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Genomics

Alga Genome Uniquely Informative

The sequence of the genome of a green alga has been determined. Surprisingly, both biologists and car manufacturers are interested.

Plants and animals diverged from a common ancestor billions of years ago. However, there are other descendants of this ancestor that retain properties–and genes–of both: Chlamydomonas reinhardtii, a single-celled green alga, is one example. It has chloroplasts which perform photosynthesis, the most notable characteristic of plants, but also has hair-like structures otherwise only found in animals. These hair-like structures, called flagella and used by Chlamydomonas to swim and agglutinate during mating, were present in the plant-animal ancestor, but have been lost from the plant lineages. Consequently, Chlamydomonas has many genes derived from ancestral green plant genes, and others descended from ancestral animal genes. Thus, this species sits between the animal and plant kingdoms, being neither entirely one nor the other and with features of both.
It was therefore not unexpected that a large consortium of more than 100 researchers, including the teams of Olivier Vallon1 and Laurence Maréchal-Drouard,2 from CNRS, came together to sequence the genome of this species. This sequence has now been published.3
Biologists like working with micro-organisms: they are easy to grow and the complexities associated with multicellularity can be avoided; they nevertheless have many of the systems and functions present in more complex, multicellular organisms. Indeed, Chlamydomonas has long been used as an experimental model, and much of our knowledge of photosynthesis comes from work on this alga.
Each Chlamydomonas cell has two flagella, used for swimming and sensing its environment. Humans have flagella, also called cilia, on a variety of cell types. They are responsible, for example, for eliminating mucus from the airways, and the motility of sperm. From polycystic kidney disease to primary ciliary dyskinesia, Chlamydomonas flagellum genetics keeps turning up genes whose human counterparts were found to cause genetic diseases, but whose functions in the cell were unknown. A simple list of genes shared between Chlamydomonas and ciliated organisms already suggests hundreds of candidate genes for human genetic diseases.
Perhaps more bizarrely, these little cells that swim around in ponds have been exciting interest for biofuel and biohydrogen research. Their chloroplasts are biological solar panels, which trap the energy in light to produce compounds with potential as fuel. In some conditions, Chlamydomonas can produce hydrogen, which could be used to generate electricity. Related species are capable of producing biodiesel with decent yields. Like all photosynthesising species, algae incorporate carbon dioxide into more complex products, but as Olivier Vallon points out, “they can grow much more quickly than plants, and have a variety of useful biochemical properties not found in higher plants.” They could be used as carbon sinks, and thereby contribute to reducing greenhouse gasses in the atmosphere. The availability of the genome sequence will facilitate research on chloroplasts, the cornerstones of these carbon and energy issues, and this work may lead to new applications involving solar energy capture and carbon assimilation.
The Chlamydomonas genome has about 15,000 genes, about half the number found in man. Comparisons with the genes in other species will help understand ancient evolution, and with the functions of many of the genes still unknown, the publication of the genome sequence promises to reinvigorate research on this fascinating microorganism.
Alex Edelman

Notes :

1. Physiologie membranaire et moléculaire du chloroplaste (CNRS / Université Paris-VI).
2. Institut de biologie moléculaire des plantes (CNRS).
3. S. S. Merchant et al., “The Chlamydomonas Genome Reveals the Evolution of Key Animal and Plant Functions,” Science. 318: 245-50. 2007.

Contacts :

Olivier Vallon, IBPC, Paris, Olivier.Vallon@ibpc.fr

Laurence Maréchal-Drouard, IBMP, Strasbourg,
laurence.drouard@ibmp-ulp.u-strasbg.fr


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