Moreover, aquaculturists had previously used sperm from paddlefish to trigger gynogenesis in other sturgeon species and vice versa, and none of those experiments had ever produced crossbreeds. The Hungarian team had every reason to be confident that their fish could not hybridize.
But when Káldy and Mozsár exposed the Russian sturgeon eggs to healthy paddlefish sperm as a control for their experiment, they were stunned to see that the vast majority of the eggs hatched into live hybrid offspring. “They called me and told me that something’s wrong, because all of the control is living,” Bercsényi recalled. “I said, ‘Jenő, you made a big mistake. Please repeat the experiment.’” And so Káldy did—but the result was the same.
“We never wanted to play around with hybridization,” said Mozsár. “It was just a negative control, which found, somehow, a way to live.”
At first, Káldy didn’t believe the fry were hybrids: Because they looked just like regular sturgeons as youngsters, they might have come from spontaneous gynogenesis or some other “more reasonable explanation,” he said. But a genomic analysis by Gyöngyvér Fazekas, a colleague at the Research Institute for Fisheries and Aquaculture, and Balázs Kovács, an aquacultural geneticist at Hungary’s Szent István University, confirmed that the team indeed had more than a hundred hybrids growing in their tanks.
As Bercsényi’s team described it in their recent paper in Genes, some of the hybrids have three copies of each chromosome, a half-genome from each parent. But other hybrids have five copies of each chromosome: They somehow received the equivalent of their sturgeon mom’s full genome plus a half-genome from their paddlefish dad. The bodies of the sturddlefish combine characteristics from both parents, but the ones with more sturgeon DNA look more like their mother—they have more of the distinctive sturgeon scales called scutes, for instance.
How can these seemingly impossible hybrids exist?
Chakrabarty’s hunch is that the answer lies in the relatively slow rate of evolution that occurs in this group of fish. The Polyodontidae (paddlefishes) and Acipenseridae (sturgeons) are the last living families in the order Acipenseriformes, and studies suggest that both have very slow mutation rates. Despite the eons of independent evolution separating them, maybe their genomes just haven’t diverged enough to prohibit hybridization. But that raises the question of why previous hybridization attempts between sturgeons and paddlefish failed.
Semmens leans toward a different hypothesis: that the successful hybridization has to do with the Russian sturgeon’s extra-large genome. Genomicists think that the ancestors of sturgeons were diploid until all their chromosomes doubled and the fish became tetraploid. But only some species—including Russian sturgeons—retained their extra chromosomes. As a result, it’s possible that the Russian sturgeon’s genome carries enough redundancies and variations on genes to help the hybrids survive mismatches in their parents’ DNA.