incapable of producing these same segments by anterior regeneration. I propose that fissiparous reproduction, possibly coupled with a low susceptibility to amputation in nature, may have relaxed selection for anterior regeneration in P. litoralis. The decoupling of fission and regenerative abilities suggests that some steps in the developmental trajectories that lead to fission and regeneration may have diverged in this species.