Pharynx associated musculature in the Stilbonematinae (Desmodoroidea, Nematoda) with focus on the spiral muscle as a synapomorphic trait between Dorylaimia and Chromadoria

Recent molecular phylogenies of Nematoda have suggested a sister-group relationship between Dorylaimia and Chromadoria, but supporting morphological evidence has so far been lacking. Using immunohistochemistry in combination with confocal laser scanning microscopy, we identified pharynx-associated musculature in Stilbonematinae, consisting of buccal dilators, somato-pharyngeal muscles, and a longitudinal spiral muscle encircling nearly the entire pharynx. To trace the evolutionary origin of these structures, we extended our investigation to closely related marine outgroups within Desmodorida but also the basally branching Chromadorida and Enoplia. Somato-pharyngeal muscles, which arise from the body wall musculature and attach to the pharynx surface, function as retractors in Chromadoria and Dorylaimia. In some Enoplia, however, they extend towards the posterior pharynx and act as protractors. These muscles are homologous and thus represent part of the ancestral body plan of Nematoda. Homologous buccal dilators and spiral muscles occur in both Dorylaimia and Chromadoria, whereas in Oncholaimina (Enoplia) buccal dilators-acting as protractors-have evolved independently. Taken together, these morphological findings strongly support a sister-group relationship between Dorylaimia and Chromadoria. A statistical analysis of the Stilbonematinae pharynx and its spiral musculature showed that the number of spiral coils is strongly correlated with pharynx slenderness (length-to-width ratio), in line with Roggen's pharynx model. In both Dorylaimia and Chromadoria, the spiral musculature surrounding the posterior pharynx likely generates injection pressure, aiding the posterior transport of ingested food into the intestine as well as the anterior movement of secretions from pharyngeal glands.