Alga reveals rich trove of data about common ancestry of plants and animals

Chlamydomonas maintained genes lost during evolution

Genes of a tiny, single-celled green alga called Chlamydomonas reinhardtii may contain scores more data about the common ancestry of plants and animals than the richest paleontological dig.

A team of researchers, including senior author Arthur Grossman of the Carnegie Institution's Department of Plant Biology and Jane Grimwood and Jeremy Schmutz of the Stanford Department of Genetics, reports in the Oct. 12 issue of Science on the results of a major effort to obtain the genome sequence of Chlamydomonas and to compare its approximately 15,000 genes to those of plants and animals, including humans.

The research shows that this alga has maintained many genes that were lost during the evolution of land plants. It also has others that are associated with functions in humans and numerous genes of unknown function that are associated with critical metabolic processes.

"Although Chlamydomonas is certainly more plant than animal, there are clear similarities between this photosynthetic organism and animals that would surprise the average person on the street," said Grossman, who holds a courtesy appointment in Stanford's Department of Biological Sciences. "Just 20 years ago no one would have guessed that an alga would have retained many of the functions we associate with humans and would be useful for developing a basic understanding of certain human diseases."

Chlamydomonas, affectionately called Chlamy, is an alga of 10 micrometers in size that is present in soil and freshwater environments. It performs photosynthesis like plants, but it diverged evolutionarily from flowering land plants about 1 billion years ago. It is even more distantly related to animals (the split between animals and plants was roughly 1.6 billion years ago).

Chlamy moves using two anterior hair-like flagella that were lost by its cousins, the flowering land plants, after the evolutionary split of the two lineages. The flagella are equivalent to the cilia and centrioles in animal cells. Centrioles are structures involved in cell division; they form a spindle apparatus that helps separate genetic material into two new cells during mitosis. Cilia are important to many animal functions.

The study identified many new proteins that are likely associated with the flagella and has distinguished those proteins of the flagella that are critical for movement and those that are associated with sensory functions (feeling the conditions in the environment). The analysis also has generated new insights about human diseases associated with ciliary dysfunction in humans, including those of the kidney and the eye.

In addition, the researchers performed a comparative gene analysis across species to explore the evolutionary history of Chlamy, and the relationship of this alga to other organisms. They found that Chlamy shared 35 percent of its genes with both flowering plants and humans, and an additional 10 percent with humans but not with flowering plants.

"The work has generated a clear roadmap for exploring the roles of numerous genes in photosynthetic function, for defining the structure and dynamic aspects of flagellar function and for understanding how the soil environment, with its large fluctuations in nutrients, has molded the functionality of organisms through evolutionary time," Grossman said.

Most of the sequence analysis was supported by the U.S. Department of Energy and the Joint Genome Institute. Some aspects of the work were supported by a grant from the National Science Foundation.

The Carnegie Insitution's Department of Plant Biology and Department of Global Ecology are on the Stanford campus.