how the bat got its wing

8/6/2019 How the bat got its wing

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How the bat got its wing.

by Stephen F. MathesonOriginally posted on Quintessence of Dust, May 2008.

Nothing can be more hopeless than to attempt to explain this similarity of pattern inmembers of the same class, by utility or by the doctrine of final causes. The hopelessnessof the attempt has been expressly admitted by Owen in his most interesting work on the'Nature of Limbs.' On the ordinary view of the independent creation of each being, we canonly say that so it is;that it has so pleased the Creator to construct each animal andplant.

The explanation is manifest on the theory of the natural selection of successive slightmodifications,each modification being profitable in some way to the modified form, butoften affecting by correlation of growth other parts of the organisation. In changes of thisnature, there will be little or no tendency to modify the original pattern, or to transposeparts. The bones of a limb might be shortened and widened to any extent, and become

gradually enveloped in thick membrane, so as to serve as a fin; or a webbed foot mighthave all its bones, or certain bones, lengthened to any extent, and the membraneconnecting them increased to any extent, so as to serve as a wing: yet in all this greatamount of modification there will be no tendency to alter the framework of bones or therelative connexion of the several parts.

from On the Origin of Species, 1st Edition (1859), Charles Darwin

The wing of a bat is an amazing thing. It's not just a wing; it's clearly a modified mammalian limb. Abat looks like a lot like a rodent with really long, webbed fingers on elongated arms.

Image fromAnimal Diversity Web at the University of Michigan.

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http://sfmatheson.blogspot.com/2008/05/how-bat-got-its-wing.htmlhttp://dx.doi.org/10.1038/nature09151http://dx.doi.org/10.1038/nature09151http://dx.doi.org/10.1038/nature09151http://dx.doi.org/10.1038/nature09151http://dx.doi.org/10.1038/nature09151http://dx.doi.org/10.1038/nature09151http://darwin-online.org.uk/content/frameset?viewtype=side&itemID=F373&pageseq=453http://animaldiversity.ummz.umich.edu/site/accounts/information/Chiroptera.htmlhttp://2.bp.blogspot.com/_bWetit67UXI/SC5Gl_wSxJI/AAAAAAAAAok/ZRdeRvsJEh0/s1600-h/bat_wings+400px+UMich+Animal+Diversity+Web.jpghttp://darwin-online.org.uk/content/frameset?viewtype=side&itemID=F373&pageseq=453http://dx.doi.org/10.1038/nature09151http://dx.doi.org/10.1038/nature09151http://dx.doi.org/10.1038/nature09151http://sfmatheson.blogspot.com/2008/05/how-bat-got-its-wing.htmlhttp://animaldiversity.ummz.umich.edu/site/accounts/information/Chiroptera.html


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Recent genetic analyses have yielded a fairly solid outline of the evolutionary history of bats, whichhave left a somewhat poor fossil record in which the earliest fossil bats look pretty much like modern

bats. It seems that bats arose relatively quickly during evolution, acquiring their distinctive feature powered flight in a few million years. No transitional forms have yet been found, which is a shame,

because this particular evolutionary transition is the kind that is otherwise reasonably approachablefor the detailed study of how changes in form come about.

The fossils can't yet show us how paws gave rise to wings, but that doesn't mean we can't test specifichypotheses regarding the paths that evolution could have taken. In fact, developmental biologists haveenormous resources that can be brought to bear on the question, by virtue of decades of research onthe development and genetics of the wingless terrestrial bat better known as the mouse. In early 2008,an interesting reportdescribed one kind of genetic change that can lead to bat-like bodies, and thefindings put some new wind in the sails ofevo-devo.

Two of the more remarkable aspects of bat wing structure are the forelimbs and the forelimb digits,what humans would call the arms and the fingers. Both are dramatically elongated in the adult animal,despite getting off to a very typical start during early development. Check it out: in the picture below,

bat and mouse limbs are compared with the image scaled so that body lengths are comparable.

Image from Figure 1 ofCretekos et al., cited below.

