Ultrastructure of the Lagena radicicola zoospore, including a comparison with the primary and secondary Saprolegnia zoospores

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Abstract
The zoospore fine structure of Lagena radicicola Vanterpool & Ledingham is compared with the primary and secondary zoospores of Saprolegnia diclina Humphrey. The L. radicicola zoospore is broadly pyriform, it has two subapically attached flagella, a pyriform nucleus that has a fine fibrillar attachment to the posterior kinetosome, and a Golgi body next to the nucleus. There are various vesicles including dense body vesicles and several small expulsion vacuoles but there are no U- or K-bodies. It has a well-organized rootlet system comprising anterior and posterior doublet microtubules, a ribbed triplet that probably provides a cytoskeletal frame for the plasmalemma, and a multistranded, band-shaped rootlet of 4 to 10 microtubules. Cytoplasmic microtubules are very sparse. Interpretation of the rootlet systems was aided by computer graphics; 3-D images of the rootlet system were produced as stereo pairs on a Kontron image analyzer. Based on cell shape, position of the flagella, and angle of kinetosomes classification of the L. radicicola zoospore as either a primary or secondary type is imprecise; however, the various kinds of rootlets are similar to those found in the secondary zoospore of S. diclina but differ in structural detail and orientation. The transition zone in the flagellum of L. radicicola is simple and lacks the typical concertina body found in most oomycete flagella. The relationship of L. radicicola to the Oomycetes and various heterokont algal groups, based on comparative zoospore ultrastructure, is discussed.