It is a common misunderstanding that the evolution has a sense of directions, a term that nerds biology is constantly trying to correct around the world.
But new research reveals that there may be some semblance of truth to this misunderstanding, at least more than we ever realised.
While not as simple as a mutation with a purpose, it now appears that not all DNA is the same when it comes to being mutable. At least not in a roadside flowering weed known as thale cress (Arabidopsis thaliana).
“We always thought that the mutation was basically random throughout the genome,” says plant scientist Gray Monroe of the University of California, Davis.
“It turns out that the mutation is very non-random & it’s non random in a way that benefits the plant. It’s a whole new way of thinking about mutation.
In order for a genetic mutation (or variant) to occur in the first place, several things have to be right for-it. First, DNA must be altered with-in germ cells, cells that pass their genetic material to organism offspring.
This can be a change in a single “letter” in a DNA sequence, for example due to UV damage, the loss of a gene, or mix up of an entire chromosome due to errors in copying and transferring genetic material.
Then this damage must elude several cellular mechanisms that prevent the transfer of these changes. This includes DNA repair systems or, for extreme mutations, programmed cell death (apoptosis).
If the mutation-evades these processes, it can be passed on to the next generation.
Most mutations that include changing a single “letter” are neutral because they do not cause significant changes in the form or function in organism.
But those that cause change, if they continue through following generations, may subject to whims of natural selection.
It was at this point that evolution was thought to do most of sorting between good mutations & duds. For example, if a mutation hinders survival of a plant or animal, it is unlikely to stay long.
While the forces of selection can limit mutations passed from generation to generation, the mutation itself has generally been regarded as an unpredictable roll of the dice in the organism’s genetic library.
“Since the first half of the 20th century, the theory of evolution has been dominated by the idea that mutations occur randomly with-respect to their consequences,” the team wrote in their paper.
Monroe & his colleagues used the plant equivalent of a lab mouse – the aforementioned thale cress – to test assumption that the mutation was indeed randomly distributed throughout a genome. They analyzed the genomes of 400 plant lines and, to their surprise, this is not what their data presented.
Instead, they found that certain regions of plant’s genome were much more prone to mutations than others.
“These are the really important regions of genome,” says Monroe. “The most biologically important areas are those protected from mutation.
This was true whether they were looking at coding or non-coding parts of the genetic code, suggesting that the effect is not due to specific DNA types but to the region as a whole.
“Evolution around the genes in Arabidopsis appears to be explained more strongly by mutation bias to a greater extent than-by selection,” Munroe & his team write, explaining that if this discrepancy had later been caused by natural selection, their analysis would have detected more unique-genetic variations than observed
In addition, the data revealed epigenetic factors, such as the way DNA wraps around certain proteins & DNA repair mechanisms predict which parts of the genome are less prone to mutation. There was already strong evidence that DNA repair is-targeted-to active genetic regions. which also supports this study.
Knowing how Thale loads the dice in terms of mutations could have implications not only for other plants, but also for understanding evolution & disease in almost all species.
“That means we can predict which genes are more likely to mutate than others and that gives us a good idea of what’s going on,” says Weigel.
“It’s exciting because we might even use these findings to think about how to protect human genes from mutation.
These results suggest that natural selection has skewed the probability of mutations in an organism’s genetic library.
Although an individual mutation remains randomly in terms of their consequence, the position is biased by a genome to promote the survival of an organism, even before the possible effects of mutation comes in-to play in the game of natural selection.