Ultrastructural studies of the cell cycle in a multicentriolar form ofBracteacoccus minor (Chlorophyceae,Chlorellales)

Summary The ultrastructure of mitosis and cytokinesis is studied in the typical and a multicentriolar form of the multinucleate green algaBracteacoccus minor (Chodat) Petrovà. These processes are essentially identical in both forms, and are similar to those in other uni- and multinucleate chlorellalean algae. The mitotic spindle is closed and centric, and a fragmentary perinuclear envelope is present. In multinuclear cells mitosis is synchronous and may occur at the same time as cytokinesis. Cleavage is simultaneous and centrifugal, starting near the nucleus-associated centrioles and apparently mediated by phycoplast microtubules of the trochoplast type. Flagellated wall-less spores are usually formed. In the typical form ofB. minor, each interphase nucleus is associated with two mature centrioles (= one set) which function as centrosomal markers. At the onset of mitosis these centrioles duplicate and segregate and eventually establish the two poles of the spindle, where polar fenestrae develop in the nuclear envelope. In the multicentriolar form, however, each interphase nucleus generally is associated with two or three sets of centrioles. Consequently, during mitosis each half-spindle is associated with two or three sets. These centrioles are not necessarily all associated with the fenestrae at the spindle poles, but one or more sets are frequently associated with the nuclear membrane, more or less remote from the nuclear poles. However, the spindle in this multicentriolar form remains essentially bipolar. Cleavage generally results in zoospores with two, four or six flagella. The behaviour of the extra centrioles during the cell cycle and their possible relationship with centrosomes are discussed.