It’s very uncommon for vertebrates to reproduce without a mate. But it does happen. It is so rare, however, that it’s almost never observed in a laboratory.
Take a few scientists at the University of East Anglia and others at the University of Gothenburg. They were a few steps into another research project looking at the diversity of cichlid fish genetics when they got a surprising revelation: One female cichlid in their lab tanks was swimming around with fertilized eggs in her mouth.
The odd thing about this discovery was that there were only other females in the tank with her, meaning that no outside fertilizer had been there to help the fish reproduce. So, the researchers set out to investigate.
At first, there was a suspicion that the fish was cloning itself in a process called parthenogenesis. That involves an egg doubling, dividing and then combining its genetic material. But that route was soon ruled out as the scientists examined the fish’s sex organs.
The sex organs of the cichlid had both normal ovaries, but also spermatocytes, cells that make sperm. This meant the fish could make offspring without the help of a mate.
Because of these differences to other cichlids of its kind, the fish produced 14 broods of young while it was under observation. Out of those, 46 offspring were hatched and 17 fish survived in total. But none of the young fish shared their mother’s unusual reproductive ability, which scientists call “selfing” instead of cloning because it involves fertilization.
Being able to reproduce without mates seems like an advantage, perhaps in situations where mates are scarce. And there are other animals, like some lizards, that can reproduce on their own through parthenogenesis. But there are likely downsides to being able to self-fertilize, researchers say.
Specifically, they point to a phenomenon called inbreeding depression, wherein there is such minimal genetic diversity that birth defects can occur in the generations that follow. The development of two different genders, after all, came about very likely to ensure more diversity in genes, they say.
Researchers say that the risk of genetic defects may be why “selfing” is so exceptionally rare in nature. But they don’t rule out that it could have happened before in a lab environment — fish are rarely swimming around by themselves, they say, and scientists focusing on other questions could easily have missed them.
Full results of the work discovering the self-fertilizing cichlid fish are published in the journal Royal Society Open Science.