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For Immediate Release: July 30, 2007
Contact: Gina Hebert, MBL, 508-289-7725; email@example.com
Rising Starlet: Sea Anemone Provides Deeper Look Into Animal Evolution
MBL, WOODS HOLE An encounter at the MBL between a California physicist and an embryologist from Hawaii has led, five years later, to a striking, new perspective on animal evolution.
Nicholas H. Putnam, Mansi Srivastava, Uffe Hellsten, Bill Dirks, Jarrod Chapman, Asaf Salamov, Astrid Terry, Harris Shapiro, Erika Lindquist, Vladimir V. Kapitonov, Jerzy Jurka, Grigory Genikhovich, Igor V. Grigoriev, Susan M. Lucas, Robert E. Steele, John R. Finnerty, Ulrich Technau, Mark Q. Martindale, and Daniel S. Rokhsar (6 July 2007). “Sea Anemone Genome Reveals Ancestral Eumetazoan Gene Repertoire and Genomic Organization.” Science, Vol. 317, No. 5834, pp. 86-94.
Photo: (Click on thumbnail for high-resolution image)
The starlet sea anemone can reproduce in different ways, including (left) asexually, by developing a second head and splitting in two; and (right) sexually, by releasing eggs. Photo by Adam Reitzel, courtesy of John R. Finnerty.
An information sheet (PDF format) on the starlet sea anemone, an animal that is rapidly emerging as an important model system for biological research, written by John R. Finnerty of Boston University.
More biological information on the starlet sea anemone: www.http://nematostella.org
Genomic resources on the starlet sea anemone: http://stellabase.org
In the July 7 issue of Science, the genome of the starlet sea anemone is analyzed by a team that includes Daniel Rokhsar of the Department of Energy Joint Genome Institute (JGI) in Walnut Creek, California, and Mark Q. Martindale of the Kewalo Marine Lab at the University of Hawaii at Manoa. The two first met at the MBL in 2002.
The genome of this tiny invertebrate turns out to be almost as complex as that of a human being, which implies that their common animal ancestor, which lived some 700 million years ago, contained most of the genes needed to build the sophisticated organisms we find today.
“It was a real eye-opener to find that these so-called simple animals, which branched off from [animal] evolution much earlier than groups like flies or worms, had this seemingly complex genome, at least in terms of gene number,” says Martindale, who is an instructor in the MBL’s Embryology Course.
The starlet sea anemone was found to have about 18,000 genes, comparable to the human genome with 20,000 genes, and about two-thirds of each animal’s genes are modern versions of genes that were in the shared ancestral genome. By comparison, fruit flies (Drosophila) and soil nematode worms (C. elegans) animals that seem more complex than the sea anemone and upon whom much of the genetic research of the past century has been based share less than half their genes with the common animal ancestor.
“One major lesson here is that there is no simple relationship between what we perceive as the complexity of a body plan and the number of genes,” says Martindale. The starlet sea anemone, Nematostella vectensis, is a species of anemone found in brackish seawater and marshes along the east coast of North America. It has a sac-like body a few inches long, a mouth but no anus, several tentacles, and a nerve net (but no centralized nervous system, which the fly and the nematode have). Along with corals, jellyfish and hydra, the sea anemone is a member of the Cnidaria group.
Why the sea anemone has kept a complex genome over the course of evolution yet has a simple body plan, while other animals have lost genes but gained physiological sophistication, opens up interesting questions. “Is it the combinatorial interactions of genes [that evolved]?” Martindale asks. “Are there more sophisticated ways that genes interact with one another that evolved later on, which allowed cell-type complexity or behavioral complexity?”
“The other way of thinking about it is, maybe some of the animals we see today, like the sea anemone, have simplified or failed to elaborate their body plans in order to fill some particular ecological niche some place where they face no evolutionary pressure to become fast predators with complex sensory and locomotive systems,” he says. “They have settled into some simplified way of making a living that works perfectly well for them.”
The study found other surprising similarities between the sea anemone and human genomes, including large blocks of linked genes. In the fly and nematode genomes, the genes have been shuffled among the chromosomes to a far greater degree. Also, the sea anemone genome, like the human genome, is full of non-coding regions called introns. About 80 percent of the introns are in the same places in the two genomes, which indicates the common ancestor had them, too.
Rokhsar, who is also a faculty member at the University of California at Berkeley, led the starlet-sea-anemone genome sequencing project with Nicholas J. Putnam of the JGI, who is an MBL Embryology Course alumnus.
The MBL has long been an international gathering place for scientists who use marine animals as model organisms. When Rokhsar and Martindale first met at the MBL in 2002, Rokhsar had just finished sequencing the genome of the sea squirt, Ciona, and was looking for a new animal for a genome project. Martindale, who had been working on sea anemone embryology for 12 years, told Rokhsar about the favorable aspects of Nematostella vectensis. “Unlike other cnidarians, you can get it to spawn in the lab,” he says. It is also one of the most ancient animals on earth, and thus could reveal information about early evolutionary history.
Martindale supplied Rokhsar with starlet sea anemones and put him in touch with other cnidarian experts, and Rokhsar and Putnam began sequencing the animal’s genome in 2004.
“The MBL is a pretty special place, where people like Rokhsar and I who don’t really have that much in common would cross paths and find common ground, and that it would lead to something pretty profound,” Martindale says. “And I firmly believe that other great things are in the process of being done here at the MBL.”
The MBL is a leading international, independent, nonprofit institution dedicated to discovery and to improving the human condition through creative research and education in the biological, biomedical and environmental sciences. Founded in 1888 as the Marine Biological Laboratory, the MBL is the oldest private marine laboratory in the Western Hemisphere. For more information, visit www.MBL.edu