Revised hypotheses for phylogenetic homology of the stomatostylet in tylenchid nematodes

Molecular phylogenetic systematics suggest that Tylenchida share an immediate ancestor with Cephalobina. This relationship has implications, contradicting classical views, for evolution of the stomatostylet and homology with the cephalobid stoma. Emerging evidence from comparative TEM and 3D modelling is the basis for hypothesising that the tylenchid stylet, specifically the cone and shaft, is homologous with cuticle associated with arcade syncytia of the cephalobid gymnostom; furthermore, the stylet knobs and associated m1 protractors, are prostegostom. The guiding apparatus through which the stylet moves is lined by epidermal syncytia and is homologous with the cephalobid cheilostom. Junctional complexes associated with the epidermal syncytia of the cheilostom and adjacent gymnostom arcade cells occur in both cephalobids and tylenchids, but in the latter the membrane complexes are folded and modified as the guide ring. Testing of the hypothesis requires clearer phylogenetic resolution as well as additional ultrastructural and developmental observations of representatives of tylenchids and outgroups.