Comparative study of axopodial microtubule patterns and possible mechanisms of pattern control in the centrohelidian heliozoa Acanthocystis, Raphidiophrys and Heterophrys.

The axopodial microtubule pattern of 9 centrohelidians belonging to the genera Acanthocystis, Raphidiophrys and Heterophrys, as well as the fine structure of their microtubule organizing centre, the centroplast, was studied to determine the rules which govern their patterns. Microtubules capable of binding a xamimum of 4 linkers are arranged in regularly distorted hexagons and equilateral triangles. The number of microtubules present in each axoneme ranges from some 140 in Acanthocystis turfacea to as few as 6 in Heterophrys marina (Stock I). In the later species each axoneme contains a single hexagon of microtubules only. In other Heterophrys species, the central hexagon is surrounded by closely packed microtubules or by microtubules arranged in pentagons; only the central hexagon is anchored in the centroplast shell, whereas additional microtubules seem to originate from secondary nucleation sites somewhat distal to the centroplast. It is argued that the distortion of the basic unit hexagon (with alternate angles close to 134 degrees and 106 degrees) indicates that the microtubules are composed of 13 protofilaments. While in the larger Acanthocystis and Raphidiophrys species, the pattern may result from self-linkage, the arrays found in the Heterophrys species seem to favour a template-determined linkage. To explain the formation of the central hexagon in Heterophrys and balanced lateral growth in the larger microtubule arrays, a 'linker-nucleation hypothesis' is proposed. The assumption is made that graded conformational changes in the microtubule subunits not only specify the position where the next linker will bind, but that this linker, through linkage, becomes able to induce secondary microtubule nucleation, which will result in balanced lateral growth of the array. The application of this hypothesis to other microtubule systems, e.g. basal body formation, is discussed.