Fine structure of mitosis and cytokinesis inUrospora (Acrosiphoniales, Chlorophyta)

Abstract

The mitosis and subsequent cytokinesis in vegetative cells of the filamentous stage of the green algaUrospora have been examined in detail and are compared to the situation in other coenocytic (un-) branched filamentous green algae. Before the formation of transverse walls inUrospora numerous nuclei arrange themselves in a band at the equatorial plane. All nuclei, including those in the same cell but not in the band, divide synchronously. InUrospora wormskioldii (Mertens in Hornemann) Rosenvinge the nuclear envelope is often more or less broken down during mitosis, but inU. neglecta (Kornmann) Lokhorst and Trask it is seen almost intact. Cytokinesis is accomplished in the region of the band by means of a diaphragm-like furrow of the plasmalemma. Arrays of microtubules ensheath the developing septum, they are most distinct at its leading edge. It seems that this particular arrangement of microtubules represents a guiding mechanism for the development of the ingrowing septum. A second function attributed to these microtubules is that they act as a supporting skeleton, which protects the developing furrow during its cleavage through the less elastic, centrally-located vacuole. It is proposed to modify the definition of the term phycoplast so that it also comprises the orientation of microtubules allied with cytokinesis in coenocytic green algae. The implications of these findings for the taxonomic position ofUrospora and for recently proposed evolutionary lines and classifications in green algae will be discussed.