Absolute configuration analysis of the flagellar apparatus in Giraudyopsis stellifer (Chrysophyceae, Sarcinochrysidales) zoospores and its significance in the evolution of the Phaeophyceae

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Abstract
To test a hypothesis that the Sarcinochrysidales is closely related and possibly ancestral to the Phaeophyceae, serially sectioned zoospores of an isolate of Giraudyopsis stellifer Dangeard were examined with the transmission electron microscope. The isolate used in this study differs from other known forms of G. stellifer in that its basal system is essentially unicellular rather than a prostrate filamentous disc. In terms of the absolute configuration and overall arrangement of flagellar apparatus components, G. stellifer zoospores are essentially identical with those of brown algae, and dissimilar to those of other Chrysophyceae. Transitional helices and a distal fiber are the only specific flagellar apparatus components of G. stellifer zoospores not also found in at least some members of Phaeophyceae. These findings strongly suggest that multi cellular chrysophytes like those presently included in the Sarcinochrysidales, and not unicellular forms, as some workers have supposed, gave rise to the brown algae. Variations among species in several flagellar apparatus components may be phylogenetically significant, allowing for an ultrastructure-based assessment of evolutionary relationships in the Sarcinochrysidales and the Phaeophyceae.