The joy of opossum sex

Opossums have an unusual way of sorting out their sex chromosomes.
14 April 2017

Interview with 

Bryony Leeke, Francis Crick Institute




It’s time to hear from another of the speakers at the spring meeting - Bryony Leeke from the Francis Crick Institute in London, whose presentation was part of the ‘newly tractable systems’ strand of talks. Kat Arney couldn’t help but catch up with her afterwards to talk about the animals that she’s studying...

Bryony - So we work with opossums. They're from South America and they're about the size of a rat which makes them a very useful model animal for studying marsupial biology because they're easy to keep in the lab because they're small and they eat a normal diet just like a rat or a mouse would.

Kat - What are marsupials and why are they interesting, and how are they different from mammals like us?

Bryony - So marsupials are basically an earlier evolutionary form of mammals. They branched away from the kind of mammals that we are about 180 million years ago during evolution. They're similar in some ways. They feed their young milk and their young develop inside of them. But they're also different in that their babies are born in a much more developmentally early stage - basically, still what would look like an embryo in a mouse.

They crawl up the mother’s tummy and then attach to her nipples and get milk from outside and finish their development there. So they're very useful to study aspects of early development because they're already external to the mums. They're easy to study.

Kat - What sort of things are you trying to look at with your opossums? You're keeping them in your lab.

Bryony - Yeah, they live with us in the Francis Crick Institute in little cages and we use them to study the sex chromosomes. So in mammals like us and also in mammals that are marsupials, males have an X chromosome and a Y chromosome, and females have two X chromosomes. We’re interested in learning how those sex chromosomes have evolved. For example, how is it possible for females to have two X chromosomes and therefore, twice as many genes from the X chromosome as a male that only has one?

We know that in mice, female mice actually cope with this by turning off one of their X chromosomes so they have the same amount of X chromosome genes as in a male. We’re interested in studying how the same process might be happening in marsupials and what that can tell us about the evolution of that process.

Kat - There are some other kind of animals. There's things like duck-billed platypuses that they're not marsupials, they're not mammals, they are something else. What do we know about them?

Bryony - So those are called monotremes and they are also mammals because they feed their young with milk. But unlike marsupials and us, they lay eggs. So their babies do not develop inside of them in a womb. And they're even stranger. They actually don’t have the same sex chromosomes that we have or that opossums have. They have five X chromosomes and 5 Y chromosomes. It’s quite a confusing picture actually. There are some really great researchers in Australia who work on them, but they quite hard to study because they're quite rare, so not much more is really known about them.

Kat - In terms of understanding how this X chromosome, Y chromosome business works, what do you still want to find out about how the opossum’s X and Y chromosomes work?

Bryony - That’s a really good question. So it’s known in a mouse that the way that the X chromosome gets shut down is that one special gene makes a very, very long RNA – lots and lots of copies of it. This RNA wraps all around the chromosome that's going to be shut off basically like tying something up on a bunch of string to compact it. Our lab has found a similar gene in opossums that seems to do the same thing, but it’s not the same gene. So they’ve evolved separately throughout evolution but to do basically the same job, which is quite cool!

We’re interested in learning about what happens next. We know about the special gene that shuts things down, but in mice, there are lots of secondary and tertiary processes that then happen to make sure that the chromosomes stays turned off. These are all epigenetic processes. So we are now looking into the same kinds of processes in the opossum to see what kind of epigenetics are keeping that chromosome silent.

Kat - Bryony Leeke from the Francis Crick Institute.


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