Morphology and ultrastructure of the bacterial receptacle in Steinernema nematodes (Nematoda: Steinernematidae).

Infective juveniles of entomopathogenic nematodes in the genus Steinernema harbor symbiotic bacteria, Xenorhabdus spp., in a discrete structure located in the anterior portion of the intestine known as the 'bacterial receptacle' (formerly known as the bacterial or intestinal vesicle). The receptacle itself is a structured environment in which the bacteria are spatially restricted. Inside this receptacle, bacterial symbionts are protected from the environment and grow to fill the receptacle. Until now, no comparative study across different Steinernema spp. has been undertaken to investigate if morphological variation in this structure exists at the interspecific level. In this study, we examined the bacterial receptacles of 25 Steinernema spp. representatives of the currently accepted five evolutionary clades. Our observations confirmed the bacterial receptacle is a modification of the two most anterior cells of the ventricular portion of the intestine. Size of the bacterial receptacle varied across the examined species. Steinernema monticolum (clade II) had the largest receptacle of all examined species (average: 46×17 μm) and S. rarum (no clade affiliation) was noted as the species with the smallest observed receptacle (average: 8×5 μm). At the morphological level, species can be grouped into two categories based on the presence or absence of vesicle within the receptacle. The receptacles of all examined species harbored an intravesicular structure (IVS) with variable morphology. All examined taxa members of the 'feltiae' (clade III) and 'intermedium' (clade II) clades were characterized by having a vesicle. This structure was also observed in S. diaprepesi (clade V), S. riobrave (clade IV) and S. monticolum (clade I).