Evolution of regulatory networks: nematode vulva induction as an example of developmental systems drift

Changes in the developmental processes and developmental mechanisms can result in the modification of morphological structures and in the evolution of phenotypic novelty. But how do developmental processes evolve? One striking finding in modern biology is the confrontation of morphological diversity in multicellular organisms with the conserved blueprint of life-the small number of conserved signaling pathways and transcriptional regulators. Evolutionary developmental biology (evo-devo) tries to explain this discrepancy between macroscopic diversity and molecular uniformity. Selected case studies in evo-devo models allowed detailed insight into the mechanisms of evolutionary changes and might help solving this problem. Here, I compare the formation of vulva development between Caenorhabditis elegans and the evo-devo model Pristionchus pacificus. More than 3 decades of work in C. elegans and 15 years in P. pacificus provide an insight into the molecular mechanisms of developmental change during vulva evolution. C. elegans and P. pacificus differ first, in the type of the signaling system used for vulva induction; second, the cells required for the inductive interactions; third, the logic of the signal system, and finally, the sequence and structure of peptide domains in otherwise conserved proteins. Nonetheless, the vulva is formed from the same three cells in both nematodes. I discuss redundancy as an evolutionary mechanism to explain developmental systems drift, a theory predicting conserved morphological structures to be generated by diverse molecular regulatory networks.