Photo by Todd McNaught
In humans, gender is determined by more than a stereotypical penchant for Monday Night Football or an extensive shoe collection. The decision typically is controlled by the configuration of sex chromosomes an individual inherits — two X's for a female and an X and Y for a male.
In fish, it's not so simple. Some species rely on chromosomes for gender choice, while others use nongenetic strategies such as the temperature of the water in which their eggs develop or even the types of social interactions they encounter.
For the tiny threespine stickleback fish, though, the mystery of gender determination has been revealed in a new study led by Human Biology Division researchers. Dr. Katie Peichel and colleagues have found that a snippet of DNA on what appears to be a primitive Y chromosome governs stickleback maleness.
Because the chromosome acquired its sex-determining function so recently in evolutionary time, its study promises to provide a unique look at the origin of Y chromosomes and sex-determination strategies. Ultimately, the work could lead to the discovery of new genes in humans that affect maleness, which may provide insight into genetic defects that influence male fertility.
The study appears in the Aug. 24 issue of Current Biology.
"Most people who work on mammals think about very stable X and Y chromosomes, which determine sex in humans," said Peichel, whose lab conducted the study in collaboration with scientists at Stanford University and in Japan and Canada. "But surprisingly, sex determination is a very rapidly evolving pathway. In fact, if you look at two very closely related species of fish, for example, they may have strikingly different sex-determination strategies, such as X and Y chromosomes or environmental factors. We think that by studying the sex-determination pathway in the stickleback, we can learn a lot about how developmental pathways of many kinds — such as how a fin or an arm develops — are built."
By studying the basic principles of animal development, fundamental researchers like Peichel often unearth valuable clues about defects in the processes that can cause diseases, such as cancer.
The threespine stickleback is Peichel's organism of choice because the fish has many underlying similarities to the human body, but its smaller genome makes it much simpler to study. Her gene-hunting studies are made possible by a powerful tool she created while a postdoctoral fellow at Stanford: the first detailed map of the threespine stickleback's genetic blueprint. The map allows Peichel and researchers around the world to pinpoint and study any stickleback gene of interest.
While at Stanford, Peichel used the map to discover that the decision to become a male or female threespine stickleback is controlled by genes, not temperature or other environmental factors. After setting up her laboratory at Fred Hutchinson, Peichel, graduate student Joe Ross and former lab technician Clinton Matson set out to examine the fish's sex-determination system in more detail. That work revealed that the stickleback DNA responsible for maleness could be traced to a stretch of the fish's genetic blueprint that has many of the hallmarks of a Y chromosome.
In humans, the Y chromosome is immediately obvious under a microscope because of its tiny size. Yet an examination of the stickleback's chromosomes under a microscope reveals no obvious sex chromosomes, said Ross, a Molecular and Cellular Biology Program graduate student.
"The human Y chromosome is about 300 million years old, while we think the stickleback Y is only about 10 million years old," he said. "During the evolution of the human Y chromosome, it's become more and more degenerated and is considerably smaller than the other chromosomes. Many scientists think that given enough time, the human Y chromosome will no longer exist, and a new system for human sex determination will evolve."
Fully evolved Y chromosomes, like the human Y, are so diminished in stature because of the strategies they use to preserve their function in controlling sex determination. Humans have two sets of chromosomes, including a pair of sex chromosomes. Both chromosome partners of each nonsex chromosome pair share many similarities, which allow them to swap parts with one another during a process called recombination.
In contrast, the sex chromosomes in male humans — an X and Y — are very different from each other, which prevents much swapping from taking place. The differences between the X and Y evolved over time to keep the Y distinct and to prevent it from losing its critical male-determining gene during the swapping process.
Ross said that evolution of a fully mature Y chromosome is likely a sequential process. He and Peichel found that the first steps of this process are evident in the threespine stickleback chromosome responsible for maleness.
"First, a gene has to evolve a sex-determining function," Ross said. "This could be, for example, a gene that gets duplicated so that one sex has an extra copy compared to the other sex."
Next, the DNA near the sex-determining gene must acquire new bits and pieces that make it dissimilar from its partner chromosome pair, thereby preventing recombination and the resulting risk of losing the sex-determination gene. The newly acquired differences in the Y typically consist of repeated DNA sequences that often cause adjacent DNA to be looped out and deleted, which causes the diminution of the Y over time.
Mismatched chunks of DNA
Through DNA analysis, Peichel and Ross found evidence of both of these Y chromosome hallmarks in the threespine stickleback. They found that male sticklebacks always have mismatched chunks of DNA on their sex-determination chromosomes while female sex chromosome pairs have nearly identical DNA. In addition, they found that in males, recombination between the presumed Y and its partner is greatly reduced compared to the set of sex chromosomes in female fish.
The researchers haven't pinpointed the gene on the newly evolved Y that is the master control gene for maleness. They suspect that like its counterparts in humans and other mammals, it will act as a genetic switch that turns on a set of additional maleness genes that reside on nonsex chromosomes. Without a Y, these genes remain inactive.
Many of these so-called "downstream" sex-determination genes are unknown in humans, Ross said. If the analogous genes can be identified in the stickleback, they may provide clues to why some men are infertile.
"Only a small percentage of male infertility can be traced to a genetic defect," he said. "That's because we don't know what the genes are."
Ross said that the evolutionary youth of the stickleback Y is fleeting.
"In the end, the stickleback Y will probably end up looking like the human Y chromosome," he said. "What we don't know is whether this will take million of years or whether it will happen much more quickly."