Multicellular Life origins

All Multicellular Life On Earth Is The Result Of A Random 600-Million-Year-Old Accident

January 12, 2016 | by Janet Fang

photo credit: Fluorescence micrograph of a small choanoflagellate colony, with one of its cells dividing. Ken Prehoda 

 

Just one random mutation 600 million years ago made it possible for our earliest, single-celled ancestors to evolve into complex organisms. According to a new eLife study, changing the function of one ancient protein led to multicellular life as we know it. 

Life on Earth consisted of single cells for billions of years. Multicellular organisms eventually arose as single cells began taking on specialized roles and working together in various arrangements that ultimately became the tissues and organs of animals, plants, and fungi. However, very little is known about the molecular mechanisms underlying the evolution of multicellularity. 

We do know that neighboring cells must coordinate with each other on their positions when they divide (an important part of replicating themselves). And one key aspect of this process is the orientation of a structure called the mitotic spindle, which distributes the chromosomes of the parent cell among the two daughter cells. Spindles that aren’t oriented properly can result in cancer, among other malformations. In many animals today, the GK protein-interaction domain (GKPID) mediates the orientation of spindles by linking the spindle to specific “marker” proteins on the edge of the cell. 

A team led by the University of Oregon’sKenneth Prehoda and the University of Chicago’s Joseph Thornton wanted to figure out which genes are behind the coordination of multiple, single-celled organisms. Our closest living unicellular relatives are free-living, single-celled organisms called choanoflagellates (pictured above). They live in the ocean, swimming around and gathering food with the help of a short tail (or flagellum). To the right is a solitary choanoflagellate in the process of dividing, with its DNA in blue and mitotic spindle in green. They sometimes cluster together into multicellular colonies with their flagella radiating outwards. 

The team used a “time-traveling” technique called ancestral protein reconstruction – which combines gene sequencing with computer modeling – to reconstruct the genomes of ancient organisms based on the DNA of their living descendants. The team found that GKPID evolved and commandeered control of spindle orientation from an ancient mechanism: The complex was assembled through a series of “molecular exploitation” events that repurposed proteins for new and different roles. 

“New protein functions can evolve with a very small number of mutations,” Prehoda explains in statement. “In this case, only one was required.” The use of marker proteins can be recreated by introducing one substitution into the ancestral GKPID that the team had constructed. This mutation allowed single cells to organize into multicellular life over evolutionary time. 

The team also found these tails are critical for organizing multicellular colonies – suggesting that the connection between flagella and the orientation of cell division was important for the transition of our single-celled ancestors to a multicellular lifestyle. The tails became less important as that single mutation made it possible for newly created cells to orient without them. /.../