Life history and morphology of Ichthyophonus hoferi in vitro.

Abstract

The varied developmental forms of Ichthyophonus hoferi induced in culture including amoeboblasts, amoeboid bodies, plasmodium-like bodies, spherical uni-or binucleate bodies, thick walled spherical multinucleate bodies and spherical multinucleate hyphal terminal bodies that were observed under different cultural conditions are described. When the thick walled spherical multinucleate bodies were seeded into MEM containing 10% fetal bovine serum at pH 7-9, there was minimal hyphal growth following germination, which produced spherical uni-or binucleate bodies subsequently developing into the thick walled spherical multinucleate bodies. At pH 3-5 in the same MEM medium the growth was characterized by abundant hyphal growth and the formation of spherical multinucleate hyphal terminal bodies at the tips of each hypha. These hyphal bodies differed from the original spherical multinucleate bodies in cell wall thickness. However, the spherical multinucleate hyphal terminal bodies produced at pH 3 or 5 could germinate to form more hyphae at these low pH levels but those produced at pH 3 when shifted to a higher pH of 7-9 did not produce hyphae but only formed the plasmodium-like and oval amoeboid bodies. When the spherical multinucleate hyphal terminal bodies were seeded into TGC medium at pH 7, the growth was characterized by production of a thick (15-20μm) and abundant hyphal growth on which were produced identical spherical multinucleate hyphal terminal bodies which had subsequent growth characteristics as those produced in MEM at pH 5. Our observations shows that differences such as the extent and type of hyphal growth and the size and structure of spherical bodies produced reflect the available nutrients and physical conditions such as pH surrounding I. hoferi. The thick walled spherical multinucleate bodies produced in culture is probably identical to the thick walled forms observed in infected fish and described previously by a number of different names such as “resting spore”, “latent cyst”, and “large multinucleate bodies”.