Domesticated betta fish have evolved a sex gene not found in wild fish of their species.
In 1975, scientists tried spaying a few hundred female betta fish. We all know what happens to spayed cats and dogs: They become sterile. Betta fish are different. A third of the surviving bettas regenerated an ovary—which, okay, interesting enough. But the remaining two-thirds did something much, much stranger: They grew testes. They turned brighter and darker in color too—like male bettas. They grew elongated fins—like males. They even started making sperm—like males, obviously. When mated with other female betta fish, these females-turned-males produced offspring that looked perfectly healthy. The only notable oddity was that the resulting broods were usually, but not always, exclusively female.