Ultrastructure of the post-corpus of Zeldia punctata (Cephalobina) for analysis of the evolutionary framework of nematodes related to Caenorhabditis elegans (Rhabditina)

The ultrastructure of the post-corpus of Zeldia punctata (Cephalobina) was compared with previous observations of Caenorhabditis elegans (Rhabditina) and Diplenteron sp. (Diplogastrina) with the goal of interpreting the morph ological evolution of the feeding structures in the Secernentea. The post-c orpus of Z punctata consists of six marginal, 13 muscle, five gland and sev en nerve cells. The most anterior of four layers of muscle cells consists o f six mononucleate cells in Z. punctata. The homologous layer in C. elegans and Diplenteron consists of three binucleate cells, suggesting a unique de rived character (synapomorphy) shared between the Rhabditina and Diplogastr ina. Contrary to Diplenteron sp. where we observed three oesophageal glands , Z. punctata and C. elegans have five oesophageal glands. We question this shared character as reflecting a common evolution between the Cephalobina and Rhabditina, because there are strong arguments for functional (adaptive ) convergence of the five glands in these bacterial feeders. Convergence is further suggested by the mosaic distribution of three versus five glands t hroughout the Nemata; this distribution creates difficulties in establishin g character polarity. Although morphological data are often laborious to re cover and interpret, we nevertheless view 'reciprocal illumination' between molecular and morphological characters as the most promising and robust pr ocess for reconstructing the evolution of the Secernentea and its feeding s tructures.