Sponge genome wrings out evolutionary clues
UC SANTA BARBARA (US)—Scientists have sequenced the complete genome of a living marine sponge dating back hundreds of millions of years.
The Great Barrier Reef marine sponge, from a 650 million-year-old group of organisms, not only points to rich genetic resources available at the dawn of the animal kingdom, but also reveals some basic information about cancer.
The findings are published in the Aug. 5 issue of Nature.
“This is a milestone sequence,” says Kenneth Kosik, Harriman Chair in neuroscience research at University of California at Santa Barbara and co-director of the Neuroscience Research Institute. “This sponge is the most basal animal for which we have a genome.”
A genome represents all biological information required to create and maintain an organism. DNA is the language of the genome, and DNA is separated into genes that give directions for the creation of discrete parts of any organism. The entire genome of the sponge, Amphimedon queenslandica, is available online.
The evolutionary origin of animals is marked by the ability of individual cells to assume specialized properties and work together for the greater good of the entire organism, Kosik explains.
“The sponge represents a window on this ancient and momentous event. Curiously, the cells of a sponge bear little resemblance to cells found in the rest of the animal kingdom.
Kosik and colleagues published a study in 2007 showing that while sponges lack neurons; the sponge genome reveals the presence of many genes found in neurons.
The sponge genome reveals that, along the way toward the emergence of animals, genes for an entire network of specialized cells evolved.
“This network laid the basis for the core gene logic of organisms that no longer functioned as single cells, but as a cooperative community of specialized cells—all geared toward the survival of a complex multicellular creature,” Kosik says.
The work also helps scientists to understand cancer, says Todd Oakley, professor of ecology, evolution, and marine biology.
“Once there is a transition from single cell to multicellular organisms, conflict is set up between the different cells of the multicellular organism. It is in an individual cell’s best interest to keep replicating, and this actually is what cancer is the uncontrolled replication of cells in the body.”
Evolution had to solve the problem of how to police this uncontrolled replication, Oakley explains. Normally, body cells don’t replicate out of control because there are policing mechanisms in place. When these policing mechanisms break down, cancer develops.
“So in the history of animals, we can see this link with cancer, because the genes that are involved in the transition to multiple cells during evolution are also known to be linked to cancer.”
Synapses have a unique structure. The sponge has no neurons; but has the genes that encode for proteins which are used in other animals to build synapses. Synapses are a set of proteins that allow two neurons to talk to each other. They also allow a neuron to talk to a muscle and make it move. No other cells have synapses.
Scientists must go far back in evolutionary time in order to understand the origins of the brain, neurons, and synapses,” Kosik says.
“You really have to go back to the beginning of animals to understand what happened that led to the awakening of the human brain. We had asked, ‘Can we look at the evolutionary origins of the synapse?’ ”
Kosik explains that the marine sponge is called the most basal of all animals because the sponge ancestor probably branched off first from the original animal, before any other existent lineage.
“You had some ancestral animal that is long-since extinct, and its descendants became these modern-day sponges that we have, and there were other descendants that became the rest of the animal kingdom—from jellyfish to baboons.
“We speak of the sponge as being this earliest branching phylum, or group of animals. What distinguishes the sponge from all the other animals is that it does not have any nervous system or synapses.”
The conclusion of that early work—the finding that genes that encode a synapse are present in the sponge—has important implications.
“The conclusion of that paper said that what evolution did was exaptation. This is a very important technical word. Evolution takes something that was evolved for one purpose and uses it for something else. Nature used this still-mysterious sponge structure to make the synapse, an exaptation of certain genes, which then became a key part of the nervous system.”
Kosik says that another reason the discovery is important is that one of the ways that some anti-evolutionists have criticized evolution as an idea is by using the term “irreducible complexity.”
Critics of evolution pose irreducible complexity as a problem. “The evolutionary biologists have very good answers to that question,” says Kosik. “One of them is the following result: A synapse looks like it is a structure that you cannot take apart; if you did, it would lose its function. In the sponge, nature clearly took it apart. And a piece of it is functioning very well in a species that has been quite successful for the last 650 million years.”
Researchers were surprised to find that, even though the sponges do not have synapses or neurons, they have the genes for synapses and neurons. The genes were there but they weren’t making the structures.
“This work raised a lot of questions,” Kosik says. “One obvious question is: What are the genes even doing there if they don’t have neurons or synapses? We still don’t know the answer to that question.”
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