Cell division in the green microalga Marvania geminata: semi-exogenous autosporogenesis, role of coated pit-microtubule complexes, and systematic significance

Cell division in the unicellular coccoid green alga M. geminata has been studied with transmission electron microscopy. Unlike the division cycle in other coccoid green algae, mitosis occurs not before the cell has expanded until about twice its original volume. Cell expansion involves the escape of part of the protoplast through a break in the cell wall. During mitosis the nuclear envelope remains intact. The spindle is not associated with centrioles, which seem to be absent throughout the cell cycle. The division site at the plasma membrane is marked by a short cortical microtubule which appears before division is initiated. The plasma membrane near the microtubule invaginates to form a coated pit-like structure. The latter continues to grow inward along with the short microtubule, and in effect constitutes the cleavage furrow. After furrowing, each daughter cell forms a wall layer at the new septum. Cells separate after the old, common wall layer is broken at the level of the septum. A model explaining cytokinetic events, in particular the furrowing mechanism, is presented. It is concluded that M. geminata may be considered as a member of a broadly conceived order Chlorococcales.