Why is a human not a starfish? A five-armed sea creature obviously looks very different from the average Williams student, but in the early minutes of life, embryos of both creatures are remarkably similar. Chickens, fruit flies, worms, and Ephmen all start with remarkably similar genetic ingredients; the question is, how do they become so incredibly different?
Gregory Wray, an associate professor of ecology and evolutionary biology at SUNY Stoneybrook, is trying to answer this question by studying how certain genes coordinate development in echinodermsâ€”the phylum of animals that include sea stars, sea urchins, and sand dollars. Dr. Wray arrived at Williams last Friday to present his research to the biology 1960s scholars in a lecture entitled “New Roles for Old Genes: The evolution of gene regulatory systems in echinoderms.”
Wray opened his talk with a biological paradox: many organisms that look nothing like one another as adults have very similar genes that are used in the first stages of development. How can you build different forms with the same machinery?
The key to the riddle is hidden in the two kinds of genes that guide a dividing cell to a mature organism. Effector genes are like light switches: they can either be on, producing proteins for a growing embryo, or off. Regulatory genes can do more, like the air traffic controllers at an airport: they express proteins that reattach to the DNA on the effector genes, turning the switches on and off to coordinate the timing of protein expression.
From vibrant slides of purple star fish and red sea feathers, Professor Wray moved on to a diagram with branching black lines showing some of the possible evolutionary relationships between echinoderms. Wray described two possible scenarios that could have resulted in a diverse array of organisms with nearly identical regulatory genes.
First, a distant ancestor may have evolved changes in its regulatory genesâ€”essentially acquiring a new staff of “air traffic controller genes” to direct the old fleet of airplanes (i.e. the effector genes) in different landing patterns. Another alternative is that effector genes may have changed in a way that created or destroyed connections with the regulatory messenger proteins. To continue the aeronautical analogy: certain airplanes may have installed new radios that can or cannot tune into the broadcasting control tower.
To illustrate these two scenarios, Dr. Wray described his work with a sea urchin regulatory gene called Orthodenticle. In almost all organisms, Orthodenticle regulates the development of an embryo’s head. In echinoderms, which don’t have anything resembling a head, Orthodenticle is involved in building the tips of the arms and the network of hydraulically powered tube feet that sea urchins use for locomotion. Using the first scenario above, biologists could explain this by saying that echinoderms branched away from their nearest ancestors when new regulatory genes started to coordinate the production of tube feet instead of heads. Alternatively, biologists could argue that nothing has changed in the regulators while the effector genes have rearranged their light switches so that they’re turned on an off in a new pattern.
While Professor Wray’s talk delved into even greater technical detail, he kept the jargon to a minimum and clearly explained the potentially dizzying complexity of the cellular processes. Looking at the larger picture, it appears that subtle changes in certain parts of the ancient regulatory genes lead to drastic changes in body plans among organisms. It’s as though an ancestral genetic circuitry had been “re-wired” many times to produce new appendages, symmetries, sizes, and colors.
When Wray entered graduate school, he initially studied neurobiology and the development of visual systems of chicks, but soon shifted to a side project on sea urchins when he encountered multiple snags with his original work. “[Echinoderms] are an experimentally tractable set of organisms to work with,” he said. “It’s such a diverse group with fun evolutionary puzzles.”
Dr. Wray recently co-authored an article entitled “Body Builders of the Sea” in the December/January issue of Natural History magazine. Along with some spectacular color photos of overlapping purple star fish, spiny red sea urchins, and sea feathers that wave eerily in the current, Wray presents his complicated research in very approachable prose that’s user friendly to those who shy away from technical language. Wray admitted that “Body Builders” took much longer to write than any of the papers he’s submitted to science journals, because he had to continually step back from the genetic details to convey the larger picture to his general audience.
Dr. Wray’s enthusiasm for developmental biology was contagious. Evolutionary history has produced a fascinating and sometimes bizarre array of creatures and body plans. According to Dr. Wray, “Natural selection works with the genetic tools at hand, forging new interactions among them that can resculpt an organism’s anatomy and alter its destiny forever.”