Cuticle ultrastructure differences among the four adult forms of Deladenus nitobei (Tylenchomorpha: Allantonematidae)

Abstract

Summary Deladenus nitobei, a hexatylinid species, is a parasite of the wood wasp Sirex nitobei and a fungal feeder, i.e., the nematode parasitises the wasp to be carried to a new environment (a dead tree) where it propagates after feeding on Amylostereum areolatum, a fungus mutualistically associated with the wasp. The complex life history involves four adult forms: mycophagous male, mycophagous female, insect-infective female, and insect-parasitic female. To understand the morphological strategies of D. nitobei, the cuticle ultrastructure of the four adult forms was observed using transmission electron microscopy (TEM). The structures were compared with those of other hexatylinid species, particularly Fergusobia spp. TEM revealed that the insect-infective females of D. nitobei have a more developed cuticle compared with mycophagous males and females, whereas insect-infective females of Fergusobia do not. The developed cuticle enables D. nitobei infective females to move actively and easily penetrate the host cuticle using their hypertrophied stylet. In terms of parasitic stage, the cuticle structure was typical of an insect-parasitic stage, as in Fergusobia, although with some differences. A remnant cuticle is not found in Fergusobia yet retained in D. nitobei. These structural differences imply that D. nitobei is in the middle of evolving transepidermal nutrient acquisition from per os acquisition, whereas Fergusobia is near the end of this evolutionary trend.