How did it happen?
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 said.
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 said, "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, he said.
In a recent publication, Raff and colleagues reported the results of experiments in which they crossbred 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.)
A hybrid sea urchin larva is made by fertilizing H. erythrogramma eggs with H. tuberculata sperm. The product 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 transition in larval form. Credit: Rudolf A. Raff
One such cross led to the 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 crossbred 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, said 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," he said.
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 said.
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.
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