Multiple Fission A Variation on the Theme

The cells of Chlamydomonas reinhardtii and Scenedesmus quadricauda divide by a variant of the common cell cycle, multiple fission. C. reinhardtii, S. quadricauda, and their relatives grown in light undergo a prolonged G1 phase during which they may grow to many (2n) times their original size. The n is determined by a combination of the growth rate and the species limitations (in most algae n can range from 3 to 5; in some species it can reach 10). With each doubling of size cells attain a size-governed control point, called commitment. Commitment is the formal equivalent of start in budding yeast and the restriction point in mammalian cells (Donnan and John 1983, 1984), which will lead to one round of DNA replication, nuclear and cellular division (Fig. 1).

The multiple fission cell cycle is shared with other algae (Pickett-Heaps 1975) and exists in two different patterns: clustered (C. reinhardtii and most of the order Volvocales) and consecutive (S. quadricauda and most of the genus Hydrodictyon) (Setlik and Zachleder 1984; Zachleder et al. 2002) (Fig. 2).

Green algae are excellent model organisms for cell cycle studies. They are usually unicellular, grow fast, and can be easily synchronized by alternating light/dark periods. The synchrony reached this way is very high, especially for the species dividing by multiple fission. While synchronized plant cell suspension cultures reach only about 60% synchrony, that is, 60% of cells in mitosis occur over an interval of 5-6 h (Nagata et al. 1992; Samuels et al. 1998;

Fig. 1 Schematic illustration of attaining/determination of commitment points to cellular division in synchronous populations of C. reinhardtii. The idealized curve represents the growth of cells in continuous light during the cell cycle; at times marked by arrows, the subcultures were put into the dark (indicated by black stripes). The microphotographs above the curve show typical cells from the synchronized culture at the time of transfer of subcultures into the dark; the vertical lane of microphotographs illustrates the microcolonies of daughter cells released from the mother cell during the corresponding dark interval on agar plates. The moments of transfer into the dark correspond to the attainment of the first (5 h of light), second (10 h of light), and third (15 h of light) commitment points; two, four, and eight cells were released during the dark period, respectively. Reprinted from Vitova and Zachleder (2005) with the permission of the authors and publisher, modified

Time, h

Fig. 1 Schematic illustration of attaining/determination of commitment points to cellular division in synchronous populations of C. reinhardtii. The idealized curve represents the growth of cells in continuous light during the cell cycle; at times marked by arrows, the subcultures were put into the dark (indicated by black stripes). The microphotographs above the curve show typical cells from the synchronized culture at the time of transfer of subcultures into the dark; the vertical lane of microphotographs illustrates the microcolonies of daughter cells released from the mother cell during the corresponding dark interval on agar plates. The moments of transfer into the dark correspond to the attainment of the first (5 h of light), second (10 h of light), and third (15 h of light) commitment points; two, four, and eight cells were released during the dark period, respectively. Reprinted from Vitova and Zachleder (2005) with the permission of the authors and publisher, modified

Fig. 2 Schematic illustration of the Chlamydomonas (clustered) and Scenedesmus (consecutive) patterns of the multiple fission cell cycle. The graphs show the fraction of cells that passed commitment point (dashed lines), finished nuclear division (thick solid black lines) or protoplast division (dotted lines), and released daughter cells/coenobia (solid lines with crosses). In the case of Chlamydomonas the curves representing the protoplast division were omitted because the nuclear and protoplast divisions overlap. Notice accumulation of nuclear divisions in the Chlamydomonas clustered cell cycle as opposed to the spreading of the nuclear divisions during the Scenedesmus consecutive cell cycle. The stripes under the graphs represent individual sequences of the common cell cycle running simultaneously within one multiple fission cell cycle. C.P.: commitment point; G1, S, G2, M: phases of the common cell cycle; "G1": G1-like phase after the cells passed commitment; G3: gap phase separating nuclear and protoplast divisions in Scenedesmus cell cycle (Zachleder et al. 1997); C: cytokinesis

Fig. 2 Schematic illustration of the Chlamydomonas (clustered) and Scenedesmus (consecutive) patterns of the multiple fission cell cycle. The graphs show the fraction of cells that passed commitment point (dashed lines), finished nuclear division (thick solid black lines) or protoplast division (dotted lines), and released daughter cells/coenobia (solid lines with crosses). In the case of Chlamydomonas the curves representing the protoplast division were omitted because the nuclear and protoplast divisions overlap. Notice accumulation of nuclear divisions in the Chlamydomonas clustered cell cycle as opposed to the spreading of the nuclear divisions during the Scenedesmus consecutive cell cycle. The stripes under the graphs represent individual sequences of the common cell cycle running simultaneously within one multiple fission cell cycle. C.P.: commitment point; G1, S, G2, M: phases of the common cell cycle; "G1": G1-like phase after the cells passed commitment; G3: gap phase separating nuclear and protoplast divisions in Scenedesmus cell cycle (Zachleder et al. 1997); C: cytokinesis

Menges and Murray 2002), C. reinhardtii cell culture can be synchronized so that > 95% of cells proceed through three rounds of mitosis within 4- 5 h. The inhibitors routinely used for synchronization of plant cells provide an addi tional means to manipulate the outcome of the cell cycle (see below). Of the species described below, C. reinhardtii is the best established model system for genetics with an improving molecular toolkit (Harris 2001). Work thus far indicates that regulators of the cell cycle seem to be conserved between algae and land plants but much simpler. Algae can serve not only as a model for higher plant cell cycle regulation, they can also be useful in other fields like the study of the relationship between cell and organellar cycles/division or the study of organellar division.

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