Want to make a new species? Put a little pressure on it…

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?

evolution of man

A depiction of the evolution of man. Image credit: owutranscript.com 

Meyer et al. [1]  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 [2], 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.

lambda phage

Artist’s interpretation of bacteriophage λ on an E. coli. cell. Image credit: theminione.com 

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.

Teacup evolution

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.

 

F1.large

Figure 1 [1]. In the ‘Allopatric’ group, red bars represent phage λ evolved exclusively with OmpF receptor while blue represents phage λ evolved with LamB. In the ‘Sympatric’ group, phage λ evolved with both OmpF and LamB were then isolated in equal proportions from cultures with one or the other receptor (red & blue bars respectively). The length of the bars shows the extent to which each group specialised, with -1 (OmpF) and 1 (LamB) indicating complete specialisation.

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…

 

References:

  1. Meyer, J.R., et al., Ecological speciation of bacteriophage lambda in allopatry and sympatry. Science, 2016. 354(6317): p. 1301-1304.
  2. Todar, K. The Growth of Bacterial Populations. Online Textbook of Bacteriology 2012; Available from: http://www.textbookofbacteriology.net.

About jcturnbullnz

BSc Genetics student and phage hunter
This entry was posted in Experimental Evolution and tagged , , , , , , , , . Bookmark the permalink.

10 Responses to Want to make a new species? Put a little pressure on it…

  1. msbdavies says:

    Hi Jo
    I’ll have to keep a close eye on my E. coli in the lab, who knows what they’re up to!

    With “constructed hybrid with both sets of specialist mutations”, do you know the exact model of this hybrid and if they tried multiple designs to include both sections (such as a duplication of the entire region)?

    Thanks,
    Briar

  2. jcturnbullnz says:

    Hi Briar

    I know right, all this time we thought it was the lab elves moving stuff around…

    The hybrids were constructed using the engineered alleles rather than material from the evolved specialists. Using MAGE (Multiplex Automated Genome Engineering) with 90-mer ssDNA fragments meant the authors could make multiple small/specific insertions or deletions simultaneously. The paper did not give an exact model for the hybrid, but the supplementary data (table S2) includes the oligo fragment that was used to modify an engineered ancestral lysogen. The literature does not mention whether multiple approaches were tried in constructing the hybrid, but the construction was repeated with a second lysogen to confirm the inviability of the hybrid phenotype.

  3. aearnshaw9 says:

    Hi Jo,

    I love the concluding remarks about how genetic incompatibility creates new species – in a very technical sense would this mean that BIG dogs and small dogs are evolving into being new species? Hmm…
    My question is how would the phage have evolved to use both receptors, especially if it is easier to evolve to be a specialist? Asides from being able to use two receptors to enter, is there any benefits? Due to the experiment, it seems difficult to remain a ‘generalist’ phage.

    Let me know if I’m not understanding correctly!
    Alyssa

    • jcturnbullnz says:

      Hi Alyssa

      Good point! I think this is an extention of the ‘dogs evolved from wolves’ discussion. Theoretically, given enough time some dogs could speciate again as wolves did, and i think the dramatic size differences in domestic dogs today would be both a pressure for sexual selection and contribute towards them eventually being reproductively incompatible.

      I’m not sure that it’s easier to evolve towards specialisation – changes will happen based on selection pressure, and specialising is (like anything) going to be a trade-off between enhanced adsorbtion to the receptor they’ve specialised to, and overall resource (receptor) availability. Knowing whether there are any other benefits aside from being able to exploit 2 receptors is beyond my knowledge on the specific genes involved, but when considering ecology on a more general scale, the ability to exploit both receptors could be extrapolated to an increased likelihood to thrive in a wider range of environments.

  4. jessealbany says:

    Hey Jo,

    I appreciate how quickly i was caught up with the topic having not learned about phage all that much myself. If the researchers do end up releasing another paper with more generations i would be interested to see if they can get some speciation happening.

    Keep us posted!
    Jesse.

    • jcturnbullnz says:

      Thanks Jesse. I don’t think actual speciation is a likely outcome for this experiment if they continued it – while they were able to achieve some elements of reproductive isolation, taxonomy has its own set of considerations when defining a species. I think they’re just scratching the surface but it’s interesting that they were able to bring about a change in phenotype so quickly, and infer a relationship between that and a precursor for speciation.

  5. annabehlingnz says:

    A great and easy to follow post.
    Particularly interesting to consider in the context of the living/non-living debate that revolves around viruses — does this demonstration of evolution among phage provide weight to the ‘living’ side?

    • jcturnbullnz says:

      Thanks Anna! In my opinion, this doesn’t lend any more weight to the viral living/non-living debate than the fact that phage have DNA in the first place.

      However, there are traits that some phage have which go beyond something relatively simple like changing which receptor they use – for example there are large bacteriophages that form a protein compartment around the DNA they inject into a bacterial cell, which pretty much mimics the protective function of a nucleus. If an adaptation is mimicking a more complex cell that’s of a eukaryote/organism that is definitely considered alive, i think that is a good consideration for when we’re sitting on the front porch in our rocking chairs at sunset wondering whether viruses are alive or not.

  6. lshewson says:

    I love viruses and bacteriophages, this post was easy to follow which I appreciate after reading journal articles for my research 🙂

    Makes me think about what uses the receptors for phages have for E.coli. Must be very important for their survival considering having them open up the risk for infection.

    • jcturnbullnz says:

      Hi Liam, thank you.

      As well as being a receptor for the lambda phage, the LamB protein is a ‘maltoporin’ sugar transporter – it makes a little path to send maltose-based sugars through the cell membrane. OmpF makes little pores that let small molecules move across the outer membrane via passive diffusion.

      I think the benefit of getting rid of a phage receptor would have to outweigh the consequences of losing those other functions, and the impact that would have on fitness/survival overall. It’s possible that there are E. coli that do disable these receptors and just die off.

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