Once We All Had Gills: Growing Up Evolutionist in an Evolving World

15. Evolution in the Tasman Sea

201 Evolution in the Tasman Sea Wading In In the early 1980s, I started earnestly hunting for the right organism as an experimental system for delving into evo-devo. I thought the ideal animal would be one in which the evolution of early embryonic and larvaldevelopmentcouldbereadilystudiedbecauseembryosandlarvae are crucial stages in development and are simple in cell numbers and types compared to adults. My first efforts were made using the familiar sea urchins of the Northern Hemisphere. I found that we could explore evo-devoatthegenelevelinseaurchinsandpublishedourfirstevo-devo paper in 1984. In it we showed that a major innovation in the expression of histone genes in sea urchin eggs had taken place with the origins of advancedseaurchinsintheMesozoic,whilebrontosaursmunchedtheir way across the landscape. We could thus correlate a unique gene regulatory mechanism with a set of macroevolutionary events in sea urchin evolution. But the events were too distant in the past to help unravel ongoing developmental evolution. So I’d have to look farther afield. In 1985, about the time our molecular phylogeny studies of the animal phyla were starting to produce results from the piles of x-ray films of DNA sequence data we had collected, I gave a seminar at the University of California at Santa Cruz. While there, I talked with John Pierce, a biologist who studied the reproduction of marine invertebrates. I told him that I was looking for a suitable marine embryo system to study the evolution of early development. He handed me a copy of a paper publishedin1975byzoologistDonAndersonattheUniversityofSydney fifteen 202 Finding Evolution, Founding Evo-Devo about an Australian sea urchin with a remarkable embryo. I read it on theflighthome.Thatplaneridewasatransformativemoment,anditwas in a way a cosmic joke. I had studied the development of the commonly used northern temperate zone sea urchin, and here was something completely discinct, a sea urchin whose development was unimaginably different from that of the sea urchins I knew so well. We all have strong cultural biases about the things we know about nature. So it is with developmental biologists. We “know” that snakes lay eggs, that mammals don’t, and that that frogs grow up from tadpoles. Nearly all would say that marine invertebrates produce characteristiclarvae distinctfromthe adultsinbodyplan. Well, all these statements are mostly true. Most snakes do lay eggs, but rattlesnakes and a number of other snakes bear live young, among mammals thefurryplatypusandthespinyechidnalayeggs,andalthoughthefrogs wearefamiliarwithinthenortherntemperatezonestartlifeastadpoles, many tropical frogs don’t bother. All this was well known to naturalists, who haven’t kept it a secret, but not to developmental biologists, who like to think of a canonical form of development for each kind of animal. Unexpected embryo and larval evolution is everywhere if we could just see it. It was only when I began looking for organisms that might have evo-devo potential that I discovered that lots of tropical frogs just skip the tadpole as passé and lay large eggs on land, which develop directly into a little froglet. And then there were similarly deviant sea urchins. Sea urchins became classics for the study of the mechanisms of fertilization and development in 1877, when Oskar Hertwig saw animal fertilization for the first time by the simple expedient of adding sea urchin sperm to sea urchin eggs–of such simple genius is fame born. In textbooks, the fertilized egg undergoes embryonic cell divisions and develops via a complex feeding larval form called a pluteus. This larva has a bilaterally symmetric body plan, which is vastly different from the pentameral adult. The Southern Hemisphere and the deep seas held a surprise–the pluteus isn’t necessary for development of a sea urchin. It’s important to understand that marine animals have two basic kinds of development. Direct development is what we see in vertebrates and arthropods,andinsomemoreprimitivephyla.Inthesephylaanembryo producesachainoflarvalformsthathavethesamebodyplanoftheadult and overtime become more adult-like. You might argue that a tadpole Evolution in the Tasman Sea 203 doesn’tlooklikeafrogorthatamaggotdoesn’tlooklikeafly.That’strue, but the underlying body structures of these larvae are the same as the body plan of the adults. Something different, though, called “indirect development,” is how most living marine animal phyla arrange their growth. In those phyla, the egg produces a larva completely distinct in body plan from the adult. For example, the pluteus is a bilaterally symmetric filter feeder, whereas in its adult form, a sea urchin has five-sided symmetry and feeds by chewing algae off the sea floor. When competent, the larva undergoes a rapid metamorphosis...