Fine structure of the phasmid of second-stage juveniles of Heterodera schachtii (Tylenchida: Nematoda)

Abstract

The phasmid of infective second-stage juveniles of Heterodera schachtii consists of an external pore, cuticle-lined duct, socket cell, sheath cell, and single dendrite receptor. Specimens vary in detail and two discrete types, designated A and B, can be distinguished, with A being larger and relatively more complex. In both types of phasmids the socket cell is roughly cup shaped, envelops the duct, and partially encloses and forms junctional complexes with the sheath cell. The sheath cell fills the cup of the socket cell and occurs near the proximal end of the duct. In the A phasmid the sheath cell is deeply invaginated, forming a receptor cavity which is continuous with a maze of extracellular channels penetrating the cell; secretions accumulate in the channels and receptor cavity. Conversely, the sheath cell of the B phasmid lacks a receptor cavity and is nonsecretory. The sheath cell of both types of phasmid encloses and forms junctional complexes with a single dentritic process. In the A phasmid the process terminates within the receptor cavity whereas in the B type the tip is embedded within the sheath cell. A typical basal body or rootlets were not observed, although a ciliary region with a circle of eight doublets surrounding three or four singlets occurs slightly proximal to the terminus. Knowledge of phasmid structure of H. schachtii provides a basis for study of phasmid development and function in a plant parasite; it also contributes to understanding comparative morphology for phylogenetic analysis of Heteroderidae.