Researchers studying a sponge-like creature say they’ve found evidence that one gene mutation more than 600 million years ago may have paved the way for multicellular life.
Mutations can lead to good or bad results, or even to a combination of the two, says Ken Prehoda, a professor in the chemistry and biochemistry department at the University of Oregon.
“Proteins are the workhorses of our cells, performing a wide variety of tasks such as metabolism,” he says. “But how does a protein that performs one task evolve to perform another? And how do complex systems like those that allow cells to work together in an organized way, evolve the many different proteins they require?
Our work suggests that new protein functions can evolve with a very small number of mutations. In this case, only one was required.”
Prehoda’s team began looking at choanoflagellates with the help of Nicole King’s group at the University of California, Berkeley. Choanoflagellates are a group of free-living, single-celled organisms considered to be the closest living relative of animals.
Top: A fluorescence micrograph of a dividing solitary choanoflagellate showing its DNA (blue) and flagella and mitotic spindle (green). Bottom: A small choanoflagellate colony with one of its cells dividing. (Credit: University of Oregon)
These sponge-like, seawater-dwelling organisms have a short, outward-facing squiggly tail called a flagellum that allows them to move and gather food. Choanoflagellates exist both in a single-celled solitary form, but also as multi-cellular colonies.
Prehoda and colleagues then used ancestral protein reconstruction, a technique devised at the by co-author Joseph W. Thornton, a biologist at the University of Chicago.
By using gene sequencing and computational methods to move backward in the evolutionary tree, researchers can see molecular changes and infer how proteins performed in the deep past.
Our Single-celled Ancestor
In the new research, gene sequences from more than 40 other organisms were put into play.
The team’s reconstruction identified a mutation that was important for opening the door to organized multicellular animals that eventually no longer needed their tails.
They also found that the choanoflagellate flagellum is critical for organizing its multicellular colony, suggesting that this may have also been the case as our single-celled ancestor transitioned to a multicellular lifestyle.
The work suggests that the tail’s role became less important when the gene for an enzyme duplicated within cells, and a single mutation allowed one of the copies to help orient and arrange newly made cells. The protein domain that resulted from this mutation is found today in all animal genomes and their close unicellular relatives but absent in other life forms.
“This mutation is one small change that dramatically altered the protein’s function, allowing it to perform a completely different task” Prehoda says. “You could say that animals really like these proteins because there are now over 70 of them inside of us.”