Cell cycle, morphogenesis, and ultrastructure in the pseudoheliozoan Clathrulina elegans.

Abstract

The development of the sessile pseudoheliozoan Clathrulina elegans Cienkowski (Desmothoraca, Rhizopoda, Protozoa) was followed by light and electron microscopy. The cell cycle comprises a heliozoan-like stage sitting in an organic perforated capsule, an amoeboid resp. biflagellate stage ensuring dispersal, and an encysted stage. Special reference is given to the morphogenetic processes during the flagellate-to-heliozoan transformation. The differentiation of the stalk is described in detail and a hypothesis of the morphogenesis of the capsule is presented. Granules in the filopods displaying a jerky movement, are identified as a hitherto underscribed type of haptocyst-like organelles. These “kinetocysts” are shown to be a useful marker for studying the turnover of the plasma membrane at “organelle level”. Evidence is presented that in the flagellate stage a surplus of plasma membrane is stored in numerous lacunae thus facilitating the rapid transformation into the heliozoan stage with its comparatively large surface area. The morphogenetic significance of microtubules during the formation of the stalk, the latter being an asymmetric extracellular secretion product, is elucidated. The role of microtubule nucleation sites, the precisely timed assembly and disassembly of microtubules and other factors which probably control development are discussed to some extent.