Asexual reproduction in sawtooth sharks: Parthenogenesis discovered in wild offspring.

These Lonely Sharks Reproduce Without Sex, and Who Can Blame Them

These Lonely Sharks Reproduce Without Sex, and Who Can Blame Them

Wild Things
Slate’s animal blog.
June 2 2015 3:26 PM

Going It Alone

smalltooth_sawfish_47
Juvenile smalltooth sawfish in Florida's Charlotte Harbor.

Courtesy of Florida Fish and Wildlife Conservation Commission.

Andrew Fields wasn’t looking for a miracle. The Ph.D. candidate at Stony Brook University's School of Marine and Atmospheric Science was just doing some routine genetic sampling of his study organism, the critically endangered smalltooth sawfish. But then Fields found something amazing: several fish with nearly identical chromosomes. He knew this was big, but just to be certain, he ran the discovery by his advisor, Demian Chapman. “Holy shit,” Chapman said.

The pair had stumbled upon some of the only animals known to have switched in the wild from sexual reproduction to asexual. These sawfish, a type of shark, were begotten by parthenogenesis, a process by which eggs develop in the absence of sperm. The researchers’ findings, published this week in the journal Current Biology, follow research in 2012 on pit vipers that had gotten pregnant in the same paradoxical fashion and given birth in the lab. But until now, no one had come across the offspring of mateless mothers doing fine on their own.

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“It was a pretty big shock to me,” says Fields. “I never expected to find virgin births.”

Now just imagine how the sawfish felt!

In the animal kingdom, virgin births are far less exceptional than you might think. They’re downright common in invertebrates, from starfish to water fleas to honeybees. Vertebrates are in on the game, too: Turkeys, Komodo dragons, bonnethead sharks, and even the massive green anaconda have been found to occasionally spawn sans fathers. New Mexico whiptail lizards reproduce this way regularly, and bdelloid rotifers—tiny, leechlike water dwellers—have lived in celibate, all-female communities for hundreds of millions of years. (And they’re perfectly happy, thank you very much.)

Why go solo? Traditionally, researchers have viewed parthenogenesis as a move of desperation: It may happen under the extreme stress of captivity or in animals that are critically endangered and have no nearby mates. The latter was presumably the case with these sawfish. When there’s no sperm to fertilize an egg, the egg instead consumes a polar body—a cell split off during meiosis—creating a half-clone. And voilà: your own mini-you!

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In the short run, reproducing this way can help stretch a species one more generation in the hopes that the offspring can come across some mates. But in the long run, it can be a dead end. After all, sex is “probably pretty important,” as Fields puts it. That is, sexual reproduction evolved as a way to recombine DNA so that offspring have enough variation to give them a shot at survival if conditions change. Foregoing sexual reproduction drastically lowers the genetic diversity of offspring, leaving a species more vulnerable to being completely wiped out.

Virgin births could be a dire omen for species like the imperiled sawfish—creatures so threatened that they are on their way to becoming the first entire marine animal family to be driven extinct by overfishing and coastal habitat loss. “This is a big warning sign,” says Fields. “It’s saying, ‘hey, we need to pay attention to these guys’—or they’re not going to survive that much longer.”

But beyond sawfish, virgin births might not always bode so badly. Parthenogenesis has some distinct advantages. Since parthenogens with a build-up of mutations will quickly die off, this method of reproduction could purge bad genes from a species, says Warren Booth, an evolutionary biologist at the University of Tulsa who wrote the 2012 paper on pit vipers and reviewed the recent study. Booth has found this to be the case in inbreeding bed bugs, which he also studies. Moreover, if a female is particularly well-suited to her environment, having offspring that are as closely related to her as possible may not such be a bad idea.

Ready for the best part? “It’s less biologically costly,” says Booth. “With sexual reproduction, you have to find a mate, you have to be compatible with that mate, you have to be able to reproduce. With parthenogenesis, you don’t have any of those initial costs.” Sign me up! (Sadly, in case you’re wondering, humans have preventative genetic mechanisms in place that make it “beyond possible” for them to give virgin births, says Booth.)

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We’ve long considered sexual reproduction to be the best form of reproduction. But maybe—just maybe—we’re wrong. After all, asexual reproduction, which includes cloning and budding, is a far older form of reproduction than sex. And the fact that parthenogenesis also happens in populations with a 50/50 sex ratio, where sex is clearly still an option, suggests that it isn’t always just a measure of necessity. Perhaps, from our sexcentric vantage point, we’ve overlooked a perfectly good form of reproduction.

But don’t take it from me. Here are the benefits of going it alone in the sawfish’s own words, to the tune of Gloria Gaynor’s “I Will Survive.”*

At first I was afraid
I was petrified
Kept thinking I could never breed
Without you by my side
But then I spent so many nights
Counting up your genetic cost
And I grew certain
That losing you, it was no loss

And then one day
From deep inside
I felt a kicking in my belly
And knew something was alive
See, I’d thought for baby-making
That your male sperm was key
But then my egg consumed my cell
And now I’ve got a little me

Go on, now go! Walk out the door
’Cause I just found out
I don’t need males anymore
Weren’t you the one who said I’d never make a tribe
Did you think I’d crumble?
Did you think I’d lay down and die?
Oh no, not I

I self-fertilized!
Oh as long as I know how to clone
I know I’ll stay alive
Now I’ve got all my brood to wean
(Hope they still pass on my genes)
And they’ll survive
They will survive, hey, hey!

*Not an actual sawfish quote

Rachel E. Gross is the science web editor at Smithsonian.