By deleting genes involved in mobility, scientists ask how bacteria can evolve to regain the ability to move.
One of the central processes required for evolution is the development of new functions, which is usually accompanied by the formation of new genes or a change in existing genes. The easiest way to accomplish this is by a small change in the gene function or time of action. This has been the target of several studies. Genes can also radically change their function. This usually involves the duplication of genes. While the “original” gene can keep it’s function, the “duplicated” gene can accumulate mutations without much of a detrimental effect to the organism.
Over the course of evolution, genes have been duplicated several times, and even the whole genome has been duplicated various times. An open question in evolution is how duplicated genes gain new functions and integrate into existing processes. Bacteria are good organisms to tackle this question due to their short life-cycle and adaptability to new environments.
Previous experiments focused on placing bacteria in different environments and investigate how their genes adapt to those new environments. These studies usually do not involve any prior manipulation the organism being study, therefore this organism will have all of it’s genes at it disposal in order to adapt to those new environments. In these cases usually there is a slight change to the function of a gene or group of genes involved in the development characteristic of the organism that had to change to adapt to these environments.
A team lead by Dr. Robert W. Jackson, from the University of Reading, took a different approach to this question. Instead of using a bacteria which possessed all the genes it would normal have, they took a bacteria that lacked genes required for them to move. In this case, for the bacteria to start moving again, which is required for their survival, a gene previously not involved in making the parts required for movement would have to gain that function.
More specifically, they took a strain of Pseudomonas fluorescence that lacks a gene necessary for the formation of flagella (fleQ) , a structure that helps the bacteria to move. These bacteria are immobile. The bacteria were then let grow on a dish which contains the nutrients necessary for the bacteria to grow. Since the bacteria cannot move, but keep multiplying, the nutrients in the area the bacteria are growing will be rapidly depleted. As such, this imposes a strong selection for motility, because the few bacteria that gain the ability to move again will be able to go to regions within where nutrients are still available, and will keep surviving and multiplying.
The researchers found that after 96 hours the bacteria had acquired two mutations on two genes, ntrB and ntrC, which allowed them to move again. Both of these are involved in nitrogen uptake and metabolism, not in the formation of flagella, with the NtrB protein activating the NtrC protein. At first glance it is unclear how mutations in these genes could have restored motility to the bacteria. The first mutation on ntrB partially restores the activity of genes involved in the formation of flagella and increases nitrogen uptake and metabolism. On the other hand the second mutation, on ntrC, decreases the activity of genes involved in nitrogen uptake and metabolism, but sill increases the activity of flagellar genes.
In short, while the second mutation ultimately leads to formation of flagella again it also decreases nitrogen metabolism. However, this is compensated by the first mutation and in the end nitrogen metabolism is not compromised. Therefore, when compared with the original bacterial which did not possess any flagella, this new generation restored that function without any obvious trade-off.
It is worthwhile noting that the strains of P. fluorescence used in these studies double their population every 14-16 hours, making a total of around 6 generations. This study shows how predictable changes in gene function can be, and how fast this can happen, requiring only 2 mutations. Although the authors point out that in Nature these processes would probably require more time and mutations.
This work showed how genes that gained a certain function during the course of evolution can be regain a function it had lost, and that in bacteria this can be achieved in a few generations. While the bacteria had hundreds of genes at their disposal, the mutations to restore the lost function always happened in the same few genes, which shared some similarity to the lost gene, suggesting that the mechanisms that lead to the formation of a new structure are likely to be repeated in different circumstances.