Sawfish “Virgin Birth” Discovery

“Life, uh, finds a way.” Any Field Museum fan worth their salt probably remembers the scene in Jurassic Park where Jeff Goldblum’s character explains that even though all the dinosaurs in the park are female, they might still find a way to reproduce. But this isn’t science fiction—Field Museum scientists have recently helped discover that smalltooth sawfish, critically endangered shark relatives, sometimes reproduce through “virgin birth.”

Smalltooth sawfish, which give birth to live young, normally produce offspring the old-fashioned way, with sperm from a male fertilizing an egg from a female. However, scientists recently learned that about three percent of the sawfish living in a Florida estuary were born without ever having fathers.

This form of asexual reproduction, called parthenogenesis, occurs sometimes when there are no males present to fertilize a female’s eggs—it’s sometimes seen in zoos when female animals give birth even when they haven’t been around potential fathers for years. Parthenogenesis is rare, but it happens with fish, lizards, birds, and insects. This case marks the first time that scientists have seen parthenogenesis in the wild in a vertebrate that can reproduce sexually. It’s also unusual because the offspring appear to be healthy— animals born through parthenogenesis are often malformed or die early because all of their genes come from one parent, so they’re as “inbred” as you can get.

Parthenogenesis provides animals in extreme situations with a way to get around a lack of mates. When cells split to become eggs, there are leftover parts that contain packets of genetic material (DNA). These packets, called polar bodies, are normally reabsorbed by the mother’s body. However, when there are no sperm cells around to provide genetic material to fertilize eggs, the polar bodies can take on that role instead. The young produced through parthenogenesis aren’t exact copies of their mothers, though—the genes that make them up are all from one parent instead of two, but they’re still combined in a unique way. 

The discovery is the result of collaboration by scientists from The Field Museum, Stony Brook University, and the Florida Fish and Wildlife Conservation Commission. Researchers in Florida sampled DNA of a sawfish population—they use scissors to cut off a tiny piece of the fishes’ fins (don’t worry, the sawfish don’t get hurt—the skin samples are one-fourth the size of the eraser on the end of a pencil), which they then sent back to The Field Museum’s genetics lab for testing.

At The Field’s Pritzker Laboratory for Molecular Systematics and Evolution, scientists compared the DNA of the different sawfish, looking for similar patches of DNA code. The more sections of code that two specimens have in common, the more likely it is that they’re related. (The analysis used to compare sawfish DNA is actually a lot like the paternity tests made famous by daytime TV shows.) When the DNA was analyzed, the scientists found that some of the specimens’ genes were very similar—too close for the animals to even be siblings.  They could only be related through parthenogenesis.

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Field Museum scientist Kevin Feldheim noted that although the “virgin birth” angle of the discovery makes it interesting, it’s “ultimately a story of conservation.” The smalltooth sawfish is critically endangered, and all sawfish species are at risk of becoming the first entire family of marine animals to become extinct due to overfishing and habitat loss.

“It’s important to pay attention to because it’s not the normal mode of reproduction,” says Feldheim. “It could mean that these populations are in trouble—if females can’t find males, they might be resorting to this.” The scientists who worked on this study are encouraging other researchers to look for examples of parthenogenesis in the wild to help keep track of endangered species and work towards conservation.

You can see Kevin Feldheim and his colleagues at The Field Museum’s Daniel F. and Ada L. Rice DNA Discovery Center—they even have a live Q&A session every weekday from 11-12.


All photos ©Florida Fish and Wildlife Conservation Commission (FWC)