David Page '78 reveals the evolutionary roots of sex and gender.

By Carol Cruzan Morton

For a guy whose work brought new respect to the scorned but macho Y chromosome, geneticist David Page '78 seems downright mild mannered. In his office on a chilly January evening, he looks out over the lights of Cambridge and the Massachusetts Institute of Technology and marvels about the differences between the sexes.

"Sometimes people ask me how sex evolved," Page says. "Most people are thinking about the act, but I'm thinking about the co-existence of two different forms within a species. It makes for a wonderful subject of study."

For scientific inquiry, the two sexes can be defined at many levels. What makes a man male? What makes a woman female? Externally, the differences may mean beard or breasts, penis or clitoris, scrotum or labia. Internally, men have testes that make sperm, and women have ovaries that hold eggs. And then there are differences in hormones, behavior, and identity.

Genetically, in humans and other mammals, sex differences boil down to a mismatched set of chromosomes. Men and women have in common 22 pairs of the puffy, cinch-waisted blobs called chromosomes. What's different is the 23rd set, the sex-determining chromosomes, which are named "X" and "Y." Women usually have two Xs; men typically have an X paired with a Y. Page started here.

The emerging story of the Y chromosome has been written, in large part, by Page and his

graduate students and postdoctoral fellows. They have shown that the Y does a lot more than define a male. Page's group has found genes on the Y that are needed for cellular processes throughout the body. He has also found that the Y may be a haven for genes that guys need in order to chauffeur their half of the genome to the next generation.

The Y chromosome doesn't have a monopoly on male-friendly genes, which are scattered around on the genome, but these genes may have a difficult time staying in the coed gene pool. It appears that when genetic members of the he-man club are kicked off the communal chromosomes, the Y may provide a refuge for these outcasts, preserving crucial jobs such as the several Y genes that seem to be necessary to produce any or enough sperm. The human Y chromosome has evolved fast and furiously, a story Page is preparing to tell more fully.

Over the past few thousand years, poets, playwrights, philosophers, and, most recently, special prosecutors have gone to great lengths to explain the various delights and woes resulting from sexual dimorphism. Researchers, too, have tackled the elusive understanding of the sexes from perspectives ranging from behavior to physiology. But as a geneticist, Page picked the loneliest place on the genome.

Once the Y chromosome was discovered to be the key to maleness in 1959, it languished under the palling weight of scientific apathy. Amid 45 other human chromosomes, all generous sausages stuffed with genes, the Y is the runt of the human genome. Page estimates 22 genes are now known on the Y. Its X partner, on the other hand, may house as many as 3,000 genes, including those that code for muscle development, blood clotting, and color vision.

The X chromosome has been intensely--and disproportionately--studied, Page believes, because of its link to many inherited traits and disorders that almost exclusively affect males. In males, no backup X covers for absent or mutated genes. Or, as Page puts it, X genes in the male "fly without co-pilots." Thus, up to 10 percent of men suffer nicks and dings in their single X in the form of color blindness, hemophilia, and more than 300 other genetic traits. Even before the current understanding of genes, people were aware of the special heritable properties of the X. "In elaborations of Judaic writings, there were proscriptions exempting from circumcision boys whose maternal uncles died at circumcision," Page says. "They recognized hemophilia as an X-linked disorder thousands of years ago."

As a piece of genomic real estate, by contrast, the Y had all the appeal of rapidly eroding beach-front property when it caught Page's attention. Although the X and Y probably started out as equal partners about 200 million to 300 million years ago, somewhere along the way, the Y became isolated and unable to engage in most of the healthy gene-swapping that shores up and sustains other chromosomes with fresh genetic material.

An isolated chromosome rapidly becomes an endangered chromosome. Biologists have shown in fruit flies that an isolated chromosome will evolve out of existence in little more than 35 fruit-fly generations. Scientists believed--until Page's work--that the Y had withered down to a single-purpose tool. For a couple of days more than seven months before the birth of each baby boy, the Y turns on the male gender switch, starting the cascade of events leading to growth of the male sex organs, hormone production, and male behavior.

Nature takes an unnecessary risk, it seems, by maintaining an isolated sex chromosome in humans and so many other species. After all, animals really don't need two different sex chromosomes to make males and females. For example, turtles and alligators have the two sexes but no sex chromosomes; sons and daughters are determined by the egg incubation temperatures. For that matter, living creatures really don't need two sexes to have two parents. All the benefits of gene swapping conceivably could come from two individuals of a single gender, such as in baker's yeast, whose gametes look identical. Such musings deepen the mystery of when and how two distinct forms, male and female, sperm and egg, X and Y, arose.

And by rigorously following such musings, Page made his mark quite early in his career. He runs an internationally respected lab at one of the country's top research institutes, the Whitehead Institute for Biomedical Research, and is an investigator for the Howard Hughes Medical Institute.

