INJECTION TUBE DIFFERENTIATION IN GUN CELLS OF A HAPTOGLOSSA SPECIES WHICH INFECTS NEMATODES

The gun cells which develop from germinating cysts in Haptoglossa prod uce a specialized infection apparatus, the injection tube. Upon eversi on this tube fires a missile-like projectile which penetrates the host cuticle and then forms an infective sporidium within the body cavity of the nematode host. The temporal assembly of this complex cell organ elle has been determined by serial-section reconstructions of maturing gun cells in a previously undescribed Haptoglossa species, The differ entiation of the partially walled inverted injection tube is an unusua l example of internal tube growth, in which membrane and wall assembly are temporally separated, There is no evidence that the shape of this inverted tube, which coils around the nucleus until it doubles back o n itself, is dictated by the disposition of cytoplasmic microtubules. However, actin-like material was associated with the delimiting membra ne of the differentiating tube, particularly in the regions of extensi on. From these studies it seems likely that the ''head and buttress'' structures previously depicted as the barbed tip of the ''harpoon-like '' penetration missile are part of a separate, structurally complex sy stem which we suggest locks the ''missile'' into position in the invag inated injection tube, From this detailed account of cell architecture , models for the likely mechanism of infection cell firing are discuss ed, and unresolved questions relating to the cell biology and biochemi stry of these complex organelles are highlighted. (C) 1998 Academic Pr ess.