Welcome back to the Nature Podcast.
This week, the mystery of the fish that shouldn't exist.
And how superluminous supernovae get so bright.
I'm Sharmini Bundel.
And I'm Benjamin Thompson.
When it comes to reproduction, things fall into two camps.
Sexual or asexual.
And while both ultimately have the same aim, namely producing offspring, they go about it in different ways.
In vertebrates.
Asexual reproduction where an offspring is genetically identical to its parent is rare, but it does exist.
For example, in an unassuming but rather unusual fish about the size of a thumb.
These fish live in waters between Mexico and Texas, but they're called the Amazon molly.
So they're all females.
That's why they're called the Amazon mollies.
They don't live in the Amazon basin.
They're called that because of the Amazon warriors from Greek mythology, which were an all-female society of warriors.
So yeah, the species is all female.
And they give live birth to clones of themselves.
This is Edward Reissmeier, part of a team of researchers who have a paper out in Nature this week looking at the Amazon molly, which was the first asexual vertebrate to be identified back in the 1930s.
Their work is looking to understand more about the genetics of asexual reproduction and help shine a light on a big question.
Why this fish exists at all, when theory suggests it really shouldn't.
Because asexual reproduction has some fairly significant downsides.
In particular, there's no way to shuffle and mix the genes being passed down to the next generation.
The disadvantages of asexual reproduction are that it is much harder to get rid of a mutation that is harmful.
Natural selection has to choose between taking the entire genome with the harmful mutation, or taking none of the genome.
And this is not ideal because there might also be beneficial mutations in that genome.
And so there's no way to separate those two and create a new genome that only has the beneficial mutation, but not the harmful mutation.
It's been thought that over time, damaging mutations will accumulate in clonal genomes and eventually an asexually reproducing species will go extinct.
This may help explain the rarity of asexual reproduction compared to its sexual counterpart, which has its own challenges but does make it easier to weed out damaging mutations.
Mathematical modelling suggests that the Amazon molly should have died out after about 10,000 years.
And yet, it's thought to have come into existence 100,000 years ago through a very specific event.
Around the city of what is currently Tampico Mexico, a Piscillia mexicana, a different fish from the same genus, and a Piscillia latipina.
So these two different fish mated.
And this wasn't the only time that these two fish have mated.
But for some reason, instead of their offspring were completely functional sexual reproducers, or they existed, but couldn't reproduce again.
So something like a horse and a donkey make a mule that can't reproduce at all.
What happened was a fish was born that could still reproduce, but only by cloning itself.
And knowing the origin story of this asexual species and still having its two parent species around was something Edward and his colleagues could take advantage of.
By comparing the genomes of the asexual Amazon molly, along with the genomes of its two sexually reproducing parent species, the team could test some theories associated with asexual reproduction.
This includes theories about the accumulation of harmful mutations that we've heard about and the idea that, due to the lack of genetic shuffling, asexual genomes pick up more mutations generally and, as part of this, begin to differ from the two parental genomes they originated from.
The team's analyses revealed that Amazon mollies are accumulating mutations faster than the two sexually reproducing parent species.
But there was no evidence of higher accumulation of harmful mutations.
Now, on the face of it, this doesn't make sense.
How can you have one but not the other?
But this isn't unprecedented.
Similar evidence has been seen in other species.
In this case though, Edward and the team have put forward their ideas of how this is happening, for the fish at least.
They think a process known as gene conversion is saving the day.
It's a process where the two copies of the genome that exist together in a cell perhaps one of those copies gets damaged and then, as a way of repairing this, the cellular machinery will replace the damaged part with the other copy of the genome.
So it's kind of like if you have two copies of something and they're a little bit different, then if you lose one, then at least you have a backup.
This copy-and-paste repair system is used throughout eukaryotic organisms and it looks like the fish have used it to keep their genomes in good shape, removing harmful mutations.
Gene conversion has previously been seen in Amazon mollyfish and has been hypothesized more generally to play a role in maintaining asexual genomes.
And in this work, the team were able to bring everything together.
The key new thing is that we were able to show that gene conversion is actually playing a role in maintaining these genomes.
So not just that it's happening, but that, In essence, natural selection has taken advantage of gene conversion to accomplish what it wants to accomplish.
Valdir Berbel Filho has written a News & Views article about the work for Nature.