Born in Harrisburg, Pa., Page grew up in the Pennsylvania countryside. Until he attended Swarthmore, he had never met a scientist. Swarthmore professors and alumni gave Page his first taste of raw science. The summer before his junior year, he worked at the Brookhaven National Laboratory on Long Island. The following summer, he lined up a heady research position under the mentorship of Robert Simpson '59, then at the National Institutes of Health (NIH) in Bethesda, Md. This was Page's first exposure to cutting-edge molecular biology. He began to design his own experiments and became obsessed with the biological puzzle, returning to Simpson's NIH lab the next summer as well.

"I was really living and breathing the edge of the un-known," Page says. "The pure excitement of being the first person in the world to know something was absolutely captivating."

Page graduated with a degree in chemistry and entered Harvard Medical School. For advice on a summer lab position, Page turned to Nobel Prize winner David Baltimore '60, then a professor at Massachusetts Institute of Technology (MIT) and now president of California Institute of Technology, who suggested David Botstein, a pioneer in genetic engineering and studies of heredity. Botstein put Page to work on what turned out to be the precursor to the Human Genome Project, the current unprecedented effort to identify all the genes on the human genome.

Page liked the pure science but still thought of himself as a physician in training. A little over a year later, however, he took a leave from medical school. He spent six months working in a remote Liberian hospital and then another year and a half in Botstein's lab. In Liberia, he endeared himself to the Toronto medical student who would eventually become his wife by chasing hoards of cockroaches from her room before her arrival. Back in Boston, Page thrived in the lab. He began wavering between research science and medicine. He finished an M.D. degree in spring 1984 and was offered a fellowship at the new biomedical research institute affiliated with MIT being built across the street. The Whitehead Fellows program was designed to jump-start promising researchers by providing a lab and an assistant. Then Page won a MacArthur Fellowship, nicknamed the "genius" prize, in 1986. The institute broke its rules about not promoting internally and named Page to its faculty in 1988. The next year, Swarthmore awarded him an honorary degree--rare for someone so young.

Page calls his involvement with the Y a fluke. In the Botstein lab, he arbitrarily selected one of a million snippets of DNA to develop a tool that other scientists could use as a landmark when exploring the human genome. The genetic signpost Page developed signaled a shared set of sequences on both the X and Y chromosomes. A use for this new tool emerged at his first scientific meeting, where he met Albert de la Chapelle, who had described the first case of sex-reversed XX males in the 1960s.

Together, they proved a theory that XX males actually carry the tiny piece of the Y chromosome that turns on the male switch in the embryo. It also explained the unusual occurrence of XY females, who they found were missing the same piece, then known as the testes-determining factor. The question of how two forms within a species evolved gets murkier when the two forms represent a continuum rather than an absolute. Complete sex reversal happens in 1 in 20,000 people. But about 1 in 2,000 people have minor abnormalities in sexual differentiation, thanks to wayward extra pieces or a missing part of the Y chromosome.

Hot on the trail of identifying the crucial maleness switch in that small piece of Y, Page narrowed the elusive gene down to one candidate, called the "ZFY." (Genes tend to be called by three- to four-letter abbreviations describing the most relevant insight or function at the time of naming.) Or so he thought. Headlines around the world heralded the newly discovered gene for maleness. Page was 31 years old and three years out of medical school, where the only formal research training he received was as a medical student on leave.

Then the bad news started trickling in, soon becoming a flood. It was the wrong gene, suggested subsequent reports from a British team. The sex-determining gene for males was the neighboring SRY. The foundation of Page's work was solid, but he had misinterpreted the data. He had been working with DNA from an XY female who was missing more than this one gene from her Y.

"You commit yourself to ideas you think are so wonderful," Page says. "Sometimes they have a useful lifetime of six months; other times, they last decades." For a time, the atmosphere of intense competition took the fun out of science for Page. But after some soul-searching, he found his bearings again in the "pure beauty of the question."

Meanwhile, in 1992, his group was among the first to clone a human chromosome--the Y, of course. (Another group had cloned chromosome 21 and published results one day earlier.) Page's lab produced the first comprehensive map of the Y chromosome and provided DNA landmarks to navigate its genetic information. Two years ago, his group reported 12 new genes, more than doubling the total number of known genes on the Y chromosome.

The genes readily sorted into two classes. Some code for proteins expressed in only testes, where sperm is made. The other category of genes makes proteins needed in all cells. Known as "housekeeping" genes, they have nearly identical counterparts on the X chromosome. (News of these shared genes, some of which help maintain body cells, tickled reporters, who delighted in the irony that the Y was home to housekeeping genes and wondered about more characteristic male genes for belching, loud snoring, obsessive channel surfing, and inability to ask directions.)

