Ultrastructure of vegetative and motile cells, and zoosporogenesis in Chrysonephos lewisii (Taylor) Taylor (Sarcinochrysidales, Pelogophyceae) in relation to taxonomy

Ultrastructural observations on the vegetative filaments, motile cells and processes leading to zoospore differentiation in Chrysonephos lewisii are p resented. Vegetative filaments are embedded in a mucilaginous envelope, pre served by Alcian blue fixation, which favours their aggregation and also th e attachment of bacteria, diatoms and various types of debris. The filament s are provided with an external wall consisting of microfibrils positive to the PATAg test and ConA-colloidal gold labelling. Inside the filaments the vegetative cells differentiate into zoospores. This accounts for the ultra structural similarity between vegetative cells and zoospores, although zoos pores are typically provided with a flagellar apparatus. They share an exte rnal cell covering or theca, vacuolar or cytoplasmic 'scale-like structures ', protruding stalked pyrenoids, scattered nucleoids of plastid DNA and abs ence of a photoreceptor-eyespot complex. The flagellar apparatus of zoospor es is characterized by two distinct basal plates, the central pair of flage llar microtubules originating some distance above the transitional plate an d basal body angle less than 90 degrees. These results provide strong taxon omic evidence that this genus should be placed in the Sarcinochrysidales se nsu stricto, rather than remaining in Chrysomeridales. The key role of the Golgi apparatus in the synthesis of different structural elements - dependi ng upon the functional stages of the cells - and in the processes of exocyt osis during zoosporogenesis is also discussed.