Ultrastructure of the esophagus of Diplenteron sp (Diplogasterida) to testhypotheses of homology with Rhabditida and Tylenchida

The ultrastructure of the isthmus and basal bulb (postcorpus) of Diplentero n sp. (Diplogasterida) was revealed through transmission electron micrograp hs of serial sections. The postcorpus is glandular and muscular. There are 26 cells in the postcorpus, including 6 marginal (two sets of three), 6 mus cle (two sets of three), 3 gland, and 11 nerve cells. Most of the cell bodi es, including the nuclei, are in the basal bulb. Unlike Caenorhabditis eleg ans, Diplenteron sp, has three gland cells. The glands are embedded in a mu scular framework in both tars, but each gland cell is much bigger in Diplen teron sp. than in C. elegans. Each of the anterior set of three marginal ce lls is located at the apex: of the esophageal lumen and overlaps slightly w ith one of the posterior sets of three marginal cells. All six marginal cel ls in Diplenteron sp. have homologs in C. elegans. The anterior set of radi al muscle cells is V-shaped and is homologous to m5 muscle cells in C. eleg ans. The posterior set of muscle cells appears to be homologous to m6 muscl e cells in C. elegans. Diplenteron sp. does not have muscle cells correspon ding to the m7 cells associated with the "grinder" in C. elegans, which is absent in diplogasterids. The single saucer-shaped muscle cell, m8, coverin g the posterior wall of the basal bulb in C. elegans was not observed in Di plenteron sp. The structure of the esophageal-intestinal junction in Diplen teron sp. is similar to that of C. elegans in being composed of five epithe lial cells. Neurons appear to be more abundant in Diplenteron sp. than in C . elegans. Ultrastructure of the esophagus in diplogasterids, rhabditids, c ephalobids, and tylenchids will be useful in testing classical and recent c ompeting hypotheses of secernentean phylogeny.