Ultrastructure of mature and fired gun cells of Haptoglossa mirabilis

Haptoglossa mirabilis is a parasitic oomycete which infects its rotifer host by means of a specialized vegetative cell, the gun cell. The dominant feature of gun cell architecture is the attack apparatus, which consists of two extracellular components, the injection tube and the missile, and an intracellular component, the vacuolar system. At the anterior end of the cell, the wall is deeply invaginated to form the various sections of the injection tube: the bore, the missile chamber, the tail housing, and the hollow tail which is folded within the housing. The missile, also complex, is attached anterior to the tail and rests within the missile chamber at the apical end of the cell. The vacuolar system includes the posterior, anterior, and apical vacuoles. On attack, the missile is fired from the cell; the injection tube is completely everted and a portion of the protoplast is propelled at high speed into the host, in the form of a walled sporidium. A model is proposed to explain the mechanism of action of...