The housekeeping genes also offered new insight into a medical condition known as Turner syndrome. Often fatal in the womb, females who are born with only one X chromosome suffer short stature, infertility, and defects in many organs. Yet males seemed to survive with one X. The newly discovered housekeeping genes on the Y suggested people need at least two copies of several genes, either on both Xs or on the X and Y.

Although housekeeping genes were not previously recognized on the Y, their presence isn't a complete surprise. After all, scientists had long postulated that the X and Y were once a matched set of chromosomes. Why wouldn't the mismatched chromosomes still share a few genes in common?

A paper published in April may present the most complete picture to date about the Y's rapid evolution. The genes on the Y chromosome have revealed three major evolutionary plot lines to Page and his associates. The first is persistence. Some genes on the Y have persevered from the ancestral X, accounting for about 1 percent of the Y's length and including the housekeeping genes. Other genes tell a story of "transposition," where a dislodged piece of another chromosome found refuge on the Y, which includes at least one gene (known as DAZ, an acronym for "deleted in azoospermia") necessary to make sperm. The third story told by the Y genes is "retroposition," a fancy word for a more streamlined version of a gene from another chromosome homesteading on the Y.

Page is completing a kind of unifying history of the sex chromosomes, dismissing with a final wave the old notion of the Y as a degenerate X and offering provocative ways of looking at both the modern Y and X. In a sense, the working genes on the X and Y chromosome provide a kind of living fossil record of their history.

From a 50&endash;50 shared responsibility, Page says, the genetic workload of producing proteins shifted to the X and diminished on the Y. When the gene activity was fully transferred to the X, the Y lost the gene, and one X gene was able to make so much protein that only one X gene was needed.

"Ninety-nine percent of the genes once shared are already at this end point," Page says. "That's why there are 100 times more genes on the X. The X and Y still have a long way to go in reaching the inevitable outcome where the genes have shifted entirely from two copies per pair to one copy per pair of sex chromosomes. Once they've shifted, there's no problem, but this unfinished business has medical consequences. I would argue that Turner syndrome is a manifestation of the incomplete evolution of the youngest parts of the X and Y."

Although Page contributed new research techniques early in his career, these days he's more of a thinker than a doer. "I don't do experiments with my own hands," Page says. "I help chose experiments, provide strategic guidance, and help interpret things. It's especially interesting when data announces an answer to a question you haven't even asked."

In a field dominated by large consortiums and huge group efforts, Page fields a small research team, gives them the best equipment and latest technologies, and waits for what he calls the "data heroes" to work their magic. In calling them heroes, Page refers to the leap of faith his students take when "choosing to take on monumental tasks and figure out how to accomplish them without going insane." For example, in two years, Bruce Lahn, the graduate student who found 12 new genes on the Y, "accomplished the equivalent of all the world's previous molecular studies of the Y," Page says.

Page can clone a catchy phrase with the same precision his research associates can clone genes. Page not only excels at explaining genetic research and its implications, he feels a responsibility to share this information with people affected in some way by it.

"In the case of the Y, evolution has operated as an opportunistic real estate broker," Page says. "Let's assume that all genes relocate periodically. If some of those genes happen to be beneficial to males but not of much use or even detrimental to females, the real estate broker of evolution says, 'Have I got a home for you.'"

But the Y is genetically unstable and occasionally loses genes in individual mutations. Four years ago, Page's group and their Finnish colleagues found that a specific defect in the Y chromosome may be responsible for 13 percent of cases of azoospermia, the complete inability to make sperm and the severest form of male infertility. On a region of the Y known as AZF, they suspect one gene in particular, DAZ. Page has let people know that these findings affect couples seeking a type of fertility treatment called intracytoplasmic sperm injection, where doctors inject a single sperm into an egg to circumvent the low sperm counts. Because men may pass along the very Y mutation that made them infertile, they risk creating an infertile son.

Lately, Page has put this combination of communication skills and social consciousness to work as chair of the Whitehead Task Force on Genetic Testing, Privacy, and Public Policy. The task force aims to stimulate informed discussion about some of the social and legal ramifications of the human genetics revolution. Last spring, the task force hosted what is believed to be the largest public symposium addressing these issues. The participants included scientists, students, media, legal experts, and ordinary citizens. On a smaller scale, Page has made many presentations to members of state government, trial lawyers, health care advocates, business leaders, the insurance industry, and the federal judiciary.

He's also exploring how humans make eggs and sperm, which are called germ cells--another way of defining male and female. In the early days of an embryo, when it is still a mass of undifferentiated cells, before it makes a heart, a liver, or a hand, it puts aside certain cells that will form the next generation's germ cells. Only then does it see to the rest of the details of shaping a human.

"In a sense, you can view the rest of the body as the germ cell container," Page says. "In evolutionary terms, it's all built around the germ cells. It's obvious that the egg came first, and the chicken came later to serve the egg. As my high school biology teacher used to say: 'We are all mere drops in a stream of protoplasm that's been flowing for billions of years.'"