A Nudge Goes Deeper

Previously, I explored a pattern that echoes front-loading:

However, note from the phylogenetic tree above that the researchers propose the origin of the writer  just prior to the split between choanoflagellates and metazoans.  This is not a big problem for front-loading, as it would be easy to front-load the appearance of this writer by endowing life forms with serine/threonine kinases.  Nevertheless, the case for front-loading would be stronger is the tyrosine kinases were much older and closer to the original cells.  What’s more, way back in 2002, I predicted:

my working hypothesis (just recently being seriously entertained) is that the original cells were front-loaded to make it more likely that multicellular states would evolve. These leads me to predict that we will find remnants or “fossils” of such front-loaded distributed among protozoa. Specifically, we will find various pieces of information, necessary for multicellular life, not necessary for single-celled existence, yet still present in many single-celled organisms. {emphasis added)

So are the researchers correct in dating the origin of tyrosine kinases just prior to the split between choanoflagellates and metzoans?  Or was yours truly correct in expecting them to be older, and thus more widely distributed?

And as we saw it turned out the tyrosine kinase circuit long predated the choanoflagellates.  Now it looks the FLE expectation has also born out with regard to another system long considered to be strictly metazoan specific:

The evolution of animals (metazoans) from their unicellular ancestors required the emergence of novel mechanisms for cell adhesion and cell–cell communication. One of the most important cell adhesion mechanisms for metazoan development is integrin-mediated adhesion and signaling. The integrin adhesion complex mediates critical interactions between cells and the extracellular matrix, modulating several aspects of cell physiology. To date this machinery has been considered strictly metazoan specific. Here we report the results of a comparative genomic analysis of the integrin adhesion machinery, using genomic data from several unicellular relatives of Metazoa and Fungi. Unexpectedly, we found that core components of the integrin adhesion complex are encoded in the genome of the apusozoan protist Amastigomonas sp., and therefore their origins predate the divergence of Opisthokonta, the clade that includes metazoans and fungi. Furthermore, our analyses suggest that key components of this apparatus have been lost independently in fungi and choanoflagellates. Our data highlight the fact that many of the key genes that had formerly been cited as crucial for metazoan origins have a much earlier origin. This underscores the importance of gene cooption in the unicellular-to-multicellular transition that led to the emergence of the Metazoa.

Gene cooption is important from the front-loading perspective, as preadaptations function to nudge evolutionary transitions along.  In fact, in a paper entitled A powerful toolkit for synthetic biology: Over 3.8 billion years of evolution (BioEssays 32:304–313), Lynn J. Rothschild comments on evolution:

To develop evolution’s toolkit requires understanding Jacob’s metaphor(11) that natural selection works as a ‘‘tinkerer’’, rather than an ‘‘engineer’’ with a pre-conceived plan, tools and materials designed for a particular end. Data on structural(6,12) and regulatory gene evolution have supported this metaphor.(13) For the tinkerer, the premium is on utilizing (exapting) existing components in novel ways.(14) But a certain amount of ‘‘preadaptation’’ – having the ‘‘right’’ parts already available – is helpful. Thus, evolution is constrained by history, creativity, and physical constraints. The result is the Rube Goldberg device we call ‘‘life.’’

The key point here?  ‘Preadaptation – having the right parts already available – is helpful.  Yes, indeed.  As I explained earlier, The hypothesis of front-loading evolution would thus predict that significant transitions in evolution would depend on preadaptation.

Also, note that in my discussion of front-loading and epithelial tissue, I too addressed the issue of key components of a metazoan apparatus being lost in choanoflagellates.  Despite the fact that they are loaded with metazoan components, they likely represent an underestimate of the state prior to the emergence of metazoan.  And keep in mind that something that looks specific to metazoans/choanoflagellates may have been lost in other lineages.  As more genomes from more protists are sequenced, look for more examples of machinery that had been considered strictly metazoan specific to be found.

HT: Alan Fox

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3 responses to “A Nudge Goes Deeper

  1. What does “HT” stand for, anyway?

  2. Oh Bilbo! A seasoned blogger like you! I’m shocked!

    “Hat – tip” as in acknowledges.

  3. Thanks, Alan. I was always too embarrasses to ask.

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