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By Root 313 0

they make our selves feel so literal, so concrete. Thinking about how many molecules can be crammed into a cell, imagining the adenines and thymines jostling for position, or the helices spooning like lovers in the nucleus, means that we can visualize who we are with startling clarity. Science writer Carl Zimmer titled his book tracing the history of our understanding of the brain and its relation to the mind Soul Made Flesh, in reference to the way that we can now see our essence in neurochemicals and gray matter. Thinking about the actual DNA, doled out in infinitesimal picograms in the genome, seems to make that translation even more tangible.

The Sequential Fruit Fly (and Mosquito and Beetle)

THE FIRST insect to have its genome sequenced was, as you might imagine, that sturdy workhorse Drosophila melanogaster. This was followed by the honeybee, a suite of other fruit fly species in the genus Drosophila, two mosquito species, the silkworm moth, and the tiny beetle that often inhabits the flour canister in your kitchen. More are on the way, and all are helping us understand the action of evolution on humans as well as our six-legged kin.

Let us begin with the fruit flies. D. melanogaster is the model species for genetic research, but other species of Drosophila lead lives that are both similar and different. Unlike the cosmopolitan D. melanogaster, D. sechellia, for example, lives only on the Seychelles Archipelago in the Indian Ocean, where it specializes on eating Morinda fruit, from a usually toxic plant. Drosophila grimshawi has elaborately patterned wings and is one of the extraordinarily diverse Hawaiian Drosophila, occurring only in a handful of remote locations. It is nearly a hundred times bigger than the puny D. melanogaster. A close relative of D. grimshawi, D. mojavensis is native to the Sonoran desert of the Southwestern United States and breeds on the spiky organ pipe cactus.

The flies were chosen deliberately to cover a broad range of evolutionary history; the different species shared a common ancestor anywhere from half a million to sixty million years ago. This is approximately the same distance between humans and lizards, all within a group of flies in the same genus. Many of the genes are similar in all of the species, but others are surprisingly different. Journalist Heidi Ledford referred to the "turmoil" of the genome that is visible only when genes are compared across species; genes appear and disappear, the time and place for them to be switched on and off is altered. Even those stalwarts the sex chromosomes had some surprises; some genes were thought to be expressed only in males because of their position on the X chromosome, but different species with the same gene did not always express it in the same way. The genes used to code for molecules that fight microbes—part of the fruit fly immune system—are much more variable than others, which makes sense given the rapid rate of change of the disease-causing organisms. Genes for detecting odors, crucial to animals that make their living and find their mates on fermenting vegetation, are also diverse. And in some cases, although the regulation of a pathway for making a protein clearly changed, the protein itself was still being produced, suggesting that so-called transcriptional rewiring might be commonplace.

In contrast to the desire to discover universal principles about the operation of genes from the workings of Drosophila, the scientists examining the two species of mosquito that have had their genomes sequenced had a much more practical motivation: they wanted to understand species that have such enormous effects on human health. The first species to be sequenced, Anopheles gambiae, is the principal vector of malaria in Africa. The second, Aedes aegypti, transmits yellow fever, dengue fever, and the less well known chikungunya virus; the latter was responsible for a recent outbreak in countries bordering the Indian Ocean that caused about 250,000 cases of illness and over two hundred deaths. Two U.S. Department of Agriculture entomologists,