Ultrastructural morphogenesis of dimorphic arcuate infection (gun) cells of Haptoglossa erumpens an obligate parasite of Bunonema nematodes

Abstract

Haptoglossa is a genus of biflagellate organisms that has been placed in the oomycetes and is characterised by producing unique infective gun cells, which usually infect by physically rupturing the nematode cuticle. Haptoglossa erumpens is a parasite of Bunonema nematodes that produces arcuate infection cells and aplanospores that are discharged following the swelling and rupture of the thallus wall and distended host cuticle. Recent isolations of H. erumpens have revealed that the germinating aplanospores develop into two similar-sized but morphologically distinct infection cells. The uni-nucleate, convexly arcuate, gun cells were observed to fire in response to host nematodes, producing a cylindrical sporidium inside the host body. These gun cells had an apical missile chamber containing a needle with a unique arrangement of investing cones. Unlike previously described gun cells, the tube tail did not wind around the nucleus but continued into the basal vacuole where it terminated. The second type of infection cell was a concavely arcuate, bi-nucleate, cell that had an unusually large and elongate annulus component in the missile chamber. These modified bi-nucleate gun cells were never observed to fire in response to contact with Bunomena nematodes. The patterns of morphological and structural variations in these infection structures in this genus are reviewed in the light of these findings.