Mutant Self-Cloning Crayfish

Fascinating Carl Zimmer piece on the marbled crayfish, a new species that is spreading rapidly across Europe. They were first noted by German aquarium enthusiasts, who call them marmorkrebs and marveled that a single female would produce hundreds of offspring without ever mating. After a major effort of gene sequencing, biologists have determined that the marbled crayfish

apparently evolved from a species known as the slough crayfish, Procambarus fallax, which lives only in the tributaries of the Satilla River in Florida and Georgia.

The scientists concluded that the new species got its start when two slough crayfish mated. One of them had a mutation in a sex cell — whether it was an egg or sperm, the scientists can’t tell.

Normal sex cells contain a single copy of each chromosome. But the mutant crayfish sex cell had two.

Somehow the two sex cells fused and produced a female crayfish embryo with three copies of each chromosome instead of the normal two. Somehow, too, the new crayfish didn’t suffer any deformities as a result of all that extra DNA.

It grew and thrived. But instead of reproducing sexually, the first marbled crayfish was able to induce her own eggs to start dividing into embryos. The offspring, all females, inherited identical copies of her three sets of chromosomes. They were clones.

Now that their chromosomes were mismatched with those of slough crayfish, they could no longer produce viable offspring. Male slough crayfish will readily mate with the marbled crayfish, but they never father any of the offspring.

In December, Dr. Lyko and his colleagues officially declared the marbled crayfish to be a species of its own, which they named Procambarus virginalis. The scientists can’t say for sure where the species began. There are no wild populations of marble crayfish in the United States, so it’s conceivable that the new species arose in a German aquarium.

If so, the wild populations arose from aquarium keepers dumping their extra crayfish into local ponds. We really need to stop doing that. Like, "I can't keep this python but I can't bear to kill it, so I think I'll just drop it in the Everglades and start an ecological catastrophe."

Since they only reproduce asexually, nobody knows how well the marbled crayfish will do in the wild. Sex has many uses, one of which is disease resistance; some pathogen might appear that will wipe out marbled crayfish before it spreads much farther. On the other hand it might take a thousand years for that to happen, or a hundred thousand. Evolution is patient, and in the short term cloning itself is working out very well for the marmorkrebs.