Swarms of tiny xenobots, ‘living robots’ made of biological material, multiply themselves by pushing individual cells together. This mode of reproduction has not been seen before in multicellular organisms.
Last year, scientists created xenobots for the first time. For this they used embryo cells of the clawed frog species Xenopus laevis. Under the right conditions in the lab, these cells rearrange themselves into small structures that move in groups. Now those same researchers discovered that these xenobots can also replicate themselves.
clump of cells
Evolutionary Engineer Josh Bongard from the University of Vermont and biologist Michael Levin of Tufts University in Massachusetts and their colleagues began the study with skin stem cells from frog embryos. They brought the stem cells together in clumps, from which spheres formed within five days. Each sphere consisted of about three thousand cells and was therefore about half a millimeter wide. Also, the spheres were surrounded by small hair-like structures. They act like flexible oars, propelling the xenobot forward in a corkscrew-like manner, Bongard says.
The team also noticed that the clumps of cells worked together in a swarm. In this way they pushed other individual cells together in the dish. Those stacks of cells then gradually formed new xenobots.
Collaboration
The team then placed groups of twelve xenobots in a dish, along with 60,000 individual cells. This showed that the xenobots were working together to create one or even two new generations. “One parent pushes single cells onto a stack, and then, by chance, a second parent pushes more cells onto that stack, and so on. This ultimately creates the child’, explains Bongard.
Each new round of replication produces smaller and smaller xenobot babies. Ultimately, the spheres are no larger than fifty cells and can no longer move themselves. With that, the ability to reproduce itself also disappears.
Optimize
In an effort to create multiple generations of xenobots, the team turned to artificial intelligence. Using an algorithm based on evolution, they predicted which shapes xenobots produced the most generations.
This simulation predicted that clusters of cells in the shape of a C would yield the most generations. The naturally spherical xenobots were then cut into a C shape. It produced up to four generations of xenobot children: twice as many as the spherical xenobots.
“By manipulating the shape of the parents into a C shape, you create a better scoop to move more cells,” says Bongard.
Big step
It has been shown for the first time that multicellular organisms can reproduce in this way, and not through growth in or on their own body. “Our work shows that there is a hitherto unknown way for life to replicate itself,” said Bongard.
Some team members hope to use the xenobots in investigating how Earth’s first organisms may have reproduced.