Mystery Solved: How Mary the "Virgin" Fish Got Pregnant Without Having Sex

In a salty coastal lagoon on the island of North Uist, Scotland, researchers discovered a pregnant virgin fish. The stickleback was nicknamed Mary, after Christianity’s virgin mother. Normally, stickleback females lay their eggs in nests, which males later fertilize with their sperm. But Mary’s eggs were mysteriously still inside her when they were somehow fertilized, resulting in 54 healthy fish embryos and 15 minutes of fish fame.

If Mary were a human, some would call the event an immaculate conception. But as scientists reported Wednesday, there’s a far less miraculous biological explanation for this biological conundrum.

A team of scientists led by University of Nottingham biologist Laura L. Dean, Ph.D., published its decidedly non-religious solution to the mystery of Mary’s virgin pregnancy in the journal Scientific Reports. There were three possible explanations for the phenomenon, two of which the researchers ruled out, leaving one very rational — albeit improbable — explanation.

Explanation 1: Parthenogenesis

A strange phenomenon that happens in some animals is parthenogenesis, a kind of asexual reproduction in which an egg starts dividing and forming an embryo, even though it hasn’t been fertilized by a sperm. Parthenogenesis is the process responsible for highly publicized “virgin births” of sharks and snakes, and since it happens in some fish too, it could have explained Mary the virgin stickleback’s awkward situation.

That is, if Mary’s kids had the exact same genes as her. If an egg divides without a sperm through parthenogenesis, the resulting embryo can only carry’s mom’s DNA. But Dean and her colleagues cut Mary open to deliver her kids by C-section, discovering, as they grew into adults, that they carried DNA from some mysterious dad.

Parthenogensis? Nope.

Explanation 2: Hermaphroditism

Hermaphroditism is another phenomenon that happens in nature that results in individuals having both male and female sex organs. It’s been documented in multiple species, like the deepsea lizardfish, some stickleback populations, or in “sequential” form in clownfish. What hermaphroditism usually means is that an individual can self-fertilize its eggs with its own sperm and have kids without anyone else’s help.

Again, this could have been a possibility for Mary the virgin stickleback, but the DNA of her kids ruled out that option. Because hermaphrodites self-fertilize, their kids only carry the same DNA as the parent. But Mary’s offspring clearly had DNA from another individual.

Hermaphrodite? Nope.

Explanation 3: Undercover Sperm

That leaves only one other explanation: that Mary’s eggs were indeed fertilized by some male stickleback — just not the way normally associated with fish. In sticklebacks, males build nests and invite females to lay their eggs there so they can later fertilize and care for them. In other words, unlike mammals, most fish don’t have penis-in-vagina sex, so eggs and sperm come together outside of the body.

That’s why it’s so bizarre that Mary got pregnant while her eggs were still inside her body, and even more bizarre that sperm cells were able to get inside as well.

The most likely explanation, the authors write, is that Mary was hanging out around a clutch of already-fertilized eggs and that some of the sperm in that nest found its way to the eggs inside her body. Females may tend to prefer males whose nests already contain other eggs (it’s a good sign that the male is worth mating with), the authors note, and so they hypothesize “that sperm most likely entered the reproductive tract of the parent fish via contact with recently fertilised eggs in a nest.”

Mary’s Legacy

Mary, unfortunately, had to be sacrificed for the sake of her kids. Of the embryos, 54 were successfully delivered, and around 20 of them still survive now in the Nottingham Aquarium, about three years after their birth.

Far more than just a freak accident of nature, Mary’s example shows that there’s a lot more flexibility in the stickleback’s reproductive system than scientists once thought. The fact that the sperm could survive in the female’s body, for example, and that the embryos could develop to a near-hatching stage, add to the complexity of the evolution of pregnancy.

“For an egg-laying fish species,” wrote Dean in a blog post, “this is almost completely unheard of and I really remember looking down the microscope at the tiny hearts of her babies beating inside their eggs, still in her ovaries, and just not being able to believe what I was seeing!”