Valve morphogenesis and the microtubule center of the diatom Hantzschia amphioxys

The siliceous valve of Hantzschia is briefly described, preparatory to following its morphogenesis. While Hantzschia is morphologically very different to Pinnularia, there are several subtle but important similarities in detail. Bundles of microfilaments, significant in generating the gliding motion of these cells, line the cytoplasmic fissure of the raphe. Valve morphogenesis in Hantzschia has been followed in live cells and by using transmission electron microscopy. Major re-organization and translocation of individual organelles, and also the nascent wall and its associated system of structural cytoplasmic components, are described. The spindle always forms on the concave side of the cell, opposite the raphes in the keels. A "polar complex" near each spindle pole consolidates into the "microtubule center" (MC) after cytokinesis. Each MC moves around its daughter nucleus to the center of the completed cleavage furrow and comes to rest on the narrow silicalemma running along the center of the cell. Microfilaments line first one and then both sides of the silicalemma before it grows outwards and begins to secrete the wall. Microtubules (MTs) extend along the silicalemma from the MC and directly over the future raphe; a compact row of mitochondria is organized along these MTs, flanking each MC. After about one hour in this central position, the whole assemblage of MC, MTs, mitochondria and silicalemma with its forming wall, move laterally across to the convex side of the parent valves. Now, the valve thickens steadily. The fibulae that hold the wall together where it is perforated by the raphe, grow out as flanges fusing with the transverse ribs across the keel. As in Pinnularia, the raphe fissure is occluded precisely adjacent to the MC, which also suppresses formation of the fibulae in this restricted region. The nascent wall structure suggests that silica is being precipitated on to a fibrous base. After valve formation, each MC migrates back to its interphase position at the concave side of the daughter cell. The MC undergoes characteristic morphological changes during these morphogenetic events and remains throughout tightly associated with a pronounced evagination of the nucleus. Ultrastructural comparisons suggest that the Hantzschia symmetry could have originally been derived from that of Pinnularia.