How the modular structure of proteins permits evolution to move forward

The development of identical egg laying organs of two nematode species is regulated by different signaling pathways

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 PLoS Biology, 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.

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 protein 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.

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The Max Planck Institute for Developmental Biology conducts basic research in the fields of biochemistry, genetics and evolutionary biology. It employs about 325 people and is located at the Max Planck Campus in Tübingen, Germany. The Max Planck Institute for Developmental Biology is one of 80 research institutes that the Max Planck Society for the Advancement of Science maintains in Germany.