Mystery of Bird Maleness Partly Solved

Birds Require the DMRT1 Gene to be Male

by Denise Chen

September 11, 2009

The X and the Y chromosome.

A popular question of expecting mothers is “Will I have a boy or girl?” And a common way doctors come up with a definitive answer is by looking at a baby’s sex chromosomes. A baby with two X chromosomes is almost always a girl; a baby with one X and one Y chromosome is usually a boy.

Scientists can also tell if birds, moths, and butterflies will be boys or girls by looking at their sex chromosomes. But instead of an X and a Y chromosome, these critters have a Z and a W chromosome. And instead of two of the same chromosome being a girl, here it’s a boy. So ZZ is the boy and ZW is the girl, in the world of birds, moths, and butterflies.

But what is it about the Y chromosome in humans and the Z chromosome in birds that shouts out “boy!”? Well, scientists have found a gene on the Y chromosome that gets the ball rolling on becoming a boy (we’ll talk about this later). But for birds, being a boy isn’t a question of having a unique chromosome that girls don’t have. Birds use a different system.

So it might be that there is a special girl gene on the W chromosome that turns an embryo into a girl. Or it might be that an extra dose of Z makes a bird male. In other words, one Z leads to a girl and two Z’s leads to a boy. It looks like the Z’s have the special touch.

In a recent study, researchers show that a gene called DMRT1 found only on the Z chromosome partly explains bird “maleness”. When a ZZ embryo gets less DMRT1, the embryos start to take on some female traits.

These studies show us that bird gender can be partly explained by genetics. Not having enough of a single gene can keep a bird from becoming a bona fide male bird. But this doesn’t rule out the possibility of a female gene being on the W chromosome. Scientists just haven’t yet found one.

The Genes That Little Boys and Girls are Made Of

In humans, the X and Y
chromosomes determine
whether you are
male or female.

Gender starts out as a genetic thing. If things go as planned, whether you are going to be a boy or girl is decided at the moment you get your set of chromosomes from mom and dad. Well, that choice is mainly up to dad.

It is the presence or absence of dad’s Y chromosome that makes an embryo a boy or a girl. So someone with an X chromosome from each parent is female while someone with an X and a Y chromosome is male.

The first clues showing that the Y determined gender as opposed to two X’s making a girl were found in cases where people didn’t get the usual XX or XY combinations. For example, people with a single X chromosome (called Turner syndrome or XO) look female. And people with two X’s and a Y (called Klinefelter syndrome or XXY) look male.

So what is it about the Y chromosome that turns an embryo male? Part of the answer is a single gene on the Y chromosome called SRY. Without this gene, female organs and cells (ovaries and eggs) develop.

But the bird version of this particular human SRY gene can’t be found. And bird equivalents of XO and XXY haven’t been found either.

The Z and the W chromosomes have a lot of genes in common. But scientists have been able to find genes that are unique to either the Z or W chromosome. The idea is that a unique Z gene might cause a male if there are two of them. Or that a unique W gene might cause an embryo to turn female.

DMRT1 is one of these suspect genes and it’s only on the Z chromosome. Scientists already know that DMRT1 is doing something really important because mammals, reptiles, and fish also have it and need it. For example, without enough DMRT1, humans don’t develop proper testes and neither do mice.

What about DMRT1 in birds? What happens when you take DMRT1 away in male birds that usually have it? Researchers found that male birds need DMRT1 to develop testes, too! With low amounts of DMRT1, male birds develop sex organs that look and seem to act like their female counterparts.

Researchers haven’t found whether this is the only gene that plays a part in bird maleness (it’s probably not). And they want to try the opposite experiment, to see whether large amounts of DMRT1 produces female birds male-like sex organs. But they have definitely shown that DMRT1 is a key gene involved in making a rooster.

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Choosing your baby’s gender

The DMRT1 Gene is So Masculine

Getting rid of a gene's RNA
can inactivate that gene.

Finding unique genes is a lot easier than testing whether or not they are important. Scientists have come up with all sorts of tricks to find out what a gene does. In this case, they used something called RNA interference to lower the amount of DMRT1 in male birds.

Each of the many genes found in a living organism has the instructions for a specific protein. And each protein has a specific job. For example, the DMRT1 gene makes the DMRT1 protein. It is the protein that turns a bird embryo male.

Cells don’t just go from DNA to protein, though. Instead they use RNA as an intermediate. RNA acts like the messenger, delivering these instructions to the right place at the right time so that the right amounts of proteins are made. Without RNA, there is no protein.

Using RNA interference, the researchers kept lots of DMRT1 protein from being made by blocking DMRT1 RNA. They used this strategy in chicken embryos and waited to see what the embryos grew up to look like.

What they noticed is that genetically male embryos grew into male chicks with testes that looked more like ovaries. The less DMRT1 a male chick had, the more its testes looked like ovaries. In fact, these female-like testes had the structure and parts of an ovary that a female chick at that same age would have.

Then, the researchers looked at the types of proteins being made in these female-like testes. Males and females make different proteins as they develop. So if DMRT1 is important for bird malenessand scientists cause less DMRT1 to be made, they expect to see more female proteins being made in a male embryo. Or at the very least, less male proteins.

What the researchers found was far less of a protein called Sox9 that is normally found in the testes. Not only that, but there was more of two proteins called aromatase and FoxL2. These two proteins are usually found in the ovaries and never found in the testes. Again, the less DMRT1 a male chick had, the less male protein Sox9 and more female proteins aromatase and FoxL2 were made.

This means that male chicks with less DMRT1 than usual couldn’t develop proper testes. A low amount of DMRT1 seems to trick males into thinking they should instead be female. So the male embryo pulls a quick fix-it stunt, rearranging the testes to be more ovary-like and to make proteins that an ovary usually produces and needs.

The next step is to raise the amount of DMRT1 in female embryos. Can the females be tricked into thinking they should instead be males? This would allow us to rightly give DMRT1 the title as a Captain Gene of Bird Maleness.

There are still many other questions remaining. Is there a Captain Gene of Bird Femaleness and what does it do? And how do different levels of DMRT1 affect those other proteins we saw less or more of in the female-like testes of this study? What are those proteins doing?

But this study shows that there is certainly a “maleness” gene on the Z chromosome. Because a male chick has two Z chromosomes and two times as much of this “maleness” gene than a female, he is on track to be truly male.

And now that scientists have part of the picture for how a bird becomes male or female, maybe they can figure out why some reptiles become male only at certain temperatures. Or how clownfish can change from a boy to a girl when the dominant female dies!

Denise Chen

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Content provided by the Department of Genetics, Stanford University.