Front-loading with teneurins

So why did I bring up the teneurins? Let’s consider the abstract of a paper that was published a few weeks ago [1]:

Teneurins are type II transmembrane proteins expressed during pattern formation and neurogenesis with an intracellular domain that can be transported to the nucleus and an extracellular domain that can be shed into the extracellular milieu.

In other words, we have a protein that connect the nucleus to the environment outside the cell.

In Drosophila melanogaster, Caenorhabditis elegans and mouse the knockdown or knockout of teneurin expression can lead to abnormal patterning, defasciculation and abnormal pathfinding of neurites, and the disruption of basement membranes.

In other words, this is protein that plays an important role in the formation of brains and nerves. The fact that is carries out the same basic functions in worms, insects, and animals strongly suggests its role in nervous system is quite ancient and may have coincided with the emergence of the nervous system itself. So when did the tenurins arise?

From the abstract:

Here we have identified and analyzed teneurins from a broad range of metazoan genomes for nuclear localization sequences, protein interaction domains and furin cleavage sites, and have cloned and sequenced the intracellular domains of human and avian teneurins to analyze alternative splicing. The basic organization of teneurins is highly conserved in Bilateria: all teneurins have epidermal growth factor repeats, a cysteine-rich domain, and a large region identical in organization to the carboxy-half of prokaryotic YD-repeat proteins. Teneurins were not found in the genomes of sponges, cnidarians or placozoa,

I broke the sentence off here to highlight that we have a protein with a conserved organization that plays a conserved role in the development of the nervous system that does not exist in sponges, jellyfish, or placozoa. Conventional non-teleological thinking would interpret this to mean that some the tenuerins appeared as a consequence of selection pressure to evolve a nervous system. That the teneurins and nervous system co-evolved. For example, a non-teleological mindset might have us consider something like T-urf13 and propose some non-coding gene was expressed and just happened to have a function that would serve the needs of an organism under selection for the development of a nervous system. Who could say otherwise, right?

Well, it turns out the story has a much more rabbit-like flavor to it. Let me complete the sentence from the abstract:

Teneurins were not found in the genomes of sponges, cnidarians or placozoa, but the choanoflagellate Monosiga brevicollis has a gene encoding a predicted teneurin with a transmembrane domain, epidermal growth factor repeats, a cysteine-rich domain, and a region homologous to YD-repeat proteins. Further examination revealed that most of the extracellular domain of the Monosiga brevicollis teneurin is encoded on a single huge 6829 bp exon, and that the cysteine-rich domain is similar to sequences found in an enzyme expressed by the diatom Phaeodactylum tricornutum. This leads us to suggest that teneurins are complex hybrid fusion proteins that evolved in a choanoflagellate via horizontal gene transfer from both a prokaryotic gene and a diatom or algal gene, perhaps to improve the capacity of the choanoflagellate to bind to its prokaryotic prey.

This protein, so important to the development of a nervous system, turns out to be yet another brain-like protein that long pre-dated the emergence of anything like a brain. In fact, the concluding sentence of this research abstract nicely fits with the framework of teleological evolution:

As choanoflagellates are considered to be the closest living relatives of animals, the expression of a primitive teneurin by an ancestral choanoflagellate may have facilitated the evolution of multicellularity and complex histogenesis in metazoa.

Yep.

[1] Tucker et al.  Phylogenetic Analysis of the Teneurins: Conserved Features and Premetazoan Ancestry. Mol Biol Evol. 2011 Oct 31.

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6 responses to “Front-loading with teneurins

  1. Hi Mike-

    If you want a laugh read the following “refutation” of front-loading:

    http://ogremk5.wordpress.com/2011/11/22/front-loading-doesnt-help-id/

  2. Joe G, why are you stalking Ogremk5 around the internet? I know he’s make you look like a complete fool numerous times over your scientific ignorance, but is that a reason to stalk and harass him? You’re going to get yourself in deep legal trouble one day.

  3. For what it’s worth, here is my version of front-loaded evolution:

    Genetic/ Evolutionary Algorithms and My Front-Loaded Evolution

  4. Mike,

    Excuse me for my alleged scientific ignorance, but for some clarification- your front-loading allows for the recruitment of a protein performing one function in one (or more) populations for some other function in some other population(s). The proteins were nudged towards a new function, with mutation and selection/ drift/ HGT facilitating the nudge.

    In this scenario we have an alleged hybrid- (for brevity) a two part coding sequence with the two parts coming from totally different organisms.

    The question is was this accidental and then multicellularity was nudged from there? IOW was multi-cellularity “waiting” for this chance event? Or was there nudging involved in the hybrid?

    To Thorton-

    I will gladly answer your questions on my blog- no need for you to pollute this one too.

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