An artificial 鈥渕inimal cell鈥� that has had all but the most essential genes stripped out can evolve just as fast as a normal cell. The finding shows that organisms can rapidly adapt, even with an unnatural genome that provides little flexibility.
鈥淚t appears there鈥檚 something about life that鈥檚 really robust,鈥� says at Indiana University in Bloomington. 鈥淲e can strip it down to just the bare essentials,鈥� he says, but that doesn鈥檛 stop evolution going to work.
Lennon and his team studied a bacterium called Mycoplasma mycoides, a parasite that lives in聽the guts of animals like cows.聽Because it gets most of聽its聽nutrients from its host, 惭.听尘测肠辞颈诲别蝉 has naturally lost聽a聽lot聽of genes.
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In 2016, researchers led by Craig Venter at the in California reported that they had stripped the bacterium鈥檚 901-gene genome back even further, to just 493 genes. The resulting synthetic organism, 惭.听尘测肠辞颈诲别蝉 JCVI-syn3B, has a 鈥�minimal genome鈥�, the smallest of any known free-living organism.
M. mycoides JCVI-syn3B can grow and divide normally, but Lennon wondered what would happen to it in the long term. Species need to change to survive, but it seemed likely that the minimal cell would have trouble evolving.
鈥淓very single gene in its genome is essential,鈥� says Lennon. 鈥淭he cell聽聽 has zero degrees of freedom.鈥� As a聽result, any mutations that arise would be expected to be harmful.
Lennon鈥檚 team began by establishing that the minimal cell聽could still mutate. It does so to聽such a degree that, even given a聽small population size of just 10聽million, every single genetic 鈥渓etter鈥� would聽be expected to mutate more than 250聽times over聽2000 generations.
The team then grew 惭.听尘测肠辞颈诲别蝉 JCVI-syn3B in the lab, allowing them to evolve freely for 300 days.
Next, the team set up some head-to-head contests. In some experiments, the minimal cells that had evolved for 300 days were pitted against the original, non-minimal 惭.听尘测肠辞颈诲别蝉. In others, the non-minimal cells went up against minimal cells that hadn鈥檛 evolved for 300 days.
In all contests, the team put equal quantities of the strains being assessed in a container and observed which one became more common, a sign of which was better suited to its environment.
The unevolved minimal bacterium was 鈥渞eally sick鈥�, says聽Lennon, and was easily outcompeted by the non-minimal version. But the version that had evolved for 300聽days did much better, recovering 80 per cent of the lost fitness (bioRxiv, ).
Crucially, the team identified the genes that changed most during these evolution contests, says at the University of Illinois at Urbana-Champaign, who wasn鈥檛 involved in the study. Some have unknown functions. 鈥淵ou have to go and ask聽yourself, 鈥榃hat does that thing聽do?鈥欌�娾�� she says.
