, Volume 84, Issue 2, pp 95-118

The ultrastructure of holdfasts, “rhizoids”, and “slime tracks” in thraustochytriaceous fungi and Labyrinthula spp.

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Summary

The fine structure of the holdfasts or “rhizoids” is described for the thraustochytriaceous organisms, Thraustochytrium motivum, Schizochytrium aggregatum, and an unidentified organism, denoted T-20, which resembles S. aggregatum and Labyrinthula spp. Labyrinthula algeriensis and L. minuta “slime track” ultrastructure is also described. The holdfasts, rhizoids, and tracks have the same basic fine structure and are collectively termed ectoplasmic nets. They are delimited by a unit membrane which is in continuity with the plasmalemma, contain no cytoplasmic organelles only membrane-limited cisternae, and contain a fibrogranular ground substance. The nets appear to arise from one or as many as 20 organelle complexes consist of an approximately disk-shaped electron-dense granular aggregate in which are embedded portions of cisternae of the endoplasmic reticulum or perinuclear clear continuum. The cisternae appear to contribute small (ca. 17 nm diameter) vesicles to the granular aggregate which coalesce to form internal membranes of the net elements. The sagenogenetosome underlies the plasmalemma where it evaginates to form the delimiting membrane of the main trunk element of the net. No continuous membrane separates the net contents from the cytoplasm, only the granular aggregate.

In L. algeriensis, L. minuta, and T-20 the net is necessary for motility of nonflagellated, nonamoeboid cells. Presence of the nets is not associated with motility in S. aggregatum and T. motivum. The possible taxonomic significance of the observations is discussed.

Contribution No. 456, Virginia Institute of Marine Science, Gloucester Point, Virginia 23062.
Supported in part by the Oceanography Section, National Science Foundation, NSF Grant GA-31014.