Ultrastructure of the zoospores of the coenocytic algaeAscochlorisandUrnella(Chlorophyceae), with emphasis on the flagellar apparatus

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Abstract
Ultrastructural features of the flagellar apparatus of the zoospores of Asochloris multinucleata and Urnella terrestris are similar to those of Chlamydomonas-like motile cells. including: distal and proximal fibers, cruciate rootlets and basal bodies offset in a clockwise absolute orientation. Unusual characteristics include a plate-like structure between proximal ends of the basal bodies and modified microtubular rootlets with the following features: (1) two microtubules constitute the X-membered rootlet in A. multinucleata and one microtubule in U. terrestris; (2) the very short extension of the rootlets from the basal bodies into the cytoplasm; and (3) absence of the striated microtubule-associated component on the two-membered rootlets. The zoospores of both algae are walled. Basal bodies appear to be anchored by a rhizoplast and/or rootlets that extend to a mitochondrion or chloroplast. Rootlets do not appear to be involved with phototaxis or in positioning of the stigma. Phylogenetically Urnella is more closely related to Ascochloris than to Protosiphon.