Scenedesmus quadricauda

Scenedesmus is a genus of common nonmotile chlorophyte green freshwater alga that has been used for decades as a model organism for the regulation of the cell cycle, photosynthesis, and a variety of toxicity studies. The daughter cells coming from a division of a single mother cell stay connected by a common cell wall in structured clusters called coenobia; in S. quadricauda coenobia could be either four- or eight-celled with the terminal cells having two spines. Individual cells contain a nucleus and a single chloroplast. The chloroplast and mitochondrial genomes of another member of the genus Scenedesmus, S. obliquus, have been sequenced (Kuck et al. 2000; Nedelcu et al. 2000; de Cambiaire et al. 2006).

Cell Cycle

Similarly to C. reinhardtii, S. quadricauda divides by a multiple fission. However, the mechanism is slightly different (Fig. 2). While C. reinhardtii divides by so-called clustered multiple fission, S. quadricauda divides by a consecutive multiple fission pattern (see above, Fig. 2) (Setlik and Zachleder 1984; Zachleder et al. 2002). The S. quadricauda cells grow during a G1 phase and attain consecutive commitment points, each of which allows one round of DNA replication and nuclear division to occur. Individual commitment points are with only a short delay followed by DNA replication and nuclear division but not by cell division (Fig. 2). S. quadricauda cells are routinely multinu-clear during the cell cycle (Fig. 3) with the cell division occurring in several rounds only after all the nuclear divisions have finished.

The S. quadricauda cell cycle has been characterized in detail in the work of Zachleder and colleagues (Setlik and Zachleder 1984; Zachleder et al. 2002; Zachleder and Setlik 1990). When put in light to allow photosynthetic growth the cells attain a commitment (for a description of commitment see above) at a critical cell size implying the involvement of a sizer. At commitment the processes start which will eventually lead to DNA replication, mitosis, and cell division. The period immediately following commitment could therefore

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Fig. 3 Fluorescent microphotographs of S. quadricauda cells at different stages of the cell cycle. DNA stained with SYBR Green I dye; a uninuclear cells of daughter coeno-bium at the beginning of the cell cycle (0 h of light); b binuclear cells of the coeno-bium after 9 h of light; c quadrinuclear cells of the coenobium after 11 h of light; d quadri- and octonuclear cells of the coenobium after 13 h of light. The terminal cells are usually smaller and their cell cycle progression is delayed compared to the central cells, here, still with only four nuclei. Arrows show where the chloroplasts have already divided; the chloroplast fission will be shortly followed by protoplast division. Photographs are courtesy of M. Vitova

Fig. 3 Fluorescent microphotographs of S. quadricauda cells at different stages of the cell cycle. DNA stained with SYBR Green I dye; a uninuclear cells of daughter coeno-bium at the beginning of the cell cycle (0 h of light); b binuclear cells of the coeno-bium after 9 h of light; c quadrinuclear cells of the coenobium after 11 h of light; d quadri- and octonuclear cells of the coenobium after 13 h of light. The terminal cells are usually smaller and their cell cycle progression is delayed compared to the central cells, here, still with only four nuclei. Arrows show where the chloroplasts have already divided; the chloroplast fission will be shortly followed by protoplast division. Photographs are courtesy of M. Vitova be called the pre-S (pre-synthetic) phase (Zachleder et al. 1997). Depending on the growth rate (e.g., amount of light and length of the light period) the cells can attain additional consecutive commitment point/s before DNA replication and mitosis take place; under optimal conditions the S. quadricauda cells can attain four commitment points which will all lead to DNA replication and mitosis. It is an intrinsic property of overlapping reproductive events that the time period from each following commitment point to its corresponding DNA replication and mitosis (S/G2 phases) is shorter than that of the previous one, probably due to the intervening of growth with the division processes in the earlier reproductive events.

A more detailed analysis of commitment point in S. quadricauda has shown that it is possible to separate commitment point for DNA replication (S phase) from a commitment point for nuclear division (M phase and S phase), with the former being related to a threshold in the accumulation of total RNA and the latter to a similar threshold in the accumulation of total protein. The two processes can be separated by timely withdrawal of light during which the cells will either only replicate their DNA or replicate DNA and divide their nuclei/cells (Zachleder and Setlik 1988). Similar commitment/restriction points for G2 phase progression were also described in Euglena gracilis (Hagiwara et al. 2001) and in mouse embryonic fibroblasts (Foijer and te Riele 2006).

Alternating light/dark regimes are a way to synchronize algal cultures. However, different light/dark regimes could also be used to manipulate the outcome of the cell cycle in order to get cell cycle patterns differing in the number of attained commitment points and/or length of postcommitment period. Combined with the use of inhibitors of protein synthesis and/or DNA replication, synchronization is a tool providing for a wide range of cell cycle patterns (Bisova et al. 2000; Zachleder et al. 2002; Zachleder and Setlik 1990). This has been used to modify the number of attained commitment points and mitosis/es and also to alter the timing of nuclear division in relation to the attainment of commitment point. Analysis of histone H1 kinase activity in S. quadricauda under these conditions clearly uncovered the presence of at least two different histone H1 (CDK-like) kinase complexes. The activity of one of them correlates with growth and reaches its maximum just before attainment of the commitment point; the activity of the other one is related to mitosis/es (Bisova et al. 2000).

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