How the modular structure of proteins permits evolution to move forward

July 26, 2011

Changes in a short protein domain can alter a whole signaling network involved in organ development– this is the key result of a comparative study of the development of the egg laying organ in two species of nematodes. However, the outward appearance of the organ remains the same in both species. The study provides support for the theory of developmental systems drift – a theory maintaining that, over the course of evolution, analogous organs of different species can retain the same shape and function while the regulative mechanisms underlying their development can change considerably.

The new results, published July 26 in the online, open-access journal , raise the question of whether the modular structure of proteins creates space for evolutionary development, even in otherwise highly conserved structures of organs and signaling pathways.

The nematode Caenorhabditis elegans (C. elegans) is a model organism of genetics. The worm is only about one millimeter long, and its genome has been completely sequenced, so scientists can trace the fate of every one of its 959 cells. In research lasting more than a decade, Ralf Sommer, Director of the Department for Evolutionary Biology at the Max Planck Institute for Developmental Biology in Tübingen, Germany, has established as a comparative model organism, a second nematode, Pristionchus pacificus (P. pacificus). At first sight, this species resembles C. elegans, but it belongs to another family. The last common ancestor of the two species lived 250 to 420 million years ago, well before the zenith of the dinosaurs.

"For [this sort of] comparison, the organisms should not be too closely related, since very small differences in the genome cannot be easily assigned to single events in the evolution," explains Xiaoyue Wang, first author of the study. "The two worms are ideal and a wide variety of genetic and molecular tools is available." The scientists studied the development of the worms' egg laying organ, the vulva, which looks identical in both species and is induced to develop from six precursor cells by signals emanating from surrounding tissues

To more thoroughly test the validity of the theory of developmental systems drift, Xiaoyue Wang analyzed the induction of vulva development from a signaling center in the posterior part of P. pacificus using genetic and molecular methods. The system was used because while similar signaling pathways are involved in vulva development of both species, they appear to exert their molecular activities in different ways: "I have found a single mutation in a stop element of the DNA, where in C. elegans the production of a ends, but in P. pacificus the protein is extended by 17 amino acids," Wang says. The protein, which functions as a receptor, obtains an additional binding site through the extension that enables it to interact with another signaling pathway.

Evolution seems to use the existing signaling pathways almost like a modular construction system: In P. pacificus, a novel binding site connects a different signaling pathway which is then used in a novel context. Wang continues, "I don't believe that what we have discovered in our study of nematodes is an unusual exception. Similar processes are known to lead to cancer development in humans. But likewise, they can initiate changes that can become subject to natural selection and eventually be propagated in the course of evolution."

The observation that changes in the regulatory mechanisms do not lead to changes in the organ could be due to redundancy. In the development of the vulva of the nematodes, for example, several mechanisms acting in parallel have already been described. The modular design of proteins makes it possible to conserve important parts of the molecule over the long time of evolution, while creating opportunities for short protein domains to change. The overall impact of these mechanisms in evolution remains to be investigated.

Explore further: An island as a reflection of the world

More information: Wang X, Sommer RJ (2011) Antagonism of LIN-17/Frizzled and LIN-18/Ryk in Nematode Vulva Induction Reveals Evolutionary Alterations in Core Developmental Pathways. PLoS Biol 9(7): e1001110. doi:10.1371/journal.pbio.1001110

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not rated yet Jul 27, 2011
Presumably, these two species had been geoghraphically separated when they started to "drift apart" genetically. Otherwise, interbreeding would have prevented the difference from evolving.
1 / 5 (5) Jul 27, 2011
So what exactly is meant by "evolution" in this case?

Is it the eventual changing of the nematode into a frog or is it simply that some internals are different but the nematode remains a nematode no matter what? What part is evolution and what part is adaptation?
Does the change in the stop initiate a corrective action leading to the changes the eventual construction to prevent exactly the same kind of bad result that occurs in humans?
Whatever the case, the nematode will remain a nematode for the next 10 billion years. If it tries to become a frog [in response to external pressures, e.g. ultra-violet light induced mutations] it will simply die.
As an aside - it's impossible to establish that these nematodes had a common ancestor 250-420 million years ago. One can only speculate based on one's axiomatic convictions. The truth will most probably be vastly different.
5 / 5 (1) Jul 27, 2011
Anybody know exactly how Kevin makes a living peddling religionist lies? Is he a preacher? Does he have a website? It would be good to remind people of this every time he shows up to 'discuss' creationism.

You wouldn't want to confess your sins would you kev? "Yes I earn income by preaching the word of god his holiness every Sunday as an ordained (or lay) _______"

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