-A curious middle school student from California
June 16, 2006
What a fun question. Platypuses are those weird animals that look like a cross between a duck and a sea otter. And believe it or not, at least some of their genes seem to be a cross between birds and mammals!
Not too much is known right now about specific genes. A project similar to the Human Genome Sequencing project is going on right now for the platypus. Once that's done, we'll have lots of detailed data that can really get at your question.
In the meantime, we can look at the chromosomes of a platypus. Let's focus on how platypuses decide to be boys or girls. It is the weirdest thing I've seen.
Nearly all mammals make the gender decision based on the sex chromosomes. If you have an X and a Y chromosome, you usually end up a boy. Two X chromosomes and you're a girl.
How does this work? Well, there is a gene on the Y chromosome called SRY. This gene triggers the development of a male. Without the gene, you are a female.
Scientists know about this because of conditions like Swyer's syndrome. In Swyer's syndrome, the SRY gene doesn't work. These XY folks look female.
A bird works similarly. The difference is that the chromosomes are called W and Z. And that a W and a Z make a girl and two Z chromosomes make a boy.
One other difference is that the gene involved isn't yet known. In other words, the bird's SRY gene hasn't been found. One candidate gene is the DMRT1 gene found on the Z chromosome although this hasn't been confirmed yet.
If true, then DMTR1 is pretty different from SRY. Either having only one DMRT1 makes you a girl or having two copies makes you a boy.
OK, so what's that got to do with a platypus? Well, a platypus has 10 sex chromosomes instead of just 2. More precisely, it has 5 pairs of sex chromosomes.
The chromosomes have been named E1-E10. A female has two each of E1, E3, E5, E7, and E9. A male has one each of E1-E10. Also, in a male, E1 is always paired with E2, E3 with E4, etc.
So this seems like a mammal in a weird sort of way. If you have the same sex chromosomes in a pair, you are a girl. Different ones and you end up a boy.
And yet, there is no hint of an SRY gene. There are a couple of other mammals where SRY hasn't been found yet either so that isn't completely unique. Except one of the chromosomes has something on it that looks like DMRT1 -- the possible bird gender-decider.
DMTR1 is on E9, one of the chromosomes that if you have two of them, you are a girl platypus. In birds, if you have two chromosomes with DMTR1, then you are a male.
So in platypuses, it works differently. DMTR1 may be involved. But something on all of the other X-like chromosomes might be involved too. Or on all of those Y-like chromosomes. I think it is safe to say that we don't know yet how it works.
One thing we do know is that E1 is pretty similar to other mammalian X chromosomes. And that E9 has DMRT1 and so is sort of like a Z chromosome. In fact, when you look at cells at a time when you can see the DNA, all 10 of these chromosomes form a chain. At one end is the "mammalian" XY pair and at the other the "bird" WZ pair.
As you can see, the platypus is very different from mammals and birds (and reptiles and...). I can't wait to hear what they find out next!