The virgin birth of a dragon
Komodo dragons and embryonic stem cells
A cool story out of London this week is the birth of five Komodo dragons. These animals are endangered so additional Komodo dragons are always a reason for excitement.
But what makes this even more exciting is that the mom had the babies without a dad. In other words, some of her eggs developed into babies without the need for sperm (this is called parthenogenesis).
This isn't unique to Komodos. Many animals can do it (although not mammals).
Scientists think Komodos may have developed this ability because of where they live. Komodo dragons sometimes get stranded on an island alone.
Because of how gender is determined in Komodos (see below for more detail), all of the babies produced this way are male. So a new colony of Komodo dragons can be founded by a single female.
OK, neat. But why should I care? Because scientists are trying to get it to work in people.
Scientists aren't trying to get rid of men (at least I don't think so). Instead they want to create embryonic stem cells that won't be rejected by the patient.
To better understand all of this, let's first talk about why eggs can't always simply develop into babies. Then we'll talk about why the process doesn't work in mammals (including us). Finally we'll talk about how scientists are getting around this problem.
Making babies without a dad
Nature has come up with lots of ways to create children without a father. Many animals can do it (although no mammals have yet been found that can).
To understand why this is so amazing, we first need to dig a bit deeper into why eggs usually need sperm to make a baby. The quick answer is because eggs don't usually have enough chromosomes for an animal to survive.
Chromosomes contain the instructions for making an organism (click here to learn more about chromosomes). Most of our cells have a pair of each of our chromosomes.
So we have two copies of chromosome 1, two copies of chromosome 2, etc. The egg and sperm cells are an exceptionthey have only one of each pair and so half the usual number of chromosomes. This is so that when an egg and a sperm combine, you end up with the right number.
So how do you get a baby with an egg that has only half the usual number of chromosomes? One way is if it is OK to only have half. In the honeybee, for example, males have half the number of chromosomes that females do.
But most of us need all of our chromosomes. So nature has come up with a bunch of ways to duplicate the DNA in the egg so that the new baby ends up with the right number of chromosomes (see the link below for more details).
One consequence of this is that all of the new babies are the same sex. Which sex it is depends on how gender is determined in that particular animal.
In many animals, sex is figured out by the sex chromosomes. In people, if you have two X chromosomes, you're a girl. If you have an X and a Y, you're a boy. In Komodo dragons (and birds), the male has two Z chromosomes and the female has a W and Z chromosome.
This is why all of the new Komodo dragons were male. Because mom is WZ, she makes eggs that have either a W or a Z. When the number of chromosomes doubles in the egg, all of her offspring end up with either WW or ZZ.
Since WW's don't live, all her offspring would be male (ZZ). The story would be different in people.
A human born without the egg getting fertilized would result in a girl. That is because all of mom's eggs have an X chromosome. If the number of chromosomes doubled, then there would be two X chromosomes. A female.
This is a hypothetical example though because the process doesn't work in people. Let's find out why.
How to make stem cells with just an egg
Female mammals can't make babies on their own. For the foreseeable future, this won't be changing either.
The reason mammals can't do this has to do with something called genetic imprinting. The DNA we inherit from our moms is very similar to what we get from our dads. But there are differences.
Scientists found these differences by studying sperm and egg DNA. They discovered that some part of the DNA is changed in certain places in the sperm and other places in the egg.
It is these differences that make fertilization necessary. A fertilized egg needs both sets of differences in order to grow.
So if you just double the DNA of an egg, you won't get both sets of differences. And the new egg won't grow into an adult.
But it will make it partway. In fact, far enough long to make embryonic stem cells.
Embryonic stem cells hold a great deal of promise (click here to learn more). A big problem, though, is that a patient will reject someone else's stem cells. So scientists are trying to make embryonic stem cells from adults.
One way to do this is to clone the adult. This has obvious ethical problems, some of which are being dealt with (click here to learn more).
Another way is to make a female's egg start to grow on its own. The first step is to get it to double its chromosome number. There are a variety of ways to do this.
The next step is to grow it a bit and collect some embryonic stem cells. They have done this and are beginning to study in detail what can be done with these cells.
It is important to note, though, that these stem cells are not from a clone. The DNA is different from mom's.
Remember, an egg has only half of the mom's DNA. When the egg isn't fertilized, the DNA in the egg is doubled. In essence, half of mom's DNA has been duplicated.
So the new cell has twice as much of half of mom's DNA. Simple, huh?
Let's give a concrete example. Imagine that mom is a carrier for red hair (click here to learn more about red hair). This means that one of her MC1R genes causes red hair and the other doesn't. Mom doesn't have red hair but carries the gene.
Each egg has a 50-50 shot of getting the red hair gene. Let's say the one chosen to make the stem cells is the one that has the red hair gene.
When the gene is duplicated, the new stem cell would have two red hair genes. If it had hair, it would have red hair. Which is different from mom.
But the cells would still be similar enough not to be rejected by the patient. This is why scientists are trying to create these cells.
Of course this process will only work for women. But perhaps one day the process could be repeated with sperm DNA put into an egg without chromosomes. Then both women and men could have personalized embryonic stem cells.
Parthenogenesis is not only cool, but it might help us solve the problem of embryonic stem cells. Thanks Komodos for showing us another way to get these cells!