Cells of the chrysophycean flagellate Olisthodiscus luteus contain bundles of aligned tubes and fibres within membrane-limited vesicles. Each element in a vesicle consists of a tapering portion 0.25µm long, a shaft 1.5µm long and a terminal fibre 0.25µm long; the shaft is approximately 15nm in diameter and has a helical cross-banding with a periodicity of 8nm. The flagellar hairs of Olisthodiscus have identical morphology and dimensions to these internal elements.
Zoospores of the filamentous xanthophycean algae Bumilleria sicula, Heterococcus spp. and Tribonema spp. have similar internal vesicles containing aligned tubes and fibres with precisely the same morphology and dimensions as the flagellar hairs: base plus shaft, 1-2µm long, two terminal fibres per hair, 0.5-0.8µm long, shaft diameter of approximately 15nm and a helical periodicity of the shaft of 8nm. The aligned tubes are absent from the vegetative xanthophycean cell, appear during zoosporogenesis and disappear during early stages of zoospore settlement.
It is suggested that the aligned tubes and fibres are potential flagellar hairs which are formed in the perinuclear space (and possibly other regions of the endoplasmic reticulum) and are then transported to the cell surface in vesicles of the ER for deposition on the flagella. A review of information available on the Chrysophyceae, Xanthophyceae, Phaeophyceae and Bacillariophyceae indicates that internal formation of flagellar hairs is probably the rule in the heterokont algal groups, and a similar process apparently occurs in at least the dinoflagellates, the cryptomonads and some aquatic fungi among other groups of organisms with hairy flagella.
