Ultrastructural investigation of the spore-forming protist Nephridiophaga blattellae in the Malpighian tubules of the German cockroach Blattella germanica

Multinuclear plasmodia of the sporogenic protist Nephridiophaga blattellae are found intracellularly and in the lumen of the Malpighian tubules of the German cockroach Blattella germanica. Spore formation occurs only in the l umen. During sporogony, about 10-35 spores measuring 5.5 x 3.2 mu m are end ogenously formed within a plasmodium. Sporoblasts arise by the fusion of ci sternae of the endoplasmic reticulum into a double membranous wall. which e ncloses a generative nucleus plus a portion of cytoplasm. Several somatic n uclei remain in the residual cytoplasm. Sporoblast and residual cytoplasm i nclude mitochondria of the tubular type, endoplasmic reticulum, and many fr ee ribosomes. During spore maturation, electron-dense wall material is depo sited between the spore membranes, and the spores gain their typical oval, biconcave form. Freeze-etched spores reveal a small. central. cap-like stru cture. which may be the site where an infectious sporoplasm could emerge. M ature spores always have one nucleus, whereas early sporoblast stages with two small nuclei were found by transmission electron microscopy. Many nucle i of different developmental stages contain granules within the nuclear env elope. The systematic position of N. blattellae is unresolved. In certain r espects it is reminiscent of Haplosporidia. However, the organisms of the t wo groups have different spore-forming processes and haplosporosomes are mi ssing in the nephridiophagids. Therefore a new phylum might have to be erec ted for members of the family Nephridiophagidae.