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?

This time, I decided to use the amino acid sequence of mouse prestin to BLAST the sequence data bases.  What did I find?

There are clear homologs of prestin in jelly fish and placozoa with 30-40% identical positions and 50-60% positions with similar amino acids.  But I could not find any homologs among the sponges.  So did prestin co-evolve into existence with the appearance of metazoan life?

Not so fast.

Let’s consider protozoa.  No luck with Tetrahymena, Paramecia, or Trypanosomes.  But wait!   Various other protozoa do have clear homologs of mouse prestin, including Toxoplasma, Chlamydomonas, Monosiga, and even slimemolds.  And in all cases, the identical positions lies in the 30% range, with similar positions in the 50% range.

So the deep homology of prestin extends to the first single-celled eukaryotic cell itself!

But it goes deeper.  Homologs of mouse prestin are also found in archaebacteria  (although restricted to methanogens) and is widespread among the eubacteria.  One can thus make the case that a prestin-like protein was found in the first cells.

So we have the broad outline of an interesting hypothesis.  A prestin-like protein existed among the first cells and played some role involving anion transport.  While it was not essential for life itself, it was extremely useful, as seen by the fact that it has been retained by so many lineages of bacteria and protozoa.  Since it was likely to be retained and propagated, it would likely find itself in that lineage where metazoan life emerged.  Once metazoan life emerged, the stage was set to unfold the auditory role of this protein and it quickly found itself in this new context.  Once this stage was set, it would eventually facilitate the appearance of echolocation.

Think of this as the hypothesis of recursive front-loading, where state A nudges the existence of state B, which in turn, can nudge state C into existence.  Such recursive front-loading would not have to occur at a particular place and time, but would be something that was eventually going to happen if we simply stepped back and considered the larger picture.  In this case, a particular anion transporter fulfilling a need for single-celled life would eventually facilitate the appearance of hearing in multicellular life, and then later, the very same protein would facilitate the appearance of echolocation.  This all raises the possibility that without prestin, the blind watchmaker would not have been a stunning success in evolving hearing, and then echolocation.


6 responses to “Recursive front-loading

  1. Very nice.

  2. How can one differentiate between homology and homoplasy?

    Also does this mean that the blind watchmaker isn’t deaf? 🙂

    (just had to throw that in seeing that you ain’t going to publish my comment anyway…)

  3. Now a critic would demand that we show that no other protein could have done prestin’s job. I think your reply would be, “I’m not trying to prove that a non-teleological process couldn’t succeed. I’m just trying to show that a teleological process could succeed.”

    So your case that a designer could have front-loaded evolution is strengthened. But eventually, somewhere down the road, some of us might want to go duck hunting. Must be the Elmer Fudd instinct. 🙂

  4. “Recursive front-loading’ doesn’t seem to make sense. How can an operation call itself? You can’t continually compress something.

  5. Bilbo,

    Now a critic would demand that we show that no other protein could have done prestin’s job. I think your reply would be, “I’m not trying to prove that a non-teleological process couldn’t succeed. I’m just trying to show that a teleological process could succeed.”

    Yes, a core belief of the non-teleologist is that “some other protein” could have easily filled prestin’s shoes. This is precisely the logic that caused non-teleologists to be surprised by the discovery of deep homology in the first place. After all, if so many other proteins could have done prestin’s job (as one example), why expect such deep homology?

    After pointing out how the logic of their demand has led evolutionary biology astray in the past, I would in turn demand to be shown how many other shoe-fillers exist. Just what percent of proteins could actually fill the role of prestin?

    Anyway, by making such a demand, the person would be tacitly acknowledging the utility of this front-loading hypothesis. In this case, the data we have indicate that a prestin-like protein is expressed in the auditory organs of insects and vertebrates. So this hypothesis would predict that if we probed for a prestin-like protein in the auditory organs of other distantly related creatures (i.e., nematodes, echinoderms), we would find it.

  6. Rich, not sure I understand your concern. A function can call itself. It can reset, restack, clear and actually call itself in multiple areas.

    The problem I guess is your point of compression. But compression problems are different than calling functions and can vary with requirements. It seems you’re assumming exponential growth of information based upon infinite number of iterations? But is that what we observe in cellular functions?

    The compression algorithm and information processing required for life in a front loaded process is partly a conserved, partly variable process that is easily stored and replicated each time. Example is ATP. ATP information building blocks are stored in the DNA Code and compressed. There is no need for recreating the wheel.

    It is I thought widely agreed that certain information is conserved across many Phyla. One being energy conversion. Photosynthesis is another example where conservation of information and variation exist, but it does not impact compression problems that we know of. Nor does it require infinite accumulation of every iteration for a sufficient and efficient process. Once a solution is found, the stack is reset, disregarded and needless information chunked off.

    Maybe I misunderstand your point.

    Are you stating that evolution is contantly recreating the same process with different proteins? That would be so costly and junk would indeed add up, which is what many evolutionist believed Junk DNA to be. But I thought most have agreed Junk DNA was a failed prediction of the unguided process.

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