There’s a peculiar posse of amphibians creeping through the damp leaf litter of the North American Great Lakes. They’re about the size of a slim sausage, splotchy brown with watery smiles and bulging eyes. Cute, maybe – but, biologically, they are rewriting the rules of sex.
Meet the unisexual mole salamanders, an all-female lineage that’s been thriving for millions of years by not just cloning but occasionally stealing DNA from other mole salamander species. They are the only known vertebrate to reproduce via a process called kleptogenesis.
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What's so special about mole salamanders?
These self-replicating sisters are members of a group that also includes five sexually reproducing mole salamander species. The unisexual sisterhood evolved from a hybridisation event, more than five million years ago, between these sexually reproducing species. Now the unisexuals parasitise their sexually reproducing cousins’ sperm for their own purposes.
How do unisexual female mole salamanders reproduce?
During mating season, sexually reproducing mole salamander males leave sticky packets of sperm around the wetlands. A unisexual female will pick one up – but here’s where things get weird. She doesn’t use his sperm to fertilise her eggs. Instead, she needs it to stimulate her egg production. Mostly she then discards the sperm and produces clonal eggs. But on occasion she’ll absorb the sperm’s genome into her egg, which either adds to her existing genetic material, or replaces a previously stolen chunk of DNA. It’s reproduction à la carte.
Some individuals have up to five different genomes floating around in their cells, making them walking mash-ups of multiple species. Researchers can’t give these unisexual salamanders a traditional species name, because they don’t play by the rules of a typical species. So, they are just known as Ambystoma spp.
And while this reproductive system is a dream for the unisexuals, it’s a bit of a nightmare for evolutionary biologists. Their genomes are so tangled it took decades to understand how they reproduce. It wasn’t until molecular techniques improved in the 2000s that researchers confirmed kleptogenesis as the salamanders’ MO.
What are the disadvantages of asexual reproduction?
Asexual lineages typically face an evolutionary dead end due to the lack of genetic diversity – they can’t adapt to new diseases or environmental change. But these salamanders sidestep the problem by exploiting the genomes of their sexual cousins, which they can remix with their own during the creation of eggs. They can also reproduce at twice the rate of their sexual cousins. So you might expect them to outcompete their sexual cousins in the wild. But researchers have discovered the unisexuals’ Achilles heel: they’re not as fit.
The salamanders’ vigour was tested by placing them on a tiny treadmill. Every three minutes, researchers removed them, flipped them on their backs and timed how long it took them to get back on their feet to test their tiredness.
The study showed that the sexually reproducing species could go about four times the distance as the all-female team. This gives them an advantage when dispersing. They can travel further in search of fresh water sources if their home pools have dried up – a clear advantage in this time of climate change. So, sex does have its benefits.
These all-female salamanders are evolutionary marvels. They have blurred the boundary between sexual and asexual reproduction and forced us to rethink what’s ‘normal’ in biology.
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Main image: a blue-spotted salamander (Ambystoma laterale). Credit: Adam Cushen, CC BY 4.0, via Wikimedia Commons