Genome Size and Flight in Bats

i-68efa548cdb44e33126c5936c96fe3ed-evolution_2008.jpgContinuing with our discussion of the Evolution 2008 conference, I was hoping to meet T. Ryan Gregory yesterday. He is listed on the Evolution 2008 program as an author of a talk on genome size. Goodnews/badnews: Gregory did not show, but the talk, given by his coauthor working in his lab, was excellent, so we didn’t need him.

The research was done, and the paper delivered, by Jillian Smith. The title of the paper was “Genome size evolution in mammals” but it was more focused on specific results Jillian had come up with regarding bats.

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The bottom line is this: Genome size does not have a lot of function or interest other than simply knowing the size of the genome you are about to delve into on a research project (so it’s like knowing how far away your field site is so you can budget travel, or how big your grocery store run is so you can decide if you can walk to the store or drive, etc.). But there are a couple of ways in which genome size might be interesting in a way that relates to adaptation and/or evolution.

One possible relationship is this: Gnome size relates to cell size for unknown reasons (presumably the “Gregory Lab” is working on this, though that wasn’t mentioned). Cell size relates to metabolism in such a way that smaller cells would make more sense in an organism that required high energy demand in certain tissues on a regular basis (or even occasionally, but on important occasions). Due to the nature of flight, it would make sense that flying animals would have a number of adaptations not only to make flight work, but to make flight efficient.

Thus, it was not surprising to discover some time ago that birds have small genomes and thus small cells. The bird story is a little more complicated than this, but that is the idea.

So Jillian looked at a particular group of bats and tried to assess overall genome size using the “Gregory Lab”‘s fancy genome measuring methods. She also tried to test for possible relationships between genome size across this range of bat species and specific aspects of flight. This resulted in somewhat ambiguous outcomes, but frankly, I think the research is still being analyzed and I would not be surprised if she eventually nailed down something interesting here.

In the end, it turns out that bats do have smaller genome sizes than expected.

Two questions were raised about this research that I want to mention, because I think they are examples of both valid issues and pedantic whining. What I mean is that these are things that everyone needs to know about and pay attention to, but at the same time, these are the kinds of things one often hears from people from “The Other Lab” at conferences, or from individuals who just like to throw around wet blankets. I’ll explain in a moment why these particular questions were in my view kinda dumb.

The first was (paraphrasing) “Individual bats vary a great deal … but you were comparing across species with no reference to within species variation. What’s up with that, huh?”

Jillian noted that this was true and that she was using species averages for things like body size.

Ultimately, it would be good to obtain samples of a couple dozen individuals each of three or four species and measure the variables across them. But genome size would not vary, so this would only be a matter of measuring the flight characteristics. That may lead to interesting results. If when looking at a few species it becomes apparent that interspecies variation exceeded interspecies variation for the relevant variables, and the means are close-ish, then it is time to take a taxon-free approach, perhaps. But otherwise, species averages are fine. In the end, this research will use species averages because that is what will work.

The second question was “There are a lot of differences between bats and other mammals … how do you know that these other differences (which I the questioner will not bother mentioning because I don’t really know what they are, probably) account for the genome size effect? Huh? What about that???” (or words to that effect).

That is another good question in a way, but it is also utterly obvious. No biologist goes out these days and says “I’m interested in this one thing, so I assume it explains everything.” No. You take this one thing, show that there is a possible link owing to a correlation (or whatever) then you make a hypothesis that there is a link (carefully, in a way that is testable and biologically sensible).

Then you set about falsifying your hypothesis. And you are the one to do this because this thing … in this case genome size … is what you have the ability to measure or otherwise address because that is how your resources and training are oriented.

If you give it a good try and can not falsify the hypothesis, then we’ve got something. A possible explanation. Now someone with a different system in hand and in mind can make different hypotheses and test them.

If different individuals or labs, or the same research groups over time, can’t falsify two or more different hypotheses related to the same phenomenon in the same system, then things get interesting. And so on. But eventually, what usually happens is that scientists run out of alternative ideas and end up accepting one or a few related explanations or descriptions of a system as likely adequate to explain.

This is the question I asked at the end: “So, to be clear, your research fails to falsify the hypothesis that bat genome size is small relative to other mammals for reasons of flight.”

Answer: “Yes. That is correct.”

So the hypothesis stands for now.


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Comments

  1. #1 PhysioProf
    June 22, 2008

    There are some flightless bats, right? Looking at their genome size would be a good test of the hypothesis.

  2. #2 Greg Laden
    June 22, 2008

    I had no idea there were flightless bats. (are you sure you are not thinking if rats?)

    But yes, that would be good. Flightless birds seem to have larger genomes than flighty birds.

  3. #3 Greg Laden
    June 22, 2008

    The story of flightless bats is here.

    There are not flightless bats, but there are bats that fly less and those that fly more (or harder, or whatever). This was taken into account in Jillian’s research by the way (a detail I left out).

  4. #4 Coturnix
    June 22, 2008

    Sometimes a typo is really funny, in a kinda appropriate way. ‘Gnome size’ is generally regarded to be quite small ;-)

    Still, very cool research. I wish I could have been there.

  5. #5 TR Gregory
    June 22, 2008

    Thanks for the nice writeup. I am sure Jill will be pleased at the kind words. To answer a few questions…

    - I don’t go to many of the big meetings anymore. If I am one of the invited speakers I do, but mostly I find the big ones (SSE, SMBE, ESEB) too long and exhausting. I also am in the middle of some things here and could not get away for the conference this summer.

    - Jill is working on a lot of other groups. We focused on “megabats” because it’s one group that 1) had never been studied (cf. “microbats”), 2) was a reasonably coherent dataset (a lot of the rest is in progress), and 3) relates to an interesting question that can be addressed within this one taxon.

    - Intraspecific variation, other possible correlates, yadda yadda, are standard questions. Obviously we are interested in these issues and will address them when we have more than 12 minutes to cover things.

    Again, thanks for the nice words — this was Jill’s first big presentation and it is good to know you thought it was excellent (I was confident it would be).

  6. #6 Barn Owl
    June 22, 2008

    Many bat species seem to have extreme longevity, given their size and high metabolic rate-especially when they’re compared to small insectivores and rodents. Don’t know whether this has any correlation to genome size, though. However, longevity does seem to correlate with lower free radical production and perhaps with efficiency at repairing oxidative stress-related damage, and it’s another way in which bats resemble birds.

    Sounds like an awesome meeting, with lots of great research presentations!

  7. #7 Greg Laden
    June 22, 2008

    TR: Thanks for the additional information.

    It was not at all clear that this was Jill’s first time at bat. Oops.. (A truly unintended pun, honestly …) And a well done presentation by a student is the sign of an excellent adviser.

  8. #8 David Marjanovi?
    June 23, 2008

    Cell size relates to metabolism in such a way that smaller cells would make more sense in an organism that required high energy demand in certain tissues on a regular basis (or even occasionally, but on important occasions).

    The surface-volume ratio seems to be what matters here.

  9. #9 kpss
    July 25, 2009

    Cell size relates to metabolism in such a way that smaller cells would make more sense in an organism that required high energy demand in certain tissues on a regular basis (or even occasionally, but on important occasions).