Some species do it in pairs, some without knowing the other parties involved, and some even do it on their own: when it comes to replication, nature is nothing if not versatile.
Now researchers say they have found that clusters of frog cells can undergo a form of replication never before seen in plants or animals. The spherical clumps, known as xenobots, can give rise to “offspring” by sweeping up loose cells and swashing them into yet more clusters.
“These things move around in the dish and make copies of themselves,” said Prof Josh Bongard, of the University of Vermont, a co-author of the research.
Xenobots were first announced last year, and are what are known as “living robots”– synthetic lifeforms made by taking a few thousand cells from frog embryos and assembling them into clusters about 1mm in size.
Xenobots have no digestive system or neurons, and naturally fall apart after about two weeks. Bongard said the xenobots in the current study could propel themselves around using hair-like projections called cilia.
“They definitely do not grow into frogs, they actually keep the form that we impose on them. And they look and act in ways very different from normal frog,” he said.
One of those ways is the production of offspring. Anything that makes a copy of itself can be said to replicate, Bongard noted. But plants and animals have previously been found to do this by reproduction – mechanisms that range from the asexual process of budding to giving birth.
Writing in the Proceedings of the National Academy of Sciences, Bongard and colleagues report that xenobots take a very different approach called kinematic self-replication – a process previously seen for molecules but not organisms.
“Kinematic self-replication in molecules was definitely important at the start of life on Earth. But we don’t know whether this form of replication, which we now see in groups of cells, played a role in the life’s origins,” Bongard said.
The team made their discovery by watching the actions of xenobots in petri dishes containing room-temperature pondwater and loose cells from frog embryos.
“[The xenobots] move around in the dish in this corkscrew pattern and they crash into other loose cells, and sort of just smush and push them [into piles],” Bongardsaid.
The team found that as the cells are sticky, they can – if a pile is large enough – form a new, moving cluster over five days: a child xenobot.
But there is a hitch. “It turns out that these xenobots will replicate once, one generation, they will make children. But the children are too small and weak to make grandchildren,” said Bongard.
Using artificial intelligence, the researchers found that if the xenobots were formed into certain shapes, such as that of the video game character Pac-Man, replication continued for further generations.
Bongard said the hope was that self-replicating machines could eventually be developed to do useful work, with the team’s computer simulations suggesting the xenobots could fix electrical circuits.
“These are very small, biodegradable and biocompatible machines, and they’re perfectly happy in freshwater,” he said, adding that near-term applications could include collecting microplastics from waterways.
In the long-term, biobots made from our own cells could even be used in the body to remove the need for surgery, said Bongard.
Prof Mark Miodownik, the director of the Institute of Making at University College London, welcomed the research, saying it was “amazing science and another step closer to animate materials”.