Human genome 10th anniversary. Using DNA to reveal a mosquito's history.

C R E D IT S ( T O P T O B O T T O M ): C A S E Y D U N N ; R E IN H A R D T M Ø B J E R G K R IS T E N S E N ; C H R IS T IN A H O L Z A P F E L A N D W IL L IA M B R A D S H A W among organisms, identifying differences within equivalent pieces of DNA in various species. Controversy often ensued when those early results didn’t agree with more traditional classifi cation schemes, such as those based on fossils or morphology. To add clarity, researchers have, over time, increased the amount of genetic material they compare, creating a fi eld called phylogenomics in which many hundreds of genes are evaluated in each analysis (Science, 27 June 2008, p. 1716). A few years ago, it cost about $12,000 per animal to sequence 1000 or so genes, says Dunn. Now, a few thousand dollars delivers many more genes. “We can do now what we couldn’t do before,” Dunn says. That includes sequencing little-studied organisms, such as micrognathozoans, so accurately that scientists may resolve the relationships of invertebrates whose lineages split off from a common ancestor 500 million years ago. There are often challenges to sequencing unusual organisms. Sometimes researchers don’t have enough DNA to work with; other times the organism has odd ratios of DNA’s four bases that make decoding samples difficult. But Giribet has already sequenced and analyzed 20 of these animals, including a whip scorpion, a ribbon worm, and several mollusks. Dunn is excited about the prospect of resolving the animal tree as never before: “It’s clear we are going to be able to base our tree on lots of data from lots of species.” Next-generation sequencing technologies are also allowing Dunn to explore the evolution of animals by documenting differences in gene expression patterns across closely related species. The goal is to fi nd out how these changes infl uence shifts in traits and behaviors across the tree of life. To do this, Dunn and his colleagues have turned to a new technique, known as RNA-Seq, that can gauge genetic activity in a sample by sequencing the complementary DNAs (cDNAs) that represent specifi c genes. The busier a gene is in a sample, the more times its cDNA will be sequenced. Dunn and Stefan Siebert, one of his postdocs, have already compared the gene activity of the swimming and feeding forms of a siphonophore, a marine colonial organism. That analysis yielded thousands of genes potentially responsible for the differences in the animal’s two structures. By repeating this experiment with multiple related siphonophore species, Dunn hopes to home in on those key to, say, the swimmer’s development. “This will allow us to identify which genes have changes in expression that are associated with evolutionary changes,” he says. –E.P. Odd creatures. Siphonophores (top) and micrognathozoans may clarify animal evolution.