world of amoeboid organisms


Order Microhelida Cavalier-Smith, 2012

Diagnosis: Very small non-flagellate protists with prominent central centrosome having two concentric shells of dense granules surrounding a pale matrix and dense central core; central core with lighter lumen, not differentiated into central disc and sub-hemispheres (unlike many centrohelids). Centrosome inner and outer dense shells nucleate an aster of microtubules close packed at their bases. Centrosome surrounded by a less dense zone (centrosphere) that excludes larger vesicles. Axopodial microtubules 3, arranged as an equilateral triad, fixed to cell surface by a dense collar composed of three symmetrically arranged dense submembrane linkers immediately above the level of the plasma membrane; each dense linker comprises two tiers of dumbbell shaped structures attached to inner face of the axopodial membrane and linking two adjacent microtubules to each other. Collar lumen largely occluded by a central spacer with 3-fold symmetry, comprising a short central rod and three radial arms, one connecting to each microtubule. Some axopodial triads continuous with centrosomally nucleated microtubules; others separate and terminating basally at the plasma membrane.
Trans-nuclear channels contain centrosome-nucleated microtubules, some connected to axopodial axonemes; similar microtubule-filled cytoplasmic channels pass through the extremely long, broad rough endoplasmic reticulum extensions of the nuclear envelope that wrap almost all the way round the centrosome. Distally to the collar axopodia lack cross-links between microtubules; cross-links exist in some triads in region between centrosome and nuclear cup or in triads in transnuclear channels. Dense ellipsoid extrusomes borne on sides (and invariably also at the tip) of axopodia and on the cell surface; not like kinetocysts in shape or internal structure; too small to resolve by light microscopy. Golgi stack(s) attached by forming face to centrosome. Mitochondrial cristae tubular; short and compact or vermicular. Microbody attached to nuclear envelope and centrosome. A cortical filogranular network links rough endoplasmic reticulum and mitochondria (which together encircle the centrosome except where the Golgi is) to each other and to the cell surface.

Family Microheliellidae Cavalier-Smith, 2012   Genus Microheliella Cavalier-Smith and Chao, 2012, see here

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