He was impressed by the research and the way it investigated the theories about what's expected to happen to an asexual genome over time and uncovered evidence of how this fish continues to exist by avoiding the accumulation of deleterious mutations.
This effect of accumulation of deleterious mutations has been tested in a few asexual lineages and has some mixed kind of results.
Sometimes you have evidence for that, sometimes you don't.
And that's the biggest finding of this paper is the fact that this species in particular, is that they seem to have this gene conversion.
You still have mutations happening all the time, but gene conversion may be a counterattacking effect to those bad mutations.
But Valdir suggests that, although this work gives a sense of how asexual species can survive, there are still questions to be answered.
For example, the Amazon molly has a strange life cycle.
These exclusively female fish are asexual, but their life cycle requires them to go and seek a male mate from a different species and they need sperm to be present for their eggs to develop.
They just don't use it for fertilisation.
This is known as gynogenesis, and this is one flavour of asexual reproduction.
There are others.
Valdir says that researchers need to look at other species to see what other mechanisms might be out there.
There are different ways to be asexual, right?
The fish represents one of them.
These fish seem to have found gene conversion as a solution, but there may be others.
Each case can have different solutions depending on the context of the asexuality itself.
But this is one of the alternatives, and I think the authors went really thorough on this.
It's amazing that we can actually have a direct test of how genomes can survive, given all the bad predictions you have about asexuality.
Edward agrees that more work needs to be done on more asexual species to figure out how they go about maintaining their genomes.
This week's paper backs up the hypothesis that gene conversion is a way to do it, but it might not be the way.
And there are other things to figure out too.
For example, their evidence suggests that the two sets of chromosomes the Amazon molly got from its parents are changing at different rates, which doesn't make a lot of sense, as they should both be subjected to the same pressures.
Edward says the team can't work out why one is mutating faster than the other, but that it's something they're trying to explain.
Overall though, he says that understanding more about these fish and how they survived and thrived for 100000 years something that has had researchers perplexed for a very long time gives us a little more insight into the myriad of ways that life exists on this planet.
It's easy, I think, as humans to say the way that humans do things is the correct way.
The pinnacle of the tree of life is humanity.
And so, the same way, I think that it's very easy for us to say well, we do sexual reproduction, and lots of other stuff does sexual reproduction.
So that's the only way to survive.
That's the only way to exist.
So seeing the ways in which life has found ways around these problems that we've come up with to explain why we're the best is just so exciting for me.
Edward Reissmeier there from the University of Missouri in the US and the Ludwig Maximilian University in Germany.
You also heard from Valdir Bebel Filiu from the University of West Florida in the US.
To read Edward's paper and Valdir's News and Views article, look out for links in the show notes.
Coming up, how an ultra-bright supernova got its extra shine.
Right now, though, it's time for the Research Highlights with Dan Foxx.
Almost 2000 light years from Earth, four stars have been spotted orbiting each other in surprisingly cramped conditions.
Astronomers have observed many instances of four or even more stars whirling around each other, setting up complex gravitational interactions.
Now a team have spotted two stars that orbit each other every few days, while a third circles the pair once every 51 days.
All three are bigger and hotter than our Sun, but are so closely packed that they would fit inside Mercury's orbit.
A fourth star loops around this inner nexus roughly once every 1000 days.
The whole collection could be squeezed inside Jupiter's orbit around our Sun, making it the most compact four-star system known of its kind.
The four stars might merge eventually and end up as a pair of dense white dwarf stars billions of years from now.
Orbit over to Nature Communications to read that paper in full.
Bacteria in saliva and the small intestine might help to shield their host from severe allergic reactions to peanuts.
According to a study in mice,
Anaphylaxis is a severe immune reaction that can occur within seconds of consuming an allergen.
Now researchers have found that in humans, saliva and a section of the small intestine contain bacteria that can degrade the compounds in peanuts that are most likely to trigger an allergic reaction.
The team isolated the bacteria and introduced them into mice.
Animals colonised with the bacteria had diminished anaphylactic reactions compared with their uncolonised counterparts.
In humans, the researchers saw that these bacteria were more abundant in those with a higher tolerance to peanuts.
The results suggest that the body's resident microbes can diminish allergic responses to food.
Read that research in full in Cell, Host and Microbe.
You may have thought that supernovae are, as standard, pretty bright.
