Development and structure of the resting sporangium wall in Coelomomyces dodgei and modification during dehiscence

The mature resting sporangium (RS) wall of Coelomomyces dodgei (Chytridiomycetes; Blastocladiales) consists of three principal layers: (I) an outer pigmented layer (1.8-2.2 microns) that contains polysaccharide, (II) a middle electron translucent layer (1.3-1.6 microns) comparatively free of polysaccharide, and (III) an inner layer (125 nm) rich in polysaccharide that surrounds the meiospores. These layers develop successively between the hyphal coat of the RS initial and its plasma membrane. Prior to meiospore formation, a convex discharge plug that later becomes the dehiscence vesicle forms beneath the inner layer along the preformed dehiscence slit in the outer wall. As dehiscence begins, the RS wall opens along the dehiscence slit, exposing the middle layer which then becomes the rigid outer refractile layer above the dehiscence vesicle. During dehiscence, the discharge plug swells out through the slit, and expands to form the dehiscence vesicle. Meiospores become active as the vesicle expands and are released through tears in the vesicle that result from continued expansion and lysis. Melanized areas observed along the dehiscence slit of RS that initiated dehiscence in the larval hemocoel indicate that the host's defense system is functional but does not recognize the parasite's hyphal coat as foreign, possibly because it lacks any significant level of polysaccharide.