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.

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, that looks identical in both species and is induced to develop from six precursor cells by signals emanating from surrounding tissues.