Ultrastructure of Polyphagus euglenae zoospores

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Abstract
Zoospore ultrastructure of Polyphagus euglenae Nowakowski, a parasite of algae, was studied using transmission electron microscopy. The elongate zoospore contains a centrally located nucleus covered by a ribosomal aggregation. Membranes, continuous with the nuclear envelope, concentrically traverse the ribosomal aggregation and form a lateral membranous labyrinth. In the posterior end of the zoospore, a single prominent microbody–lipid globule complex (MLC) lies adjacent to the nucleus. Oval microbodies cover the anterior aspect of the lipid globule, and a fenestrated cisterna (the rumposome) covers the posterior aspect of the lipid globule. Several of the elongate and branched mitochondria that bind the ribosomal aggregation are near the microbodies, and these mitochondria are also part of the MLC. The flagellar apparatus includes a parallel oriented kinetosome and secondary centriole, as well as a rootlet that extends laterally between the kinetosome and the plasma membrane and passes over a portion of the rumposome. Regularly banded and homogeneous configurations of the rhizoplast are found. Cored vesicles and vesicles containing heterogeneous inclusions are scattered in the outer regions of the cell. In the diaminobenzidine cytochemical technique for the visualization of catalase, microbodies are lightly stained, and this staining is inhibited by aminotriazole. Inflated regions of the nuclear envelope and ribosomal aggregation cisternae also contain electron-dense reaction product. Because staining of endomembranes is inhibited by sodium azide but not by the inhibitor of catalase activity, aminotriazole, it is concluded that catalase, which may have low activity, is present only in microbodies. Because of the distinctive structure of the flagellar apparatus, ribosomal aggregation and cisternal system as well as the position of the MLC, zoospore ultrastructure of P. euglenae is clearly distinct from any other chytrid zoospore that has been studied.

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