After all, they can outshine entire galaxies.
Or so years ago, some new kids came onto the block super luminous supernovae which are about 10 to 100 times brighter, which raises the question what not on earth is powering them.
There are two possible theories that have been put forward, though neither of them quite explains what's been seen.
Now a new paper in nature offers a fresh perspective.
Reporter Nick Petridge-Howe spoke with one of the paper's authors, Joseph Farrar, and asked him to shine a light on the two main ideas that astronomers had proposed.
So there's two basically, schools of thought for why superluminous supernovae are so bright and also trying to explain another strange aspect of them, which is that most supernovae sort of have a smooth evolution.
But superluminous supernovae.
In addition to being way too bright, their evolution is also very bumpy.
Their brightnesses go up and down in a strange way.
So there's two ideas.
The first is that, before the star died and became a supernova, it shed some of its mass into its surrounding environment.
And then, when the supernova happened, the shockwave traveled through that matter, causing it to glow.
And that can explain some of the over brightness as well as the bumps, because you're sort of running into these randomly distributed shells of matter.
The second theory is that when the star died, the core collapse forged a very extreme object called a magnetar.
So basically, when the core collapses, all the mass of the core, which is about the mass of the sun, gets squeezed into the size of a city.
And the compression is so high that protons and electrons get sort of forced together into neutrons.
And you get a very, very small, dense star made entirely of neutrons called a neutron star.
Now, because the star goes from being very big to very small very quickly.
It's spinning and that spin increases.
It's spinning really quickly.
And then the magnetic field gets amplified through the neutron star.
So when you have a neutron star spinning very quickly with a very powerful magnetic field, That's a magnetar.
And we think that they're formed in these explosions.
And then they release huge amounts of energy into the supernova from within.
So, whereas the interactions with matter around the star sort of power the supernova externally, the other theory is that magnetars power the supernova like a big battery from within.
So you've got these two ideas, interaction with matter or them being powered by a magnetar.
And these could help explain the evolution, those changes in brightness over time, as you're observing it.
So what would be the ideal thing to see to say whether it's one of these ideas or the other?
Well, that's a good question.
Up until now, we didn't really know exactly.
I mean, if it was a magnetar, the best thing would be to see the magnetar somehow, but that's not exactly possible.
And that's actually been the big problem with these two theories.
We can have models, right like formulas that tell us what to expect in the data, and we can fit those, but that's not a confirmation.
That just tells us, If it works like this, the model would tell you this.
So the ideal thing to look for would be to take your idea like either it's a magnetar, either it's surrounding material and ask what alternate predictions do these theories make?
And are any of those predictions sort of met?
So does the math sort of fit what you see in space, I guess?
Exactly.
Yeah.
We want something just beyond a model fit.
We want additional predictions. prediction an additional effect that we can measure that ties back to the same principle that's driving the over brightness and as i understand you got an opportunity to try and do a bit of this because there was an observation you had with the catchy name 2024 afav can you tell me a little bit about this observation and how it could potentially allow you to understand these theories 24 AFAV was one of the most bizarre supernovae I think we've ever seen.
When it exploded, it rose over 40 days to a superluminous peak.
So right away, it told us this is a superluminous supernovae.
And then, after the peak, it started executing this incredibly strange sequence of wiggles in the light curve, like the brightness plunged and then it skyrocketed up again, and then it did this multiple times.
And we spotted a pattern, which is really cool. the bumps in the light curve were not random.
They seemed to be getting closer together in a specific pattern.
And so we made predictions for future bumps, and then used Las Cumbres Observatory's dynamic observation capabilities to track during when we expected to see the bumps.
And we did.
So we got this nice continuous light curve and we could see the bumps as they were happening, thanks to this prediction.
That's really really crazy, because it's really unusual to be able to sort of predict what an object in space is going to do in real time, especially when it's like a brand new prediction.
So you caught this early on enough to, I guess, predict what might happen and then see if your model, your predictions, then fitted what actually happened, to understand whether one of these two different theories works out.
Yeah.
I mean, we caught the supernova very early.
It was discovered by the asteroid terrestrial last alert system, Atlas like probably a couple of days after explosion.
So obviously the key question is did either of these two ideas that you mentioned you know, the magnetar or this sort of cloud around it, did either of these fit the observations of what you saw?
