Mitosis, cytokinesis and colony formation in the colonial green algaAstrephomene gubernaculifera

Interphase cells of Astrephomene resemble those of other volvocalean algae, except for the insertion of nearly parallel flagella into a depression or shallow pit. During prophase, virus-like particles become associated with the condensed chromatin where they remain throughout mitosis. Except for polar fenestrae, mitosis is closed. Two centrioles are located laterally to each pole of the curved spindle. Microtubules insert into the chromosomes at three layered kinetochores. Spindle microtubules remain surrounded by the nuclear envelope through telophase. Phycoplast microtubules radiating from each pair of centrioles predict the plane of cytokinesis. After several divisions a curved plate of cells develops with the centrioles of each cell toward the convex surface. Additional divisions produce a hollow sphere but, unlike members of the Volvocaceae, no inversion occurs. Cytoplasmic bridges, resembling similar structures in the Volvocaceae, are formed by incomplete cytokinesis. Since the bridges in the Volvocaceae are thought to be important in the process of inversion, their presence in Astrephomene suggests that this alga has evolved from ancestors that invert.