All The Single (Salamander) Ladies
A population of mole salamanders in the Midwest is throwing a curveball at our understanding of sex and reproduction. Some populations of this salamander are unisexual—they’re females that can reproduce without males.
The unisexual mole salamanders aren’t just cloning themselves, however. “These salamanders really are taking advantage of the system,” says Katy Greenwald, an associate professor of biology at Eastern Michigan University. “They’re all female, so they don’t bear that cost of producing males, but they’re occasionally incorporating DNA from other species. In some ways it seems like this potentially totally unique evolutionary win-win—which might ultimately let us learn a little more about the evolution and maintenance of sexual reproduction.”
These salamanders may not need males, but they do need sperm to reproduce. Various species of male salamanders leave packets of sperm lying around, and the mole salamander females steal them. This particular population of unisexual mole salamander can reproduce from five different salamander species that live nearby, and a single clutch of eggs could hold offspring with genetic ties to several different species.
Greenwald joins Ira to explain what advantages living a single-sex life may have for the mole salamander.
Katy Greenwald is an associate professor of Biology at Eastern Michigan University in Ypsilanti, Michigan.
[MUSIC PLAYING] IRA FLATOW: In the Midwest, the population of mole salamanders is changing up our understanding of sex and reproduction. Some of these salamanders are unisexual– they’re females that can reproduce without males. But they’re not just cloning themselves, they make use of sperm from other species in their reproductive process.
Katy Greenwald is associate professor of biology at Eastern Michigan University in Ypsilanti, Michigan. She’s one of the subjects of our latest Macroscope video up on our website at ScienceFriday.com. Welcome to Science Friday.
KATY GREENWALD: Oh, thanks so much for having me.
IRA FLATOW: So set the scene for us– what are these salamanders, where do they live? Go ahead, tell us all about them.
KATY GREENWALD: Sure. So these salamanders, we generally call them unisexual salamanders because they’re all female, there are no males. And they are pretty broadly distributed throughout the Great Lakes region out into New York and New England and then up into southern Canada. So they’re actually generally quite abundant and populous salamanders in that part of the world.
IRA FLATOW: So if they’re unisexual, how do they reproduce?
KATY GREENWALD: Well, they reproduce in a way that, as far as we know, is actually globally unique, so there’s nothing else that we know that does this. And what they do is they breed in ponds. They’re part of a genus of pond-breeding salamanders, and the typical pond breeders have both males and females, and the males produce these little sperm packets, called spermatophores in the ponds.
And the females then pick those up and there’s internal fertilization and they lay their eggs. So the unisexuals are actually in these same ponds with these other species, and they will grab those spermatophores produced by the males of the other species.
IRA FLATOW: And as far as we know, can we find a specific mutation or something that allows them to do this?
KATY GREENWALD: That’s a great question. So what’s really interesting about them is despite the fact that they are stealing genomes from up to five other species, they are actually all a single evolutionary lineage that’s probably about five million years old. So it looks like something that just kind of cropped up one time, evolutionarily speaking, in this lineage, and then it’s just allowed them to be really remarkably successful.
IRA FLATOW: I’m Ira Flatow. This is Science Friday from WNYC Studios talking with doctor Katy Greenwald. So how many genomes do they have if they’re mating, so to speak, with all these other salamanders?
KATY GREENWALD: We find them with anywhere from two to five complete sets of chromosomes.
IRA FLATOW: Two to five sets.
KATY GREENWALD: Yes, so what we call diploid, that’s two sets, all the way up through pentaploid, that is five sets. And those five sets, or those multiple sets, can come from any one of five other species. Interestingly, actually, the most common ones that we find have three sets of chromosomes, which is–
IRA FLATOW: Three sets.
KATY GREENWALD: –pretty unusual for animals.
IRA FLATOW: So what is the evolutionary advantage of this way of life?
KATY GREENWALD: This is really neat because one big question in evolutionary biology is why so many organisms reproduce sexually. The big question comes down to kind of a numbers game. So a unisexual or an asexual lineage that’s all-female can grow twice as fast as the sexually reproducing lineage.
This is actually known as the cost of producing males, so if you have a lineage that’s all-female, every female can reproduce on her own. You don’t have to create males. You don’t have to have males taking up resources that could be used for females.
You’ve got costs associated with finding mates and so on. So sexual reproduction is actually quite costly, and so there have to be some big advantages, right, to outweigh those costs. And those are thought to be the additional genetic variation that you get from sexual reproduction, as well as being able to get rid of bad mutations.
IRA FLATOW: So if all these females are living in the pond, aren’t we going to lose the males because you’re not making any more of them?
KATY GREENWALD: That is also a very good question. What it looks like may be happening, at least at a number of my sites where we have 90% of the salamanders in the pond are these unisexual, is that I think if we had very long term data, we might see that in fact they do kind of compete themselves out of existence, at least temporarily, until the sexual species are able to build up their numbers again, and then that might allow the unisexuals to then come back on the scene.
IRA FLATOW: So you may have a crash in the population that then stabilizes and rebuilds itself.
KATY GREENWALD: Exactly.
IRA FLATOW: What do you want to know most about this?
KATY GREENWALD: Oh, I have a lot of questions about this. So the most interesting question as far as the mode of reproduction in my mind is that sometimes the females include the male’s genes and sometimes they don’t. So they will actually produce clutches of eggs where they’ve picked up that spermatophore but then not used it. They’ve just basically cloned themselves.
Sometimes, though, they do add the male’s genome. And sometimes they actually swap it in, so they drop one of their own and add his. And sometimes the egg masses include multiple modes of reproduction. So they may have more than one of those things happening. And that, I think, is a really fascinating system and would allow us to kind of get at this question of when sexual reproduction is adaptive versus when it’s not.
IRA FLATOW: That is quite mysterious, and I wish you luck, and come back when you find out, OK, Katy?
KATY GREENWALD: Thank you, thank you. I’ll do my best.
IRA FLATOW: Katy Greenwald, associate professor of biology at Eastern Michigan University. That’s in Ypsilanti, Michigan. And she’s one of the subjects of our latest Macroscope video up on our website at ScienceFriday.com. It’s a great video. You’ll see the salamanders. Terrific stuff up there on our website at ScienceFriday.com. Thank you for joining me today!
KATY GREENWALD: Thank you.