Creatures as diverse as worms, sea urchins, flies, fish and humans may all march to the beat of the same drummer when it comes to developmental timing. According to a study published today in the journal Nature, researchers at Massachusetts General Hospital and Harvard Medical School have discovered that a tiny RNA gene known to regulate development in the roundworm Caenorhabditis elegansis found in the genomes of all animals whose left and right sides are basically identical. (This excludes more primitive animals such as jellyfish and corals.) The commonality of this molecular timekeeper, which is encoded by a gene dubbed let-7, suggests that it evolved in the last common ancestor of these animals nearly a billion years ago. If substantiated, the finding could prove critical to understanding how creatures control the orchestration of cells as they develop from fertilized egg to adult.
Team leader Gary Ruvkun and his colleagues first described let-7 in February, having discovered it in C. elegans. Subsequent work documented its ubiquitous presence among bilaterally symmetrical animals. Specifically, they found that let-7 is expressed at certain times--such as just before metamorphosis in the fruit fly--thus hinting strongly at a role in controlling developmental transitions. But let-7 doesnt act alone. Rather it is part of a "genetic cascade" involving so-called target genes and effector genes. Intriguingly, the researchers were able to identify let-7s target gene in the fly and in vertebrates, which suggests that the entire genetic cascade has been conserved over the course of evolution. Yet whether let-7 is used in the same way in all of these different animals remains to be seen, according to CNRS researcher Andre Adoutte. "With time, the regulators and effectors of let-7 in several phyla will no doubt be identified," he writes in an accompanying commentary. "Then we will be able to answer the question of whetherthis timekeeping mechanism is truly universal."