Scientists Baffled by Genetic Diversity in Weird Fish That Fertilizes Itself

Mangrove killifish of Florida is only one of two vertebrates that can ‘intimately befriend itself,’ so how come they’re not an army of clones?

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Mangrove killifish. Photo shows one individual of this species. It is about the size of a pinkie  finger, has a long slim silvery-brownish body, lateral finds quite near its gills and dorsal fin way in the rear, and a round, short tail.
Mangrove killifish, a master of navel-gazing.Credit: Cardet co6cs

Not only can the mangrove killifish fertilize itself – it prefers to. That should translate into a high degree of genetic uniformity in any given population of them. But scientists report the pinkie-sized fish are not the army of clones they had expected from a species that proliferates one-on-oneself, literally, and doesn’t even want to have sex. Now they’re trying to figure out how this can be.

In other beings, an egg with the mother’s genes gets fertilized by a sperm with the father’s genes. The offspring gets half its genes from mother and half from father.

If a given gene from both is the same, the offspring is called homozygous in that gene; if the gene from mother differs from the gene from father, the offspring is called heterozygous in that gene.

Mangrove killifish (Kryptolebias marmoratus), and the other self-procreator that “intimately befriends itself,” as the scientists politely put it – are hermaphrodites. The egg with the mother’s genes gets fertilized with, well, the sperm with the mother’s genes. Mother’s egg meets with mother’s sperm – and the rest is navel-gazing history.

Which begs the question of how this fish, which is anomalous in a bunch of other ways too, achieved the kind of genetic diversity theoretically reserved for species that routinely have sex (with others, that is). The scientists present their postulations in Genome.

Sexy stranger in town

As weird animals go, the mangrove killifish is in a class of its own.

Most water dwellers can only survive in a narrow range of saltiness. Freshwater fish die in the sea; sea-dwelling fish die in freshwater. Not this screwball. The mangrove killifish thrives in any salt concentration from freshwater to twice as salty as the sea.

Where most fish gasp, flop around and die on land, mangrove killifish can live for up to two months out of water, breathing through their skin like amphibians. Why they do that is another matter: the fact is, they can.

Now, “selfing” – procreating with oneself – has its upsides, explains Luana Lins, a postdoc at Washington State University: Other circumstances being apt, you can start a family on the spot.

But the whole point of sex, porn producers aside, is to increase genetic diversity, which theoretically increases the survivability of a species if the environment becomes inclement.

Mangrove killifish in actionCredit: YouTube

Say you’re a fish and the water grows (irreversibly) warmer. If you’re all genetically the same in temperature tolerance, at some stage of this increase you will all die. If there is genetic diversity, one can at least hope some of the fish will be more temperature tolerant, and that the species will survive.

This killifish is some survivor, to put it mildly. So on the one hand Lins and the team who sequenced the fish’s DNA could expect diversity. But they “self” themselves, so they could also expect them to be the same.

Yet Lins and colleagues at WSU, Stanford University and the University of Alabama found “a remarkable amount” of genetic diversity, after comparing 15 different killifish lineages.

A more florid cousin, the Blue Lyretail killifishCredit: Thinkstock

“We found way more areas that were heterogeneous, heterozygous, than we expected if you just followed the logic of something that has been mating with itself for a long time,” said Lins. “That opens up a can of worms. How is that happening?”

We don’t know. Stay tuned. Maybe they mutate a lot. Or maybe the killifish notice when there’s a stranger in town and it makes them hot.

As the team observed, when the hermaphroditic killlifish are exposed to killifish of a different lineage – whether a rare male, or unrelated hermaphrodites – they are likelier to lay more eggs than when they only see their brother/sisters of the same lineage.

So, a stranger in town makes them lay more. There’s fish food for thought.

Also, most killifish populations have a small percentage of males that will manage to fertilize somebody.

But, how the devil does a killifish notice a stranger in town with exotic DNA that could enrich its gene pool? “We don’t know,” said Lins. “There are a lot of unknowns, and I think that’s the fun of science. We’re all trying to figure out what is going on with these fishes.”