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By Root 255 0

almost anywhere except deep in the ocean, and they vary enormously in size, shape, food habits, and virtually every other aspect of life. A queen ant can live for decades in her nest, while tiny midges that circle over fast-running Appalachian streams can dispatch a whole adult lifetime, complete with finding a partner, mating, and laying eggs, in a prompt 45 minutes. That diversity makes it much easier to answer questions about the genes responsible for traits such as life span or body size, because we have so many different types of animals to compare. Even if we had genomes for all the primates, say, or even all the mammals, it wouldn't be as useful as having genomes for as many types of insects, because compared with insects, one monkey is pretty much the same as another when it comes to appearance and even behavior. A monkey is a lot more like a mouse than a grasshopper is like a flea. And of course insects are important vectors of diseases from malaria to typhus, as well as linchpins of our agriculture through pollination and pests because of their fondness for the same foods we eat. Without them, we cannot understand what makes life tick.

What's more, because we shared a common ancestor with insects so long ago, we can use them as a way to explore how we arrived at similar-seeming destinations with such radically different modes of transportation. For example, we are social and spend time and energy taking care of our young. Honeybees are social and spend time and energy taking care of their young, too. We share a fair proportion of genes with honeybees—but are the genes associated with social behavior the same in both of us? If they are different, how do they get similar results? If they are the same, why did the genes persist through evolutionary time in us and them, but not in thousands of other species?

Size, Junk, and Garbage

BEFORE exploring which insects have had their genomes sequenced and what those sequences tell us, it is necessary to look at a different kind of large-scale genetic information we can get for living things: genome size. Before we knew much about the chemical bases that comprise the DNA inside a cell, we could at least determine the amount of DNA itself. Indeed, calculating an object's size is one of the first things we do with something new, whether that something is a previously undiscovered mountain, a recently incorporated township, or a newborn baby (why the vital statistics of weight and length are so often included on birth announcements is a mystery, at least to me, but it testifies to our obsession with measurement).

Ever since the DNA molecule was discovered in the late 1800s, scientists were interested in the amount of it in different kinds of animals and plants. In the 1940s and early 1950s, the "DNA constancy" hypothesis, which stated that the nuclei of cells in various tissues contained about the same amount of DNA, and that this was roughly twice the amount contained in sperm cells, was used to test, and eventually support, the notion that DNA was indeed the source of genetic material.

Once this idea was accepted, it seemed plausible that the more DNA a species had, the more genes it possessed, and therefore the more complex it could be. Intuitively, people looked at genes like money in the bank; the more you have, the more you can buy. Scientists thus expected that smaller, simpler organisms such as amoebas or flatworms would have less DNA per cell than hamsters or birds of paradise. Much to their surprise, this turns out not to be the case. The amount of DNA—weighed in picograms, or trillionths of a gram—is not related to the apparent complexity of the animal or plant in which it resides. Knowing genome size is useful in deciding which organisms should have their genomes sequenced, for the purely practical reason that sequencing smaller genomes is cheaper.

Animals vary seven-thousand-fold in their genome size, and as you might expect, insects are champions of this variation. Among mammals, the smallest genome of 1.73 picograms resides in the Asian bent-winged bat,