Genome Size and Flight in Bats

The best of last June
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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