Developmental biologists have some pretty good ideas about how this might arise physiologically:certain growth factors (called bone morphogenetic proteins, or BMPs) are known to control limbgrowth, and some BMPs seem to be turned up in developing bat fingers. But the genetic mechanismsunderlying these processes are unknown.

Enter Chris Cretekos and colleagues, then working in a group in Houston headed byRichardBehringer. They set out to examine the genetic underpinnings of the elongation of the forelimbs(arms) of bats, using the formidable tools of mouse developmental genetics. And, clearly, they alsosought to directly test one of the central hypotheses of evo-devo: that changes in regulatory DNAsequences (as opposed to changes within the genes themselves) are a potent source of variation inevolution. Consider the beginning of their abstract:

Natural selection acts on variation within populations, resulting in modified organmorphology, physiology, and ultimately the formation of new species. Although variationin orthologous proteins can contribute to these modifications, differences in DNAsequences regulating gene expression may be a primary source of variation.

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http://dx.doi.org/10.1126/science.1105113http://dx.doi.org/10.1073/pnas.0509716103http://dx.doi.org/10.1073/pnas.0509716103http://dx.doi.org/10.1073/pnas.0509716103http://dx.doi.org/10.1101/gad.1620408http://dx.doi.org/10.1101/gad.1620408http://en.wikipedia.org/wiki/Evo-devohttp://dx.doi.org/10.1101/gad.1620408http://dx.doi.org/10.1101/gad.1620408http://dx.doi.org/10.1101/gad.1620408http://dx.doi.org/10.1073/pnas.0509716103http://www.isu.edu/bios/Professors_Staff/cretekos_c.shtmlhttp://www.bcm.edu/db/db_fac-behringer.htmlhttp://www.bcm.edu/db/db_fac-behringer.htmlhttp://2.bp.blogspot.com/_bWetit67UXI/SC-b9vwSxKI/AAAAAAAAApI/UpDYRK1xSMo/s1600-h/bat+vs+mouse+limbs+comparison+from+Cretokos+2008.jpghttp://en.wikipedia.org/wiki/Evo-devohttp://dx.doi.org/10.1101/gad.1620408http://dx.doi.org/10.1073/pnas.0509716103http://www.bcm.edu/db/db_fac-behringer.htmlhttp://www.bcm.edu/db/db_fac-behringer.htmlhttp://dx.doi.org/10.1101/gad.1620408http://dx.doi.org/10.1126/science.1105113http://dx.doi.org/10.1073/pnas.0509716103http://dx.doi.org/10.1073/pnas.0509716103http://www.isu.edu/bios/Professors_Staff/cretekos_c.shtml


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From C.J. Cretekos et al., "Regulatory divergence modifies limb length betweenmammals,Genes & Development22:141-151, 15 Jan. 2008

Besides their expertise in mouse genetics, the authors brought two major assets to their study: 1) theyhad already carefully mapped the developmentof the short-tailed fruit bat (Carollia perspicillata,"our model Chiropteran"); and 2) they knew a lot about the genetic control of limb length in other

mammals. In particular, they knew that the protein Prx1 is known to influence limb elongation, bycontrolling the expression of other genes. So they hypothesized that changes in the activity or level ofPrx1 might underlie the difference in limb length between bats and mice, and they were well-equippedto do the experiments.

First, the authors examined the Prx1 gene in the two species, and found that the overall structure ofthe gene is very similar in both mice and bats, and that the actual coding sequences of the two genesare almost completely identical. (Aligning the coding sequences showed that more than 99% of theamino acids are the same in both species.) In other words, the part of the Prx1 gene that codes forprotein is almost certainly not a source of variation between mice and bats. This could mean that Prx1doesn't have anything to do with the difference between forelimb length in these two species, or itcould mean the the difference is generated, at least in part, by variation in the regulation of the gene.

Cretekos et al. postulated that altered Prx1 regulation might be involved, and designed a coolexperiment to address this possibility.

