Comparative and experimental embryogenesis of Plectidae (Nematoda).

Comparative analysis of early embryogenesis indicates that considerable differences exist among nematode species. To better understand to what extent the well-studied development of Caenorhabditis elegans is representative for nematodes in general, we extended our earlier studies to other families of this phylum. Here we report our findings on seven species of Plectidae. We found that Plectidae embryos share a number of developmental similarities with one branch of nematodes (Secernentea), including C. elegans, but not with the other branch (Adenophorea), and thus support conclusions concerning their phylogenetic position drawn from molecular data. However, Plectidae also show developmental differences to other Secernentea, suggesting an early separation from them. Prominent characteristics of Plectidae are (1) strict left-right divisions of somatic founder cells generating a prominent early bilateral symmetry and (2) a very early start of gastrulation with immigration of a single gut precursor cell. To determine whether gastrulation with two gut precursors is crucial for C. elegans embryos, we induced it to gastrulate with a single blastomere like in Plectidae. As this alteration is compatible with an essentially normal subsequent embryogenesis, cleavage of the gut precursor before gastrulation is obviously not required. As major differences exist among nematodes concerning the potential to compensate for eliminated early blastomeres, we tested this feature in one Plectus species. We found that Plectus does not replace a lost cell but behaves like C. elegansin this respect, in contrast to our previous findings in Acrobeloides nanus, another member of the Secernentea.