Figure 1. An example of me responding a little quickly in public to a recently published paperF'ing Bulls**t MT“@WvSchaik: 'Host Demise as a Beneficial Function of Indigenous Microbiota in Human Hosts' paper http://t.co/aHifo7Rx2y”— David Baltrus (@surt_lab) December 16, 2014
This being said, I'm writing this post because another paper was published recently that linked together natural transformation and microbe-microbe killing. Honestly, I haven't had the chance to read the paper yet because I was sitting in airplanes all day yesterday. I'm guessing that I would probably have similar thoughts as I did about the type VI secretion paper last year. These thoughts come out in public sometimes:
Figure 2. An example of me responding to a recently published paper after sitting on airplanes all day and without reading the paperAnd the hand waving evolutionary explanations for linked regulation of competence and "trait X" continue. https://t.co/hCd1hXgehe— David Baltrus (@surt_lab) February 3, 2016
All right, so what's my beef with these studies? Let me be completely clear, I respect the authors and inevitably I have no problem with the experimental design or the actual reported science. In the type VI secretion case, and probably with this new paper, I have absolutely no problem with the experimental design or with the genetics. I don't think the papers are wrong science-wise. I'm writing about these papers because I spent the better part of 5 years in graduate school huddled in the fetal position thinking about the evolutionary effects of natural transformation in bacterial populations. My problems with a lot of these papers are usually directed solely at the evolutionary interpretations and spin within discussion sections and press releases.
There's a historical legacy that surrounds researchers of natural transformation in bacteria, where there are a couple of entrenched camps that tend to argue past each other. These fights usually flare up around disagreements that conflate questions about original evolutionary benefits of natural transformation and benefits of natural transformation that are measurable today (after these systems originally evolved). After many years of thinking about this, I'm actually agnostic when it comes to the original evolutionary scenarios for natural transformation. Rosie Redfield is one hell of a thinker and I defer to her about such things (so I guess that firmly places me within one camp). I tend to be more interested in wondering about how strong the selective forces on natural transformation are within present day bacterial populations and on trying to figure out realistic parameters that could affect our evolutionary interpretation of gene exchange in bacteria.
Like I said above, I think the genetics and molecular biology within these papers are tip top and have absolutely no quarrel with those. I get caught up when the discussions start to extrapolate from the controlled conditions of the lab environment into natural populations. Natural transformation certainly leads to gene exchange in natural populations of bacteria. However, suggesting that pathways are linked in regulation because of evolutionary benefits of natural transformation is a leap of faith that no paper out there has been able to tackle as of yet. What leads to the disconnect? There is a long standing tradition across many, many, many papers that describe evolutionary "just so" stories whereby we witness results in the lab or under certain conditions and think/assume that natural selection must act that way across many different conditions or environments. My comments about "hand waving" are usually directed at such extrapolations.
This is getting long, so I'll save the nitty gritty for another time. However, long story short, these extrapolations hinge on critical parameters of these experiments being similar in the lab and under natural conditions. There are no natural populations of bacteria for which we have realistic estimates of things like A) the DNA pool available for natural transformation B) natural selection pressures over space and time within and between bacterial populations that are exchanging DNA C) the repeatability and direction of these selection pressures D) how often cells encounter other cells that they can kill in nature E) having killed these cells in nature, how often these cells take up DNA F) evolutionary costs of natural transformation in nature G) I'm missing something because it's early in the morning but there are other parameters. When one sits down to write mathematical models that account for all of these above parameters, it ends up being REALLY difficult to find parameter space whereby natural transformation is GENERALLY beneficial. That's not to say that gene exchange doesn't matter within natural populations (as it certainly does) but it's hard to find situations where there are clear results where natural transformation is beneficial even a majority of the time. Under laboratory experiments like the ones in the type VI secretion paper (and probably the bacteriocin paper, again, haven't read yet) all of these parameters are actually controlled for pretty cleanly:
A) the DNA pool is controlled by the experimenters so that there is no contaminating DNA from other strains/species that could compete with genes of interest for uptake
B) Natural selection is really strong because these experiments are typically selecting for antibiotic resistance where cells pick up the relevant DNA survive and those that don't die. The same would be true if we experiments were set up to investigate phage predation, etc...
C) Typically in these lab experiments, there is only one direction for selection to act and the environment doesn't change over time (i.e. there is only one antibiotic that the cells need to become resistant to, and therefore one locus that they need to pick up through natural transformation)
D) lab experiments are usually biased so that cells are encountering cells that they can kill at pretty much optimal frequencies (50/50).
E) lab experiments are done under conditions whereby cells are highly competent for natural transformation
F) there are few costs for natural transformation systems in these lab experiments because the experiments themselves usually occur in relatively cush situations for bacteria (media containing abundant nutrients, controlled temperatures, etc...) and only take place under limited amounts of time. In fact, for most bacteria, if you passage them under lab conditions for extended periods of time they usually lower competence levels (which suggests an evolutionary cost). Like I said though, the lab experiments are only performed over limited amounts of passage time.
So to sum this all together, I apologize for any perceived slights. That's not my intention (yeah, I know get out the bingo cards). If you feel I've been too personal, please let me know and I'll try and fix anyway I can. There are many great groups focused on understanding natural transformation in bacteria and I respect much of their work. I've just spent way too much time worrying about evolutionary scenarios that usually pop up in discussion sections of these papers without (what I perceive) is firm grounding within evolutionary biology. These papers usually aren't usually set up to be direct tests of evolutionary theory, but it's very easy to write about how we think evolution should work. These papers usually end up being very good at describing the if natural transformation works for gene exchange under certain scenarios rather than how it's actually happening in nature. That's completely OK, just be careful about extrapolating.
*c'mon, that one was way too easy