Mitosis and cytokinesis inTribonema regulare (Tribophyceae,Chrysophyta)

Summary The ultrastructure of mitosis and cytokinesis of the uninucleateTribonema regulare has been investigated by employing transmission electron microscopy. Prophase is characterized by settlement of a pair of centrioles at the presumptive poles of the spindle, metaphase by equatorial bulging of the nucleus, anaphase by non-synchronous separation of the chromosomes, and telophase by a persistent, strongly elongated, interzonal spindle. Throughout mitosis, at each pole dictyosomes are associated with the polar gaps of the nuclear envelope that otherwise remains intact. Cytokinesis does not immediately follow mitosis; from the static images it can be concluded that it is necessary for the daughter nuclei to approach each other before cytokinesis is initiated by complete division of the protoplast via plasma membrane cleavage. Afterwards, a ring of cell wall material is deposited close near the lateral wall in the plane of protoplast separation followed by a simultaneous or centripetal development of a single integral partitioning septum. Once the septum is completed, the cylindrical portion of the H-shaped segment is manufactured. The phylogenetic position ofTribonema amongst those algae, which may have evolved from unicells into filaments, is discussed.