Vol 449 | Issue no. 7163 | 11 October 2007 AUTHORS

water in addition to Abstractions MAKING THE PAPER organic materials LAST AUTHOR Carolyn Porco and warmer tem- Genetic mutation is central peratures, we may to evolution, but mutations have stumbled upon that improve one aspect Spacecraft’s images suggest one of a habitable zone in of a protein’s function can Saturn’s moons may host water. our Solar System. compromise another. Until This is an explorer’s recently, gene duplication The Cassini spacecraft took seven years to dream come true.” was thought to lead to the reach Saturn. But for Carolyn Porco, who The next step development of genes with novel functions, leads the Cassini imaging team at the Space was to determine as one copy of the gene would be free to Science Institute (SSI) in Boulder, Colorado, the locales of the evolve while the other performed its original the images it sent back were well worth the jets. Porco asked function. But this, it turns out, rarely happens wait. Most exciting of all was the revelation SSI planetary sci- — more often, an original gene’s functions that one of the planet’s moons may have the entist Joseph Spitale to triangulate the surface are simply split between the two copies. essential ingredients to support life. locations of each jet. To ensure that the meas- Sean Carroll, at the University of Wisconsin– Madison, and his student Chris Hittinger A veteran of the 1980s Voyager space mission, urements were made without prejudice, she devised a series of assays to trace how two Porco was well aware that the outer Solar System didn’t tell him her hypothesis of an association genes — GAL1 and GAL3 — in the yeast is not the barren wasteland it was once thought between the hot spots and the jets. Saccharomyces cerevisiae evolved from a single to be. Images from Voyager, for example, had Spitale’s measurements “hit the jackpot”, gene after duplication in a distant ancestor shown some of Jupiter’s and Neptune’s moons Porco says — all of the prominent jets emerge (see page 677). They suggest that genes to be geologically active. But this knowledge from one of the four tiger-stripe fractures, that evolve in this way do so to overcome didn’t dampen the thrill of Cassini’s discovery and most coincide with one of the hot spots, constraints present in the original gene. that Enceladus, one of Saturn’s 60 moons, spews confirming a causal relationship between the jets of vapour containing organic material and south pole’s anomalous heat and jet activity Most of your work has been with fruitflies. tiny, icy particles from its south pole. This spec- (see page 695). Why do this work in yeast? tacular finding demonstrates present-day geo- Geysering activity powered by pressurized To find out how duplicate genes had changed, logical activity on a small, cold moon. liquid water trapped beneath the south pole we needed to be able to measure very small Enceladus is only about 500 kilometres in is a possible mechanism for the jets; another differences in organismal fitness. We didn’t diameter, but the jets can shoot thousands of is water vapour evaporating from warm ice. think this would be possible in fruitflies kilometres into space. The particles they contain Unfortunately, no single observation from — we needed the greater power of billions of eventually make their way into Saturn’s E ring — Cassini can answer this question. Although it offspring quickly, and yeast provided this. the diffuse outermost ring, which is composed will take several lines of evidence, Porco says of microscopic icy particles. Long before Cas- that the prospect of finding an extraterrestrial Was it surprising that most of the two genes’ sini arrived at Saturn, scientists had suspected habitat suitable for life is the greatest thrill any differences were regulatory in nature? Enceladus to be the E ring’s source of material, scientist could hope to experience. Yes. The protein Gal1 is galactokinase — an enzyme — and Gal3 acts to regulate its but they never expected this dramatic phenom- Cassini’s mission extends until 2010. A transcription. The original protein would have enon. And the drama matches the implications: handful of additional Enceladus flybys are performed both these functions. We thought “It’s not out of the question that these jets are the planned, and Porco and Spitale predict that the divergence of these proteins was the main result of liquid water,” says Porco. other hot spots will be found in the fracture story, but the data told us otherwise. Gal1 still Infrared measurements by Cassini showed area. Planning future observations while ana- has a lot of regulatory activity, and Gal3 only south pole ‘hot spots’ almost 100 kelvin warmer lysing incoming data is all-consuming, says recently lost the last of its enzymatic activity. than the surface temperature expected from Porco, adding that “Cassini hasn’t been a mis- thermal equilibrium with sunlight. The hot sion so much as a way of life”. That lifestyle This duplication is thought to have occurred spots are associated with four distinctive linear looks set to continue: Porco already thinks 100 million years ago — why did it take so cracks — dubbed ‘tiger-stripe’ fractures — in the Enceladus merits another mission. “In my long for enzymatic activity to be lost? moon’s surface. “We were in a tizzy,” says Porco, mind, it’s the go-to place for investigating I can think of many examples of genes “because, if we could confirm the presence of issues of astrobiological interest,” she says. ■ deteriorating relatively quickly. But in this case, Gal3 still has to bind galactose in its regulatory role, so perhaps that constraint limited the loss of enzymatic activity. FROM THE BLOGOSPHERE On Nature’s social networking chance to learn more about training for peer reviewers; what Besides genetic duplication and divergence, website, Nature Network, we’ve what goes on here, straight happens when manuscripts are might there be other evolutionary ‘tools’? created a new group called from the editors themselves. submitted that are not written There are other mutational pathways, so yes. ‘Ask the Nature Editor’ (http:// In the coming months, we’ll in journal style or not well For instance, Chris showed that one way to network.nature.com/group/ have other Q&A rounds written; and the pros and cons make a better regulatory element is to delete bases so that transcription-factor binding askthenatureeditor). Here, focused on different publishing of submitting modified famous sites get closer together. several editors have agreed to topics, such as careers in works of art as suggestions for answer your burning questions scientific publishing and online the journal’s cover. Why did you write The Making of the Fittest, about publishing in Nature (and communications tools. If there are any topics you’d an evolution book for the general public? the other Nature journals), Current topics of discussion like the forum to cover in future, The pace of discovery in evolutionary science peer review and the scientific in the Q&A forum (http:// please post your suggestions at has really quickened. The quality of evidence publishing process. We invite network.nature.com/forum/ the URL above. We look forward and the clarity of how evolution works are you to join the group — it’s your askthenatureeditor) include to seeing you there. ■ now easier to talk about. And, happily, people Visit Nautilus for regular news relevant to Nature authors ➧ http://blogs.nature.com/nautilus and see want to know about evolution. ■ Peer-to-Peer for news for peer-reviewers and about peer review ➧ http://blogs.nature.com/peer-to-peer.

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