‘Fight or Flight’ Might be Ancient

One way to think about front-loading is that a lot of the “heavy-lifting” was done very early in evolution, such that it is all “down hill” from there.  Well, it looks like the basic logic of the autonomic nervous system, which allows organisms to shift between a non-stressed state and a state needed to respond to threats (adrenalin) was in place very early in evolution:

Recursive front-loading

Since prestin represents a splendid example of convergent evolution at the molecular level, which in turn, supports the position that the blind watchmaker can be guided, one has to wonder about the origin of prestin itself.

To these ends, I have run across the following paper:

Proc Natl Acad Sci U S A. 2003 Jun 24;100(13):7690-5.

Expression of prestin-homologous solute carrier (SLC26) in auditory organs of nonmammalian vertebrates and insects.

Weber T, Gopfert MC, Winter H, Zimmermann U, Kohler H, Meier A, Hendrich O, Rohbock K, Robert D, Knipper M.

Prestin, the fifth member of the anion transporter family SLC26, is the outer hair cell molecular motor thought to be responsible for active mechanical amplification in the mammalian cochlea. Active amplification is present in a variety of other auditory systems, yet the prevailing view is that prestin is a motor molecule unique to mammalian ears. Here we identify prestin-related SLC26 proteins that are expressed in the auditory organs of nonmammalian vertebrates and insects. Sequence comparisons revealed the presence of SLC26 proteins in fish (Danio, GenBank accession no. AY278118, and Anguilla, GenBank accession no. BAC16761), mosquitoes (Anopheles, GenBank accession nos. EAA07232 and EAA07052), and flies (Drosophila, GenBank accession no. AAF49285). The fly and zebrafish homologues were cloned and, by using in situ hybridization, shown to be expressed in the auditory organs. In mosquitoes, in turn, the expression of prestin homologues was demonstrated for the auditory organ by using highly specific riboprobes against rat prestin. We conclude that prestin-related SLC26 proteins are widespread, possibly ancestral, constituents of auditory organs and are likely to serve salient roles in mammals and across taxa.

Fascinating.  So homologs of prestin have not only been identified in both other vertebrates and non-vertebrates, but are also expressed in their auditory organs.  Okay, so we can say that a prestin-like protein was already in place to carry out auditory function in the last common ancestor of mammals and insects.  That’s some pretty deep homology.  But does it go further?

Driving biology to more and more complex forms

As I explained before, “The hypothesis of front-loading evolution would thus predict that significant transitions in evolution would depend on preadaptation.”

Recently, I discussed one such candidate – the origin of mitochondria.

A new paper has come out that strengthens this case for preadaptation:

Protein nudges organismal evolution

We have seen that prestin is a motor protein found in the outer hair cells of the inner ear of the mammalian cochlea.  It vastly enhances auditory sensitivity by converting the energy from an ion gradient to force such that if you eliminate this protein in mice, there is a greater than 100-fold loss in auditory sensitivity.

Of course, what is most remarkable about this protein is its convergent evolution in certain bats and dolphins, where the prestin protein from both species share at least 14 independently derived amino acid sites. These changes apparently played key roles in the independent evolution of echolocation.  The molecular design of this protein seems to have facilitated the appearance of echolocation, which takes auditory sensitivity to the next level.  What’s more, there are other signs of convergent evolution at the anatomical level:

Echolocation requires exceptionally high frequency hearing and, though echolocating whales and bats generate their calls differently, their cochleae show multiple convergent anatomical features [4]. In particular, the cochlear OHCs in both taxa are shorter and stiffer than in other mammals [4], and this inferred adaptation for processing ultrasound is supported by audiograms that reveal correspondingly higher frequency thresholds [5].  (Yang Liu, James A. Cotton, Bin Shen, Xiuqun Han, Stephen J. Rossiter and Shuyi Zhang. 2009. Convergent sequence evolution between echolocating bats and dolphins. Current Biology Vol 20 No 2).

Prestin would thus appear to be a good candidate for something I describe in The Design Matrix.

Epic Fail

To Celebrate….

…my recent [cough] “accomplishment”: