The male fly does not just shake its wing at the female at random. Drosophila melanogaster is thought to have evolved in Africa and must have had to set itself apart in those rain forests just as much as drosophilids did in Hawaii. If a male melanogaster does not sing just the right song, a female will emit a counterbuzz, a rejection buzz, which is an international fruit-fly message that males of every species seem to understand. Sometimes she will bat him away during his performance or extrude her ovipositor in his face, a sight that seems to have a discouraging effect. But if the male is singing her song and if she is a virgin, the rest of the action proceeds in the distinctive melanogaster series of steps, which are, as Benzer says, “only too embarrassingly anthropomorphic.” The male has an erectile penis. The female has a vagina. Copulation typically lasts twenty minutes. (“How anthropomorphic can you get?”)
Courtship and copulation are behavior of a higher order than the kind of behavior that is driven by clock genes. Courtship requires a whole series of complicated steps, a long chain of different pieces of behavior, and each step makes the next step more likely: One thing leads to another, as we say. Flies inherit every step in this dance. When a male courts a female, first he taps her with his foreleg, as if to get her attention. Then he follows her around, and starts singing. After the song, he sticks out his proboscis, as if to ask, Do I really want this? Is this female? The right species? He kisses her, licks her, and finally attempts to copulate. Richard Feynman invented a way of drawing the interactions of subatomic particles with arrows to show them approaching each other, and more arrows to show the parting of the ways. Back in the days when Benzer still thought of flies as simple particles of behavior, he sometimes drew the flies’ sequence of courtship steps in the style of a Feynman diagram with a helix in the middle. It was a joke in the same spirit as his countercurrent machine—describing living things as particles of behavior—but of course courtship and copulation are behavior of a much higher order than the behavior of particles. And again, the fly inherits all of this behavior along with its body. Behavior is part of the package.
Benzer could not imagine an instinct more interesting to investigate, and he thought that with genetic dissection he might be able to tease apart the steps. “The trick is designing a screen,” explains Ashburner of Cambridge. Benzer’s countercurrent machine was an ideal screen for phototaxis, Ashburner says (“Very, very simple but very elegant.”). His flight tester was an ideal screen for flight mutants (“Again simple but elegant.”). Screening for time-blind mutants was a little harder, because one had to figure out a way of checking thousands of flies for a fly that had a weird sense of time. “But Ron Konopka did that,” says Ashburner. “And damn painful some screens can be! And there are still aspects of complex behavior which are extremely difficult to—to—to—I mean, just logistically difficult to get mutants in. Certain aspects of sexual behavior, where you have the males posturing to the females. Or licking her bum, or whatever. You know. You could imagine you could do it, but I mean in technical terms it would be logistically an horrendous task to—”
In 1971, the year Benzer and Konopka published their discovery of clock genes, Jeff Hall joined the laboratory as a postdoc and began trying to figure out how to screen for mutants of courtship and copulation. Of all Benzer’s students, Hall was the one with the deepest background in Drosophila. He had been working since his undergraduate days with fruit flies, fly bottles, and funnel-in-beer-bottle morgues. Hall and Benzer decided that the simplest way to go into the problem would be to screen for what Hall was soon calling, in an ironic and rueful tone he learned from Woody Allen, savoir-faire mutants, flies that have no luck in love. To find them, Hall borrowed a set of mutants from another fly laboratory—a line of mutants in which the males never fathered any children. Hall put these males together with normal females and watched to see what would happen. The males could sing the love song of the fruit fly and they could follow it up—they could talk the talk and walk the walk—but they were sterile. They had defects in their reproductive systems.
Those were not very interesting mutants. They were as boring as flies with blind eyes or bad wing muscles. On the other hand, Hall could not help feeling impressed by the power of the instinct these flies had inherited. He had assumed that a blind fly, for instance, would be a savoir-faire mutant. But in a fly bottle a blind virgin male raised in total isolation can find a virgin female, even if she is blind too. Apparently he sniffs out her aphrodisiacal advertisements, her pheromones. The two mutants meet and pass on their genes.
Hall had also assumed that if a male could not fly he would not be able to hold out his wings and make the fly’s tremble song. But even the flightless mutants that Benzer found in the bottom of the flight tester were willing and able to sing a love song down there. When these flightless mutants spy females, Benzer says, they vibrate their wings “in a quite normal way, yet, when dropped off the end of a rod into open space they just clunk right down to the top of the table.” Once, years later, a student in Hall’s laboratory made male double mutants that could neither see nor smell. Then he cut off their wings. He introduced them to female double mutants that could neither see nor smell. A few of the double mutants still mated.
EACH TUFT of grass brings forth seed “after his kind,” as it is written on the first page of the Book of Genesis. Every life-form brings forth “after his kind,” the great whales in the waters and the birds above the waters. And every living thing that swims, creeps, or flies bequeaths to the next generation not only the form of its kind but also the instincts of its kind, including the instinct of generation itself: “And God blessed them, saying, Be fruitful and multiply, and fill the waters in the seas, and let fowl multiply in the earth.” This has always been one of the primary miracles, the burden of the first page.
Benzer and his students were trying to find a point of entry into these instincts by looking for places where the instincts had gone off on a new bent. Sometimes they noticed mutant flies that were half male and half female. Years before, Morgan’s Raiders had noticed these flies too. They are called “gynandromorphs,” from the Greek gynē, meaning “woman,” and andr, “man”: morphs of male and female. In some gynandromorphs the right half of the body is male and the left half is female. Every cell in the male half has one X chromosome and every cell in the female half has two. In some gynandromorphs—gynanders for short—the split runs down the middle, passing right through the head: the right eye is male, the left eye is female. In other gynanders the sexual border cuts across the body on a diagonal. The female melanogaster is bigger than the male, so the female parts of the gynandromorph are bigger than the male parts. Also, a female melanogaster’s abdomen is brown; a male’s is black (melanogaster means “black belly”). So the gynanders abdomen is a motley brown and black. “Glory be to God for dappled things,” the poet Gerard Manley Hopkins wrote, praising “skies of couple-color as a brinded cow,” “landscapes plotted and pieced,” “all things counter, original, spare, strange.” Of all things counter, original, spare, and strange, the gynander is one of the strangest—maybe too strange for Hopkins.
Sexual mosaics. Notice the mismatched eye colors and wing sizes. From Sturtevant, “Origins of Gynandromorphs.” (Illustrations credit 9.1)
To Benzer, gynanders were novelty-shop items. He once made a gynander with one good eye and one blind eye and put it into a vertical tube with a lightbulb overhead. A normal fly with two good eyes will climb straight up a tube toward the light. It will also climb up if the room is dark, because Drosophila has a sense of gravity as well as a sense of sight. But if Benzer turned on the light, his gynander would climb the tube in a corkscrewing path, because it would keep turning itself to one side, cocking its bad eye toward the light, trying to balance the input from the two sides of its head. If its right eye was bad, the gynandromorph traced a right-handed helix; if the left eye was bad, the gynandromorph traced a left-handed helix. “Sometimes,” Benzer wrote, “it is difficult to resist the t
emptation, out of nostalgia for the old molecular-biology days, to put in two flies and let them generate a double helix.”
Decades earlier, Sturtevant had realized how to use gynanders to make a form of what embryologists call a “fate map,” a map of the fate of every point in the early embryo, showing which part of a fly develops from each cluster of cells. At a very early embryonic stage called the blastoderm, a fly egg is covered with a smooth layer not of shell but of cells, ten thousand cells. In a gynandromorph embryo, half of these cells are female and half male. Gynander eggs are like Easter eggs in which each and every egg has been hand-dipped into two bowls of dye. The sexual boundary line may run crosswise, slantwise, any which way across the gynander egg, but it divides the surface into two parts, male and female.
Sturtevant had once worked out a way to trace the male and female parts of each gynander back to its point of origin on the surface of the blastoderm. To try out his idea, he had examined 379 gynandromorphs of Drosophila simulans, a close cousin of melanogaster. He had drawn pictures of each one, and noted which part of each was male and which female. Then he had put these sketches away and gone on to other projects. In 1969, two fly men—one of them a postdoc of Sturtevant’s student Ed Lewis—borrowed that now yellowed sheaf of drawings and used them to finish the project. They drew an oval map of the blastoderm’s surface and plotted the point of origin of the first left leg, the second left leg, and the third left leg; the head, the eyes, and the wings; the sections of the dorsal and ventral abdomen.
Sturtevant was dying while they worked on the fate map. They finished it just before he died.
Next, Benzer and Hotta made a fate map of the melanogaster egg. Like many of Sturtevant’s friends, they had always regretted that the map unit of genetics is named after Morgan and not after the man they called Sturt. To honor Sturtevant’s memory, Benzer decided to measure distances on his fate map in sturts. The distance from the point of origin of the fly’s first left leg to the point of origin of the fly’s second left leg, for instance, is ten sturts. Benzer could not quite pronounce the name of the new map unit without giggling. “But, you know,” he says, “that was a sentimental thing with me, naming it after Sturtevant.”
Now Benzer’s postdoc Jeff Hall began using gynandromorphs and fate maps to explore sexual instincts. He and another postdoc in the Benzer lab, Doug Kankel, knocked out a gynander with ether, mounted it in a kind of white goop (one brand name is Tissue-tek), and deep-froze it. Then, with a microtome, they sliced it into thin sections—so thin that they got as many as thirty or forty slices from a single fruit fly. They stained each section in such a way that the male cells stayed colorless, while the female cells turned dark brown. By looking through the microscope at these stained sections of the fly’s brain, Hall and Kankel could see which pieces of the nervous system were male or female, right down to individual neurons. In this way Hall would later map the portion of the brain that has to be female if a fly is going to elicit courtship from a male, and he also mapped the portion of a fly that must be male (a spot in his midsection) if he is going to try to copulate with her. Even if the cuticle of a gynander’s head is female, with female eyes, female ears, and a female head capsule; even if she has a female thorax and a female thoracic ganglion; and even if her wings are female, she will still hold out her wing and make it tremble in the love song of the male fly if just one critical focus inside her brain is male. On the other hand, a gynander with a body that is almost all male will still be receptive to a male’s song and dance if just one critical focus in its brain is female.
A fate map. This egg-shaped object is a fly embryo at the early stage called the blastoderm. Benzer’s diagram shows how each part of the adult fly comes from a specific site on the blastoderm’s surface. After making this fate map, he and his students traced pieces of behavior back to the surface, too, including many of the dance steps of courtship and copulation. From Benzer, “Genetic Dissection of Behavior.” (Illustrations credit 9.2)
Benzer could add each of these pieces of behavior to the fate map the same way he and his students mapped each piece of anatomy. Each week, their egg-shaped fate map was decorated with more landmarks of anatomy and behavior, all inscribed in the same spidery, dead black lettering that Benzer had used since Arrowsmith.
The project mattered as a way into the strange territory between genes and behavior, territory that would come to absorb Benzer more and more deeply. In a sense, the territory defined what was original about his project. Animal breeders and ethologists could see that instincts pass on from generation to generation. But only molecular biologists could go inside and see how the path from gene to behavior begins inside the embryo.
At that time the way an embryo grows and develops (a subject known to biologists simply as development) was still a complete mystery. The rules and origins of development were as obscure as the rules and origins of behavior. No one knew where to search for answers, and no one even knew what the answers should look like. Genes linked with the early development of the embryo had been on the maps for sixty years by then, but the problem was essentially imponderable. No one realized that the genes that Ed Lewis was mapping in Sturtevant’s old lab space would crack open the problem.
Lewis had arrived at Caltech in 1937 to write a Ph.D. thesis under Sturtevant. Ever since 1937 he had been sitting in the same Fly Room working on a few particularly bizarre lines of mutants. One was bithorax, which has an extra set of wings: a four-winged fly, first discovered in a half-pint milk bottle in 1915. Another mutant was Antennapedia, which has legs growing out of its head where its antennae should be. By the 1970s, after examining and crossing hundreds of thousands of deformed mutant flies, Lewis had begun to understand something about the roles that bithorax and Antennapedia play in the fly’s development. These genes control the body plan of the back half of the fly, everything below the head. Slowly, without publishing much of his research, Lewis mapped the genes in a great mural on his lab wall, and generations of Caltech administrators let him keep at it. “In any other institution—” Benzer would declare decades later at a rollicking campus party in Lewis’s honor, in front of snapping, clicking, and whirring cameras, after the world had finally caught up with Lewis’s work. “In any other institution—” By then Lewis had long since retired from teaching, but Caltech had allowed him to keep his old laboratory. He still worked almost as hard as ever as the Thomas Hunt Morgan Professor of Biology, Emeritus. Only a school devoted to pure research and to the memory of T. H. Morgan would have allowed Lewis to peg away decade after decade while he mapped bithorax. It was a project that sounded quintessentially irrelevant and proved central—like Benzer’s.
In a normal fly, the thorax has three segments. The first segment has a pair of legs. The second segment has a pair of legs and a pair of wings. The third segment has a pair of legs and a pair of balancers, called halteres. Lewis discovered that bithorax’s problem is not a single mutation but a cluster of mutations on the third chromosome. In a fly embryo these mutations confuse the identity of one segment with the identity of the next. When Lewis mapped them patiently decade after decade, he found that the genes in the bithorax complex are arranged in the same order along the chromosome as the parts of the body they affect. That is, if one goes down the chromosome from top to bottom, one comes to genes that affect the growth of the fly from the top of the head to the tip of the abdomen. The genes that control the development of the head and the antennae are at one end of the complex, and the genes that control the development of the tip of the abdomen and the anus are at the other end. What is more, these genes turn on one after the other as the fly embryo is growing, in anatomical order, beginning with the head and working down toward the anus; and if they switch on in a different order or if one of them misfires, the body plan of the fly is disarranged. In this sense, the fly itself is a map of its genes. As one drosophilist writes, “It is as if the insect’s entire body is the expression of a giant chromosome made visible to the naked eye.�
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This complex of genes would eventually lead molecular biologists inside the problem of development the way Benzer’s mutants would lead them inside the problem of behavior. What Lewis found in flies would turn out to be fundamental throughout the tree of life. New tools of molecular biology would augment the old tools of genetics and mapping to produce breakthrough after breakthrough; and the same tools would work equal wonders with Benzer’s mutants. Clock mutants and the savoir-faire mutants would provide the first set of picture windows into the workings of genes and behavior at the level of the anatomies of atoms.
Time, Love , Memory Page 14