[
International Worm Meeting,
2007]
One of the most important diagnostic character for nematodes related to C. elegans is the pattern of sensory organs—rays (genital papillae) and phasmids—in the male tail. This pattern is highly conserved within a species, but it varies between species and groups of species. Our objective is to establish the ray pattern in the stem species of rhabditids and to reconstruct the evolutionary changes which occurred within this taxon. Specifically, we look at the number of rays, the arrangement of homologous rays, and the position of phasmids relative to rays in adults and during development using DIC microscopy, MH27 antibody staining of adherens junctions, and cell ablations. Projecting the character differences onto a phylogenetic tree for rhabditids, we determine the number and kinds of evolutionary changes. We find that the number of rays was fixed early in rhabditid evolution to 9 pairs, just as the number of digits was fixed early in vertebrate evolution to 5. Later, several losses of rays but no gain occurred. Our methods allow us to homologize individual rays in the different species and thus to reconstruct the changes in their position. A significant change is the pronounced posterior and dorsad displacement of the first ray in diplogastrids. The position of the phasmids was posterior of all rays in the rhabditid stem species. Several changes to a more anterior and more dorsal position occurred within rhabditids. The change is in every case due to a migration of rays and/or phasmids or their precursors during development. The migration can happen early in L1 or L2 (as in diplogastrids), or very late during ray morphogenesis in L4 (as in Brevibucca saprophaga). Some ray pattern characters support relationships which are otherwise only supported by molecular data. Most importantly, an anterior position of the phasmids supports a relationship of Rhabditoides inermiformis and R. regina with Pleiorhabditis.