Chlamydomonas reinhardtii

Chlamydomonas reinhardtii is a motile single celled green alga about 10 micrometres in diameter that swims with two flagella. See Chlamydomonas.

These algae are commonly found in soil and fresh water. They have a cell wall made of hydroxyproline-rich glycoproteins, a large cup-shaped chloroplast, a large pyrenoid, and an "eyespot" that senses light. Normal Chlamydomonas can grow on a simple medium of inorganic salts in the light, using photosynthesis to provide energy. They can also grow in total darkness if acetate is provided as a carbon source.

The C. reinhardtii wild type laboratory strain c137 (mt+) originates from an isolate made near Amherst, Massachusetts, in 1945.

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C. reinhardtii as a model organism

  Chlamydomonas is used as a model organism for research on fundamental questions in cell and molecular biology such as:

• How do cells move?
• How do cells respond to light?
• How do cells recognize one another?
• How do cells regulate their proteome to control flagellar length?
• How do cells respond to changes in mineral nutrition? (nitrogen, sulfur etc.)

There are many known mutants of C. reinhardtii. These mutants are useful tools for studying a variety of biological processes, including flagellar motility, photosynthesis or protein synthesis. Since Chlamydomonas species are normally haploid, the effects of mutations are seen immediately without further crosses.

In 2007, the complete nuclear genome sequence of C. reinhardtii was published.^[1]

Reproduction

Vegetative cells of the reinhardtii species are haploid with 17 small chromosomes. Under nitrogen starvation, haploid gametes develop. There are two mating types, identical in appearance and known as mt(+) and mt(-), which can fuse to form a diploid zygote. The zygote is not flagellated, and it serves as a dormant form of the species in the soil. In the light the zygote undergoes meiosis and releases four flagellated haploid cells that resume the vegetative life cycle.

Curious fact: Under ideal growth conditions, cells may undergo two or three rounds of mitosis before the daughter cells are released from the old cell wall into the medium. Thus, a single growth step may result in 4 or 8 daughter cells per mother cell.

The cell cycle of this unicellular green algae can be synchronized by alternating periods of light and dark. The growth phase is dependent on light, whereas, after a point designated as the transition or commitment point, processes are light-independent. ^[2]

Evolution

Chlamydomonas has been used to study different aspects of evolutionary biology and ecology. The fact that: it has a short generation time, it is both a heterotroph and a facultative autotroph, it can reproduce both sexually and asexually and that there is already a lot of genetic information available for the species has made it an organism of choice for many selection experiments.

Some examples (non exhaustive) of evolutionary work done with Chlamydomonas go from the evolution of sexual reproduction, ^[3], the fitness effect of mutations ^[4], and the effect of adaptation to different levels of CO2. ^[5]

Genetics

The attractiveness of the alga as a model organism has recently increased with the release of several genomic resources to the public domain. The current draft (Chlre3) of the Chlamydomonas nuclear genome sequence prepared by Joint Genome Institute of the U.S. Dept of Energy comprises 1557 scaffolds totaling 120 Mb. Roughly half of the genome is contained in 24 scaffolds all at least 1.6 Mb in length. The sequences of all three C. reinhardtii genomes are available.

The ~15.8 Kb mitochondrial genome (database accession: NC_ 001638) is available online at the NCBI database. [1] The complete >200 Kb chloroplast genome is available online. [2] The current assembly of the nuclear genome is available online at. [3]

In addition to genomic sequence data there is a large supply of expression sequence data available as cDNA libraries and expressed sequence tags (ESTs). Seven cDNA libraries are available online [4]. A BAC library can be purchased from the Clemson University Genomics Institute [5]. There are also two databases of >50 000 [6] and >160 000 [7] ESTs available online.

C. reinhardtii DNA transformation techniques

Gene transformation occurs mainly by homologous recombination in the chloroplast and heterologous recombination in the nucleus. The C. reinhardtii chloroplast genome can be transformed using microprojectile particle bombardment and the nuclear genome has been transformed with both glass bead agitation and electroporation. The biolistic procedure appears to be the most efficient way of introducing DNA into the chloroplast genome. This is probably because the chloroplast occupies over half of the volume of the cell providing the microprojectile with a large target. Electroporation has been shown to be the most efficient way of introducing DNA into the nuclear genome with maximum transformation frequencies two orders of magnitude higher than obtained using glass bead method.

Notes