| There are several hypotheses of evolutionary relationships among taxa comprising Secernentea, including Cephalobina, Rhabditina, Diplogastrina and Tylenchida. We looked at different bacterial feeding nematodes from these taxa using different tools to perceive their evolutionary relationship. The stoma of Zeldia punctata (Cephalobina) includes four sets of muscular radial cells ma, mb, mc, md in contrast to Caenorhabditis elegans, which has two sets of radial epithelial cells (e1, e3) and two sets of muscle cells (m1, m2). The nuclei in the sets ma, mb, m1 and m2 are "8-shaped" due to fusion. Cell lineage of a nematode closely related, Cephalobus cubaensis, supports the hypotheses that in cephalobids the e1 and e3 cells become hypodermal cells or are programmed to die. We concluded that ma and mb are homologous to m1 and m2 in C. elegans, and we hypothesized that ma and mb could be homologous to primary and secondary sets of stylet-protractor muscles cells in the plant parasitic Tylenchida. We compared the stoma of Bunonema sp. and Teratorhabditis palmarum in relation to C. elegans . The stoma of Bunonema sp. has three sets of radial epithelial cells (e1, e3, e4) and two sets of muscle cells (m1, m2). There is no probable homologue to e4 in C. elegans. In Bunonema sp., there are six separate m2 cells, but further posteriorly the corresponding adjacent pairs of cell processes and nuclei fuse. The stoma of T. palmarum has the same arrangement of cells as C. elegans (e1, e3, m1, m2), although the cells are not fused, only the nuclei of sets in m1 and m2 are fused in pairs. Our results support the hypotheses that T. palmarum is part of a clade separate from other Rhabditidae. Bunonema sp. is the first species studied to display three sets of epithelial cells in the stoma, a character that may be an autapomorphy, since it is not known to occur in other extant taxa. Studying the early embryogenesis of different nematodes, we noticed that spatial patterns at the four-cell stage can be symmetrical or asymmetrical and the germ line can be isolated earlier or later during divisions. The pattern of distribution of those characters shows a great level of congruence with our molecular-based tree, setting Cephalobidae/Tylenchida as a clade exclusive from a Rhabditina/Diplogastrina clade. In addition, we have shown embryonic characters to be less conserved in Secernentea than was previously predicted. |
