A fascinating event that can occur in evolution is speciation; mutations bringing about new species that are significantly different to their ancestor. But how does this happen, what causes it, and how can we watch it happen?
Observing the processes of evolution can be a daunting task. After all, in an experiment you need to be able to measure the results, but evolution occurs over a long, long, LOOONNG time… doesn’t it?
Meyer et al.  got around the challenge of waiting for organisms to evolve by studying those with a much shorter lifespan. Bacteriophage λ (lambda) is a virus that reproduces by infecting its bacterial host Escherichia coli.
You’ve probably heard of E. coli. – they live in our gut, often with us none the wiser, but some types can make us sick. E. coli. is readily available and easy to grow, and boy do they grow quickly! Each new generation of E. coli. can be ‘born’ in less than 20 minutes , and can have a different genetic fingerprint to their parent due to things like mutations and genetic material being switched around during reproduction. This short generation time allows scientists to observe THOUSANDS of generations worth of change within a few weeks.
But back to the star of the show (E. coli. is just the host after all): bacteriophage λ.
This virus (also known as ‘phage’) has the ability to attach itself to an E. coli. cell, inject its DNA, then use the host machinery to replicate itself. Once there are many copies of phage λ the cell bursts open, releasing new phage progeny into the environment. Phage λ does this by having binding proteins on its ‘feet’ (we call them tail fibers) that match up to receptor proteins on the cell’s outer membrane. Most λ phage can only match to one receptor called ‘LamB’, but some types of λ can use another receptor as well.
Meyer and his colleagues wanted to explore what would happen if they took a type of phage λ that could match with two different receptors (LamB and OmpF), and let it go through many cycles of reproduction with E. coli. cells that have the LamB receptor, OmpF receptor, or both. The type of E. coli. cells that were provided represent a selection pressure for each set of phage to change the way that it attaches to host cells – evolution towards being better able to utilise the resources available.
The phage that could use both receptors was grown with each of three different types of E. coli. (LamB only, OmpF only, or both receptors), with six replicates of each scenario. Every 8 hours the phage population was moved to a new culture of bacteria, to prevent the E. coli. progeny from evolving resistance to the phage and affecting its ability to adapt. They wanted to see how phage λ would respond to only having one receptor available, and what it would do when it had access to both.
With only one option available, phage λ almost always evolved to specialise to that receptor. All six of the phage populations grown with the LamB-only E. coli. lost their ability to use the OmpF receptor, and five out of six grown with the OmpF-only cells became unable to use LamB.
Pretty easy to make a decision when you only have one option! However, when both receptors were available, most λ populations developed weaker specialisations to either of the two receptors, and some retained the function to use both.
More LamB specialists evolved than OmpF, which the authors attributed to fewer mutations being required to become a LamB specialist, making it a simpler adaptation to achieve. This was determined by looking at the DNA sequence of J, a key gene in host recognition, from one phage progeny of each experimental group and comparing differences in the sequence that could bring about a change in function.
To test that these genetic sequence changes were responsible for the phage specialising on one receptor, Meyer et al. created phage with engineered genomes containing the mutations observed for each group. These modified phage were found to specialise or generalise in the same way as their evolved counterparts. A constructed hybrid with both sets of specialist mutations was not viable, indicating genetic incompatibility between the two specialists.
While this experiment didn’t go so far as to bring about the creation of a new species, it demonstrates how easily mutations can render two members of a species distinct from, and even incompatible with, each other. Genetic incompatibility is one of the criteria considered when defining speciation, with two members of different species being unable to produce offspring. While the phage λ progeny in this experiment were still far too genetically similar to qualify as distinct species, they are an example of how selective pressures can bring about adaptive mutations in a relatively short period of time, which has the potential to lead to speciation events. Let’s give them a few more weeks and see what happens…
- Meyer, J.R., et al., Ecological speciation of bacteriophage lambda in allopatry and sympatry. Science, 2016. 354(6317): p. 1301-1304.
- Todar, K. The Growth of Bacterial Populations. Online Textbook of Bacteriology 2012; Available from: http://www.textbookofbacteriology.net.