An Ultrastructural Study of Sporidium Formation during Infection of a Rhabditid Nematode by Large Gun Cells of Haptoglossa heteromorpha

Recently fired gun cells of Haptoglossa heteromorpha, an aplanosporic nematode parasite, were examined ultrastructurally. The everted tubes of the fired cells had penetrated the cuticle of a nematode, and infective sporidia were developing inside the host body. The nematode cuticle was penetrated by the narrow, walled part of the tube below the needle chamber. The lower unwalled part of the tube tail formed the sporidium. The developing sporidium had a multilayered fibrous outer coating and the plasma membrane was separated from the wall in places. Sporidia contained biphasic membrane-bound vesicles that had been generated by the Golgi dictyosome during gun cell development. Immediately following gun cell firing, the nuclear envelope of the sporidium nucleus was not apparent, and the sporidium nucleus contained clusters of electron-dense particles concentrated in the nucleolar region. We compare the structures and organelles found in the mature gun cell with those in the fired cell and attempt to identify the membranous layers around the sporidium.