COMPARATIVE ULTRASTRUCTURE OF AURANTIACTINOMYXON AND RAABEIA, ACTINOSPOREAN STAGES OF MYXOZOAN LIFE-CYCLES

The ultrastructure of two actinosporean stages, Aurantiactinomyxon sp. (A), and Raabeia (R) forming part of the life cycle of Myxobolus cult us YOKOYAMA, OGAWA & WAKABAYASHI, 1995 has been studied. In both A and R, binucleate early stages were observed, their proliferation was not detected. In A, intracellular early stages were also found. Earliest stage of pansporocyst formation in R was a uninucleate cell enveloped by an outer one which revealed surface villosities. The latter may be instrumental in facilitating transport of nutrients to sporoblast cell s. Cell junctions exist in A and R between pansporocyst cells (gap jun ctions), attaching valvogenic cells to capsulogenic cells (gap junctio ns) and between valvogenic cells (septate junctions). The very simples t myxozoan cells represent the ''polar bodies'' resulting from the mei otic process; in its course, synaptonemal complexes were recorded in R . The structural features, even those little known ones recorded in A and/or R (fibres spaced at 11 nm running on the surface of the immatur e polar filament, meridional ridges on the upper part of the capsular primordium, the apex of polar capsule consisting of inner dense cone c overed by a layer of microtubules, malformations in the development of the capsule, close association of the sporoplasm vegetative nuclei an d infective cells, iodinophilous vacuole-like agglomerations of glycog en in the sporoplasm) show the unity of cell structures in myxozoa exp ressed in the course of the actino-myxosporean alternation. The unique features observed were viral inclusions in A capsulogenic cells, spec ial modifications of RER cisternae and vesicles with stacks of small d iscs. Infective cells in the sporoplasm can be compared with the inner cells of myxosporean proliferative cycles, both constituting stem cel ls endowed with extraordinary morphogenetic potential.