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This mutant crayfish clones itself, and is taking over Europe

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By Carl Zimmer February 6 2018  read full NYTimes article here

….Over the past five years, Dr. Lyko and his colleagues have sequenced the genomes of marbled crayfish. In a study published on Monday, the researchers demonstrate that the marble crayfish, while common, is one of the most remarkable species known to science. Before about 25 years ago, the species simply did not exist. A single drastic mutation in a single crayfish produced the marbled crayfish in an instant.

The mutation made it possible for the creature to clone itself, and now it has spread across much of Europe and gained a toehold on other continents. In Madagascar, where it arrived about 2007, it now numbers in the millions and threatens native crayfish.

….In 2003, scientists confirmed that the marbled crayfish were indeed making clones of themselves. They sequenced small bits of DNA from the animals, which bore a striking similarity to a group of crayfish species called Procambarus, native to North America and Central America…

…It 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….

….Thanks to the young age of the species, marbled crayfish could shed light on one of the big mysteries about the animal kingdom: why so many animals have sex.  Only about 1 in 10,000 species comprise cloning females. Many studies suggest that sex-free species are rare because they don’t last long.

In one such study, Abraham E. Tucker of Southern Arkansas University and his colleagues studied 11 asexual species of water fleas, a tiny kind of invertebrate. Their DNA indicates that the species only evolved about 1,250 years ago.

There are a lot of clear advantages to being a clone. Marbled crayfish produce nothing but fertile offspring, allowing their populations to explode. “Asexuality is a fantastic short-term strategy,” said Dr. Tucker.

In the long term, however, there are benefits to sex. Sexually reproducing animals may be better at fighting of diseases, for example.

If a pathogen evolves a way to attack one clone, its strategy will succeed on every clone. Sexually reproducing species mix their genes together into new combinations, increasing their odds of developing a defense.

The marbled crayfish offers scientists a chance to watch this drama play out practically from the beginning. In its first couple decades, it’s doing extremely well. But sooner or later, the marbled crayfish’s fortunes may well turn.

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