Ultrastructural Changes in Trichospores of Smittium Culisetae and S. Culicis During in Vitro Sporangiospore Extrusion and Holdfast Formation

ABSTRACT

Ultrastructural changes associated with in vitro sporangiospore extrusion (“germination”) and holdfast formation in Smittium culisetae and S. culicis (Trichomycetes) were examined upon treatment of trichospores with potassium at pH 10 (Phase I) followed by a drop in the pH to 7 (Phase II). Phases I and II correspond to sequential conditions encountered by trichospores during passage through the respective midgut and hindgut regions of the insect larval host. In untreated trichospores of S. culisetae, spore bodies were aligned at the apex of the trichospore. Canals in the sporangiospore wall were also present at the apex, along with an interwall layer interposed between the sporangial and sporangiospore walls. No ultrastructural changes were observed at Phase I. Trichospores extruded sporangiospores apically during Phase II and the interwall layer extended outward with the emerging sporangiospore to form the sleeve. Untreated trichospores of S. culicis contained spore bodies scattered throughout the cytoplasm. Canals and an interwall layer were present at both the apex and base of the trichospore. During Phase I, sporangiospores were extruded apically or basally following migration of the spore bodies to either pole of the trichospore. Holdfast formation was restricted to Phase II in both S. culisetae and S. culicis. Disappearance of the spore bodies suggests that they contain the initial holdfast material for attachment to the gut lining. Sporangiospore canals serve as the probable route for expulsion of holdfast material. Holdfasts of S. culisetae adhered to a diversity of substrates in vitro.

Key Words:

• Trichomycetes
• Smittium culisetae
• Smittium culicis
• trichospore
• sporangiospore
• holdfast
• ultrastructure
• insect larva