C. merolae has been largely used as a model for the study of division of both mitochondrion and chloroplast (Kuroiwa et al. 1995, 1998; Miyagishima et al. 1999, 2001a,b,c, 2004; Takahara et al. 2000; Nishida et al. 2003, 2004; Yoshida et al. 2006). Mitochondria in higher plants and animals divide by the combined action of mitochondrion division and dynamin rings. On the contrary, chloroplasts in higher plants divide by the combined action of protoplast division and FtsZ rings. It has therefore been proposed that during evolution dynamin replaced FtsZ in the mitochondrial division (Erickson 2000; Margolin 2000). However, the C. merolae mitochondrion divides by a combined action ofmitochondrion division, dynamin, and FtsZ rings (Beech et al. 2000; Takahara et al. 2000; Nishida et al. 2003). Similarly, the C. merolae chloroplast divides by a combined action of protoplast division, FtsZ, and dynamin rings (Miyagishima et al. 2003, 2004; Misumi et al. 2005) implying that the organization is ancestral and C. merolae is the only model organism that retains the ancestral set of organellar division proteins.
With a completed, compact genome sequence and nuclear transformation by homologous recombination available, C. merolae is a promising model organism. It has already proven its usefulness for the study of organellar di vision. It will be interesting to see whether it can also become a model for the study of the cell cycle, which would no doubt be attractive due to its unique evolutionary position right after the split of primary red and green lineages (including land plants). As discussed above, algae offer quite a few model organisms for the study of different aspects of cell growth and cell cycle regulation. In the three models with sufficient genomic information, the sets of cell cycle genes are very similar. The genomes encode for a complete set of cell cycle genes including plant-specific B-type CDK. Conveniently, most of these genes are present in single copy. This implies that (1) B-type CDKs were present in the ancestor of primary algae very early after the acquisition of chloroplast or even before it, (2) the cell cycle regulation of these unicellular organisms is more complicated than that of yeasts, and (3) the complex cell cycle regulation by gene families seen in land plants is not necessary for coordination of cell and chloroplast divisions.
All of the discussed models would be valuable in the characterization of plant cell cycle regulation and more specifically in unraveling why plant-specific B-type CDK evolved. We can also expect some more insight into the coordination of the cell cycle with the division cycles of mitochondria and chloroplast.
Acknowledgements This work was supported by the Grant Agency of the Czech Republic (grant 204/06/0102), the Grant Agency of the Academy of Sciences of the Czech Republic (grant IAA500200614) and the Institutional Research Concept (no. AV0Z50200510).
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