C. reinhardtii is a unicellular freshwater green alga with two flagella, one haploid nucleus, and one cup-shaped chloroplast containing one or more pyrenoids. The cell is enclosed within a cell wall consisting mainly of hydroxyproline-rich glycoproteins (Harris 2001). The C. reinhardtii genome is available (Merchant et al. 2007). At an estimated 125 Mb it is comparable in size to the genome of Arabidopsis thaliana, but possesses significantly fewer gene families, facilitating forward genetic analysis. Both the chloroplast (Maul et al. 2002) and mitochondrial (GenBank accession U03843) genomes have been sequenced.
Wild-type C. reinhardtii cells grow phototrophically in minimal media without any carbon supplement. Additionally, they are able to take up acetate as a carbon source for heterotrophic or mixotrophic growth. This has allowed isolation of mutants blocked in photosynthesis, making C. reinhardtii an excellent model for the genetic analysis of this fundamental process. C. reinhardtii cells are of two mating types, mt+ or mt-. When the cells are starved of nitrogen and exposed to blue light they differentiate into morphologically identical gametes and enter the sexual cycle. Gametes of opposite mating types recognize each other by sex-specific agglutinin proteins on their flagella. Mating pairs adhere to one another's flagella followed by morphological change in the flagellar tips and dissolution of the cell wall by a gamete-specific lytic enzyme. The mating partners then fuse to form a diploid zygote (Harris 2001). In favorable conditions, the mature zygote goes through meiosis and germinates, yielding four haploid progeny. Separation and analysis of the meiotic progeny (tetrad analysis) is the basis of traditional C. reinhardtii genetics. Indeed, tetrad analysis was first used in C. reinhardtii. The nucleus can be easily transformed using glass beads (Kindle 1990) or electroporation (Shimogawara et al. 1998); chloroplasts (Boynton et al. 1988) and mitochondria (Randolph-Anderson et al. 1993; Remacle et al. 2006) can be specifically transformed by biolistic bombardment. Tetrad analysis, together with the ease and speed of forward genetic screens in haploids and the availability of stable DNA transformation of all three genomes, forms the foundation of C. reinhardtii as a genetic model organism.
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