A comparative account of cyst coat ontogeny in saprophytic and fish-lesion (pathogenic) isolates of the Saprolegnia diclina – parasitica complex

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Abstract
The ultrastructural development of encystment vesicles (formerly bar bodies) and the outer cyst coat has been examined in primary and secondary spores of a range of isolates of Saprolegnia, mostly from the S. diclina – parasitica complex, of both saprophytic and fish-lesion origin. The development of both primary encystment vesicles (PEVs) in zoosporangia and secondary encystment vesicles (SEVs) in primary cysts appears essentially similar. The vesicles are probably derived from the rough endoplasmic reticulum system. The tubular primary spines and solid secondary boathooks seem to form autonomously within the vesicle matrix. Upon settling all of the preformed encystment vesicles are liberated from the zoospores to give rise to an electron-dense, amorphous, outer cyst coat, in which the spines are embedded. Fish-lesion isolates of S. parasitica have longer tubules on their primary cysts than saprophytic isolates and have distinctive bundles of long (<10 μm) boathooks on their secondary cysts compared with the single short (< 1.0 μm) boathooks typical of the saprophytes. However, even in fish-lesion isolates considerable variation in SEV and boathook sizes was observed. Staining by the Thiéry procedure revealed a high concentration of polysaccharidelike material with both the encystment vesicle cortex layer and spines, although treatment with various glucanases and proteases failed to digest either component. Spherical fibrous vesicles also appear in primary cysts and their contents are partially removed by protease digestion. These appear morphologically similar to the glycoprotein adhesive vesicles described in other oomycetes and appear to be discharged by secondary zoospores. The complex encystment apparatus in Saprolegnia is compared with that described in other oomycete and chytridiomycete fungi and the possible phylogenetic significance discussed. The role of the long boathook spines in relation to the infection of fish is also considered.