PHYSIOLOGICAL-CHANGES ASSOCIATED WITH SPORANGIOSPORE EXTRUSION FROM TRICHOSPORES OF SMITTIUM-CULISETAE

Trichospores of the fungus,Smittium culisetae (Trichomycetes), require a two-phase sequence of stimuli for sporangiospore extrusion: Phase I (20 mM KCl, pH 10) and Phase II (pH 7). Phases I and II correspond to conditions within the respective midgut and hindgut regions of mosquito larval hosts. In this study, trichospore pressure increased 1.0 MPa during Phase I but sporangiospores were not extruded. Potassium uptake was insufficient to account for the pressure increase. Sporangiospore extrusion from trichospores was stimulated by a drop in pH at Phase II. Lowering the osmotic potential of the Phase II solution with sucrose inhibited sporangiospore extrusion to varying degrees, with total inhibition occurring at osmotic potentials ≤ -2.7 MPa. At an osmotic potential of -2.1 MPa, the interwall layer at the trichospore apex swelled at the point of sporangial rupture without emergence of the sporangiospore. Interwall layer swelling during Phase II may rupture the sporangial wall of the trichospore and allow the sporangiospore to be extruded by its Phase I pressure. © 1990 Academic Press, Inc.