Yeah, so the first thing we tried was the interaction with matter around the star, and it sort of didn't work very well.
The reason for this not working well in this specific case is because the bumps that we observed were bizarrely sinusoidal.
And so that's right off the bat very challenging for an interaction model to explain because the interaction tends to be very random and irregular.
So we tried the magnetar model and the magnetar model is nice.
It can explain the overall shape and brightness of our object, which is really good.
But the classic problem with the magnetar model is that it can't explain the bumps.
And so we had to come up with a whole new idea for how to explain the bumps in this specific object.
And the key clue that we had was that the bumps were getting closer together.
And that is very, very strange.
There's sort of very limited number of astrophysical processes that can cause something like that.
One of those is some sort of gravitational infall, like if you've ever seen the inspiral of two neutron stars or two black holes producing a gravitational wave chirp.
That chirp is an increasing frequency that results from the radii of the two orbiting objects getting smaller.
And so we started playing with this idea that there is something similar happening here, where there is something spiraling into the magnetar.
The idea is basically that there's an accretion disc around the magnetar that is processing.
So if you imagine this magnetar, this is very small, very dense, highly magnetized star.
It has a big disc around it.
Like you can sort of picture it in your head, like a big donut.
And the donut is tilted relative to the magnetar, the spin of the magnetar.
So the magnetar is spinning in one direction and the donut is tilted relative to that.
Now, due to this tilt, some precession mechanisms will cause the donut to spin.
So the donut might start facing you, but then it will rotate around and face away from you.
And then it will keep doing this sort of periodically.
And this is called precession of the disc.
As it processes, it is blocking and reflecting the light or otherwise modulating the light from the magnetar along our line of sight.
So the accretion disc goes behind and reflects some of that light.
It goes in front and blocks some of it.
And that gives us these over brightnesses and then under brightnesses that are the wiggles and then, as the accretion disc falls in the precession, frequency increases and that gives us this strange chirp in the light curve.
And we needed this whole new mechanism to explain this, because a chirp has never been seen before in a supernova.
So we had to start thinking really creatively and outside the box, And so is this a win for the Magnetar proponents.
Is this good evidence that this is what is going on?
Or is there more work to be done to know if this is going on for every superluminous supernova?
None of what we have discovered here precludes that an interaction with material around the star can be happening in other supernovae.
And in fact, it is also likely happening in this one as well, just at a lower level.
But the real problem was that you had these two theories that both kind of worked to explain this mystery, but we didn't have a way to even confirm that, like for sure that they are valid.
So, even though there's these two competing ideas, it's not a matter of one's more right than the other.
It's a matter of we don't have evidence favoring either scenario.
And then what this gives us is this confirms that magnetars can power superluminous supernovae.
So it's like we went from we have this idea that they can to we're now pretty sure that they can.
And we have an example of an object where we can see the infant magnetar powering the supernova illuminated by this processing accretion disk around it.
And so what would you like to do next, I guess, to understand this phenomenon more?
Oh, there's a ton of awesome stuff.
So first of all, you know, this is a very, very rare phenomenon.
Of course, out of 15000 known supernovae, we have a couple hundred superluminescent supernovae and one chirp.
But the Vera C. Rubin Observatory just came online.
We just started getting discoveries from it, and it is going to increase our sample of supernovae by many orders of magnitude.
So we're going to go from having one chirped supernova to probably dozens of them.
And that's going to be a big sample that we can explore to see, does this idea really work at scale?
And what other effects are we missing?
But there's just so many open questions and there's so much more work to be done.
Our analysis was very like inspirational, like here's how we need to maybe treat these systems moving forward.
Here's an idea.
But there's going to be a lot of really exciting theoretical and observational work to really put these ideas through its paces and unveil these incredibly interesting objects to us in more detail.
That was Joseph Farrer from the University of California, Santa Barbara.
For more on that, including a video showing how these superluminous supernovae may work.
Check out the show notes for some links.
And that's all for this time.
We'll be back later in the week with the briefing podcast.
But until then, if you've enjoyed the show, do let us know.
You can leave a review on your podcast app of choice, or you can reach out to us on social media.
We're at Nature Podcast.
And we're on email too, podcast at nature.com.
I'm Benjamin Thompson.
And I'm Sharmini Bundel.
Thanks for listening.
