Chlamydomonas is a genus of unicellular green algae (Chlorophyta). These algae are found all over the world, in soil, fresh water, oceans, and even in snow on mountaintops. Algae in this genus have a cell wall, a chloroplast, an “eye” that perceives light and two anterior flagella with which they can swim using a breast-stroke type motion. More than 500 different species of Chlamydomonas have been described, but most scientists work with only a few.
The most widely used laboratory species is Chlamydomonas reinhardtii. Cells of this species are haploid, and can grow on a simple medium of inorganic salts, using photosynthesis to provide energy. They can also grow in total darkness if acetate is provided as an alternative carbon source. When deprived of nitrogen, in the presence of blue light, haploid cells differentiate into isogamous gametes. The two mating types (designated plus and minus) are simple Mendelian traits mapping to a single locus (mt ) on chromosome 6. Mating type switching is never observed in C. reinhardtii although other species in the genus, such as C. monoica, undergo mating type switching. When gametes of opposite mating types are mixed they fuse to become a diploid zygospore, which forms a hard outer wall that protects it from adverse environmental conditions. When conditions improve (or when the scientist restores nitrogen to the culture medium and provides light and water), the diploid zygote undergoes meiosis and releases four haploid cells that resume the vegetative life cycle. Routine methods are available to separate the four products of meioisis, allowing the powerful genetic technique of tetrad analysis. It is a little known fact that tetrad analysis was first described in Chlamydomonas.
Chlamydomonas is used as a model system for research on many fundamental questions in cell and molecular biology: How do cells move using flagella? How do they respond to environmental stimuli such as light? How does photosynthesis work, and what is role of the chloroplast genome? How do cells recognize one another during mating?
The sequence of all three genomes, nuclear, chloroplast and mitochondria, has been determined. We are frequently asked what strain was used for genomic sequencing. This was CC-503 cw92 mt+, a cell-wall deficient strain that facilitated the isolation of high quality, high molecular weight DNA for sequencing. Most of the EST libraries sequenced at Stanford were prepared in the wild-type strain CC-1690 21 gr mt+. Both CC-503 and CC-1690 derive from the same original field isolate from Massachusetts in 1945, but they have been separate since at least the mid-1950s. S1 D2 (CC-2290), a strain used for some ESTs and comparative sequencing, is a field isolate from Minnesota, dating from the 1980s. The most commonly used strains of Chlamydomonas in the literature are 137c plus and minus (CC-125 and CC-124 respectively). These are derived from 21gr from Sager’s laboratory, but during decades of use in Paul Levine’s laboratory these strains developed spontaneous mutants in NIT1 and NIT2, genes required for nitrate utilization. Thus CC-124 and CC-125 require a reduced nitrogen source (usually ammonia) for growth whereas 21gr+ can be grown on either or both of ammonia or nitrate.
It is recommended that users consult the comprehensive guide to Chlamydomonas as a research tool by referencing The Chlamydomonas Sourcebook, Second Edition, published November 2008 (copyright date 2009). Available from Elsevier Science and Technology. From the home page, a search for “Chlamydomonas” will retrieve the information on how to order. The three volumes can be purchased individually, or as a set.
Volume 1, by Elizabeth H. Harris: Introduction to Chlamydomonas and Its Laboratory Use
Volume 2, edited by David B. Stern: Organellar and Metabolic Processes
Volume 3, edited by George B. Witman: Cell Motility and Behavior
The first edition of The Chlamydomonas Sourcebook, if you can find it, is still an excellent repository of techniques and “Chlamy lore”.
Index to Elsevier’s Companion web site, with methods files for photosynthesis research, and many videos that supplement volume 3.