The embryonic moult in diplogastrids (Nematoda) - homology of developmental stages and heterochrony as a prerequisite for morphological diversity

Video analysis of developing eggs showed that diplogastrids, in contrast to the majority of nematodes, moult from J1 to J2 before they hatch from the egg. This embryonic moult leads to a conflict between the terminology for the J1-J4 postembryonic stages and the definition for the embryonic period that ranges from fertilization to hatching. The nature of developmental stages is discussed on the occasion of this terminological problem. Stages as “embryo”, J1, J2,… are defined as periods of time between certain developmental events and are called instars, whereas other stages as “tadpole stage” refer to characters. Character-defined stages refer to the temporal aspect of developmental characters. Only character-defined stages can be homologized between species. It is proposed to ignore the conflict of the J1-J4 terminology with the embryonic-postembryonic dichotomy as an embryonic stage cannot be satisfyingly defined. The biological relevance of the heterochronic shift of the first moult into the egg period is the possibility to omit secretion of the J1 stoma and pharynx cuticle as the diplogastrid J1 does not feed. Thereby the time slot for stoma morphogenesis that must be finished before cuticle deposition is prolonged. Within Diplogastridae this time slot facilitated the evolution of a tremendous diversity of complex stoma morphologies that is unique within “Rhabditida”.