Evo Devo Learns a Larval Lesson
The Life Cycle of a frog. Many life forms on Earth go through drastic changes in form. This process is called Metamorphosis.
In Ridley Scott’s 1979 slimy monster masterpiece, "Alien," the extraterrestrial life form discovered by Sigourney Weaver and crew goes through two startlingly different phases after it hatches. Is such a change during the life of an animal mere SciFi license? Not really. In fact, many earthlings go through similar drastic changes in form. Think, for example, of the caterpillar and butterfly, or the tadpole and adult frog.
Scientists have studied the life history of animals, part of a field called development, for many decades. Other scientists have studied how life arose and evolved on Earth. But for the first time since the early part of this century, the two fields are coming together, in a new discipline called by its practitioners "Evo Devo," short for evolutionary developmental biology.
Rudolf A. Raff, professor of biology at Indiana University and a leader in Evo Devo, says the marriage of the two fields, along with the recent explosion in genetics, could tell us something about what a real alien might be like.
"Is there anything that we can learn that would allow you to make any predictions about life elsewhere? I think there is, even if the genetic systems aren’t the same. There are going to be rules that you suspect are going to apply across life in many places. Some of those are based deep in chemistry. But at the other end, you might suspect that a lot of life history features are going to be the same. I think that there may be lessons that we learn that are not strictly Earth-bound in any sense."
These lessons are painted with a broad brush, cautions Bruce Runnegar, a professor of paleontology, and the principal investigator for the University of California, Los Angeles, NASA Astrobiology Institute Lead Team.
Artist conception of how life evolved and developed on Earth. Studying the interactions between development and evolution on Earth will help us to understand what to look for when searching for life outside of our biosphere.
"I don’t think we’ll find out what extraterrestrial life looks like," he says. "I don’t think you can say that we’ll have something that’ll have four legs or fur." Instead, Runnegar says, the study of life on Earth will lay out some basics of how life might have evolved on another planet, but few specifics.
Raff agrees. Understanding evolution and development won’t tell us what protects alien skin (Earth organisms use slime, scales, feathers and fur) or how many legs an extraterrestrial life form might have. There are just too many accidents of evolution on Earth. For example, terrestrial Earth mammals walk on four limbs not because it’s the perfect way to support a body, but because they inherited four limbs from their four-finned ancestors, lobe-finned fish.
But the better we understand the interactions between development and evolution-how development evolves and how it constrains evolution-Raff argues, the better we will be able to know what to look for as we search for signs of life outside our biosphere. "I think that there’s just an enormous wealth of considerations, as to what you would expect an alien to be like, that we begin to learn about through these kinds of studies of evolution and development."
Adult Heliocidaris erythrogramma sea urchin. Sea urchins undergo a dramtic metamorphosis on their way to becoming adults.
Raff studies sea urchins, organisms that go through a change of form as drastic as that of the monster in "Alien," albeit much less scary. Sea urchins hatch from eggs as microscopic floating larvae. These delicate, elegant-looking creatures have two mirror-image halves, just like beetles, birds and bats. They float for weeks near the ocean’s surface fanning even tinier organisms into their mouths.
Then the change occurs. A small bundle of cells inside the organism begins to grow. All of the other cells die, and the tiny bundle settles to the ocean floor to grow into something that looks like a calcified Koosh Ball several inches in diameter. Instead of two mirror-image halves (bilateral symmetry), the adult urchin is organized more like five equal pie slices (pentaradial symmetry), a nearly spherical version of its cousin the starfish. Adult urchins creep slowly around the ocean bottom using a five-toothed structure to scrape food from rocks.
So here’s an organism that hatches from an egg as a bilaterally symmetrical larva and becomes a pentaradial adult. "It’s clear from both morphological [body shape] and particularly molecular studies that their ancestors were bilateral," Raff says. "Here’s this big transformation of body plan. So the real question is: How did it happen?"
Feeding pluteus larva of
One way to begin an attack on that evolutionary question is to study urchins with different larvae. Raff has studied two closely related Australian urchins. One species makes a normal larva, which spends six weeks floating, and therefore dispersing widely.
The second urchin develops by way of a much larger larva, having 100 times the mass of the first type. It contains all the food it needs to develop into a juvenile adult without needing to feed. It doesn’t even have a complete digestive system. It also lacks the long arms that help the usual urchin larva remain in the water. And it develops into an adult in only four days.
The fast-developing urchin gains a jump on its cousin because it can begin bottom feeding much sooner. But the downside is a much reduced range. "Having a feeding larva that goes off in the water for weeks helps a species disperse over a wide distance. If you’re a direct developer, and you develop fast, then you stay at home," Raff says.
And staying at home has its costs. Say conditions change in a particular part of the urchin’s environment. The widely dispersed species has many populations elsewhere. "But if it’s a direct developer with a narrow range," Raff says, "and it becomes extinct in that location, well, that’s it. Game’s over." Paleontologists have found fossil evidence that direct developers face a higher risk of extinction, Raff adds.
Hybrid sea urchin larva, made by fertilizing H. erythrogramma eggs with H. tuberculata sperm. The larva resembles a starfish larva, but will develop into a sea urchin. The hybrids allow Raff and his colleagues to isolate genes involved in the evolutionary transtion in larval form.
In a recent publication, Raff and colleagues reported the results of experiments in which they cross-bred these two species of urchins. (When urchins reproduce, they just release sperm and eggs into the water. This makes it easy to experimentally mix sperm and eggs from two different species.)
One such cross led to death of the fertilized egg. But the opposite cross produced fertilized eggs that develop into full-fledged larvae. The surprise was that these hybrid larvae resembled starfish larvae more than urchin larvae.
The cross-bred urchin larvae tell Raff something about changes that led to the evolutionary branching of urchin larvae from the ancestral starfish-type larvae and to the secondary branching of the two distinctly different urchin larvae.
Such understandings of the history of life on Earth-built on Evo Devo and the new genetics-is important to our search for life on other planets, says Runnegar.
"I think everybody agrees that it’s a very exciting development, because for the first time we’re really beginning to understand mechanistically what goes on in development instead of just by allusion or allegory, as it was in the past. We’re actually beginning to know which genes are involved and how they work and how they interact with other genes."
Raff’s crossbred-urchin experiment will allow his lab to follow exactly this trajectory, toward an understanding of which genes direct the developmental pathways in larvae of the two urchin species, he says.
Raff has already launched the mission. By comparing the genomes of the two species and the hybrid larvae, Raff and his colleagues hope to home in on the particular genes or sets of genes that get switched on and off to direct the development of such different larvae.