Three years ago, Ken Kosik, a Harvard Medical School neurologist who studies Alzheimer disease, packed up files and equipment from his lab in Cambridge and moved 5,000 kilometers west, to the University of California, Santa Barbara. There, he settled into a new laboratory, where he hoped to extend his focus from clinical research into the basic evolutionary questions about the origins of nervous systems, which had always interested him.
Where to start? Kosik consulted Todd Oakley, an evolutionary biologist at UC, San Diego, who suggested the sponge. On the tree of life, cnidarians (such as jellyfish and anemones) branched immediately after the sponge ancestor. Since cnidarians have a nervous system, the researchers reasoned that sponges - which lack organs, nerves, and complex tissues - might carry the molecular, evolutionary rudiments of synapses.
The number of sponge species is estimated at about 6,000, including finger-like, bright orange sponges, massive interwoven fans, and delicate flutes that sway with the current. Some measure up to 2.5 meters long and wide, and live thousands of meters below sea level.
Kosik and Oakley decided to painstakingly catalogue all the genes associated with the postsynaptic structure in other animals, and then do degenerate PCR to try to clone out those genes from sponges. Then, "an extremely serendipitous thing happened," says Kosik. He and Oakley learned that a former UCSB postdoc, Bernie Degnan, had just completed sequencing the genome of Amphimedon queenslandica, a velvety gray sea sponge that anchors itself in shadowy crevices in coral reefs.
So the researchers, including Degnan and other collaborators, probed the sponge's genome bioinformatically. They were amazed to find nearly all the genes necessary to construct a postsynaptic complex. So, a nerveless animal with which humans haven't shared a branch on the evolutionary tree for an estimated five or six hundred million years carries most of the molecular building blocks needed to build a postsynaptic scaffold (PLoS ONE, 2:e506, 2007). "When we saw one or two of [the synaptic genes] we were very interested, but not overwhelmed," says Kosik. "As we saw more and more of them, the excitement built."
Halfway around the world, University of Göttingen geobiologist Gert Wörheide squeezed a similar insight from the porous animal. In another Indo-Pacific sponge, Astrosclera willeyana, Wörheide and his collaborators found a protein homologous to carbonic anhydrases, which are enzymes used for everything from skeleton building and processing metabolic wastes to pH regulation and transmembrane ion transport (Science, 316:1893-4, 2007).
So, are sponges the new animal model? Sponges habitually regenerate cells, suggesting that they might serve as good models for stem cell research, notes Degnan, now a professor of evolutionary and developmental biology at the University of Queensland in Australia. But sponges have major limitations, he adds. They are hard to keep alive in labs because of their need for extremely high water flow, and they have a relatively slow generational turnover, which complicates genetic studies. "Maybe there is the magic sponge out there that is easy to keep alive in the lab and has a lot of features that are really interesting, but I don't know what sponge that is," Oakley says.