From muscle to gland: The reorganization of the terminal bulb within the symbiotic Stilbonematinae (Nematoda, Desmodoroidea)

The Stilbonematinae live in symbiosis with ectosymbiotic bacteria covering their cuticle, which evidently constitute their food. In different Stilbonematinae genera, two pharynx types are found, depending on the arrangement of the bacterial coat. Species descriptions show that most Stilbonematinae species with a thick multilayer of symbionts have a two-part pharynx with a predominantly muscular posterior bulb. In contrast, in cases of a thin monolayer of bacteria, the nematodes predominantly show a three-part pharynx with a distinctly swollen muscular corpus at their anterior end. This indicates a shift of the main pumping structure from the terminal bulb to the anterior corpus. Consequently, the amount of contractile filaments in the terminal bulb should decrease. Using phalloidin staining in combination with confocal laser scanning microscopy, light microscopy, and transmission electron microscopy, we measured and compared the filamentous actin (F-actin) volume in the posterior bulb in several Stilbonematinae species representing both pharynx types. Two-part pharynges had a larger relative F-actin volume in the terminal bulb than three-part pharynges. In the latter, prominent gland tissue occupied most of the space between the reduced muscles. This supports our hypothesis of two distinct feeding modes: ingestion of large amounts of food in species with a two-part pharynx (“gourmands”) requiring a muscular terminal bulb vs discriminant grazing on a thin bacterial coat in species with a three-part pharynx (“gourmets”).