They already knew that the Prx1 gene in mice contains known regulatory elements in particularlocations within the gene. (Such elements are often located in the DNA sequences that precede thecoding region.) When they looked at the bat gene, they found similar elements in the same location,

but these elements showed some intriguing variation: when the two regions were aligned, they sharedonly 67% identity, meaning that a third of the DNA bases were different in mouse and bat. They didsome nifty cell biology to show that this region did function as a regulator of the expression of Prx1,then did something that biologists could only dream about before the genomic era: they altered themouse genome by replacing the mouse regulatory region with the corresponding region from the batgenome. In other words, they gave a mouse a piece of a bat's genome, without actually changing the

coding sequence of any gene.

The result was dramatic, although it won't sound that way at first. The mice with the bat DNAdisplayed forelimbs that were 6% longer than normal. Why is this a dramatic result? Well, first of all,think about a 6% change in a major structural attribute. If adult males in a certain country average5'10" in height, a 6% increase would mean an increase of more than 4 inches. But more importantly,the Prx1 gene is known to account for about 12% of forelimb length mice that lack the genealtogether show a 12% reduction in forelimb length. That 6% change reflects a huge change in Prx1activity, a change that was completely due to alterations in regulatory DNA sequences without anychange in coding sequence.

If that's not impressive enough, the authors went on to examine the importance of this regulatoryregion in mice, by deleting it altogether. The result was very surprising, but very interesting: limblength in mice was completely unaffected by the loss of this chunk of regulatory DNA. (The region

we're discussing is 1000 bases in length.) This means that the Prx1 gene of both bats and micecontains a regulatory region that is completely dispensable for normal development but that can bealtered to generate significant changes in limb length, which points to significant evolutionarypotential in genetic regions that seem unimportant. Here's how the authors say it:

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http://dx.doi.org/10.1101/gad.1620408http://dx.doi.org/10.1101/gad.1620408http://dx.doi.org/10.1101/gad.1620408http://dx.doi.org/10.1101/gad.1620408http://dx.doi.org/10.1002/dvdy.20400http://dx.doi.org/10.1002/dvdy.20400http://www.dkimages.com/discover/Home/Animals/Mammals/Bats/Families/American-Leaf-nosed-Bats/Short-tailed-Leaf-nosed-Bat/index.htmlhttp://dx.doi.org/10.1101/gad.1620408http://www.dkimages.com/discover/Home/Animals/Mammals/Bats/Families/American-Leaf-nosed-Bats/Short-tailed-Leaf-nosed-Bat/index.htmlhttp://dx.doi.org/10.1002/dvdy.20400


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Maintenance of redundant enhancers for essential developmental control genes wouldallow changes in expression pattern to arise from mutations that alter regulatory activity

while preserving the required gene function.

So, why is this significant? Here are two aspects of the story that are worth highlighting.

1. The results provide strong (and rare) experimental support for the ideas of the evo-devo school. The currently-heated debate over the merits of evo-devo is focused on the central evo-devo claim that morphological evolution (i.e., evolutionary changes in form) is driven to a large extent

by changes in the regulation of gene expression, and less so by changes in the structures of theproteins that are encoded. To simplify, evo-devo postulates that significant evolutionary change likethat discussed here is more likely a result of the varied use of a protein toolkit than a result ofmodification of the toolkit itself. Cretekos et al. have presented a case in point, and one that isconsidered outstanding in that it documents a morphological gain; many previous examples showedonly losses.

2. The results provide a sharp picture of what Darwin's vision of "successive slightmodifications" means in terms of developmental biology. In this case, the modifications (of a

redundant regulatory region) can yield significant anatomical remodeling without altering proteinstructure at all.

The article was a notable advance for evo-devo and for evolutionary science, but soon there will surelybe many others like it. Desperate or ignorant creationists will always find a way to avoid facing theexplanatory power of common descent, but scientists are just plugging away, and for everyblog post

by a creationist ignoramus, there are 30 unheralded publications in the biological literature thatadvance our understanding of common descent and the mechanisms that generate biological novelty.

And they're fun to read.

Article(s) discussed in this post:

Cretekos, C.J. et al. (2008) Regulatory divergence modifies limb length between mammals.Genes & Development22:141-151.

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how the bat got its wing

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