A female marbled crayfish.

About 30 years ago, a freshwater crayfish kept in an aquarium in Germany reproduced without a mate – perplexing both its owner and scientists. Since then, clones of the animal have been invading wild ecosystems throughout Europe and Africa.

The genome of the marbled crayfish (Procambarus virginalis) has now been sequenced by a team from the German Cancer Researcher Center to uncover the animal’s unique evolution and adaptation skills. The findings may also offer insight into the cloning process of cancerous tumors, the researchers say.

Freshwater crayfish belong to the order of decapod crustaceans, which also includes crabs, lobsters, prawns and shrimps. However, the only crustacean genomes available are for the water flea and sand flea, and there are no genome sequences for decapods, so there is still a lot to be learned about the various species of crustaceans.

The marbled crayfish (called Marmorkrebs in Germany) is believed to have originated when two slough crayfish, imported to Germany from Florida for the aquarium trade, mated. Somehow, female marbled crayfish acquired the ability to self-replicate – or produce cloned offspring without fertilization – a natural type of asexual reproduction known as parthenogenesis.

The species has quickly grown from an aquarium pet to an invasive pest, with reports of the animal documented in the wild in Germany, Italy, Sweden, Japan and Madagascar. In the past 10 years, the marbled crayfish population has expanded its area in Madagascar 100-fold, threatening local freshwater communities, including seven endemic crayfish species.

The crayfish have been able to quickly and easily adapt to a diverse range of freshwater environments, and will feast on anything from rotten leaves, to insects and snails, further enhancing their spread.

The German team of researchers, led by Frank Lyko, has now established the first complete genome of the marbled crayfish to better understand the animal’s invasive spread, and determine how its clonal genome could potentially be used as a model for tumor research.

Lyko and colleagues established the complete genome by sequencing DNA from 11 marbled crayfish – including those that originated from the initial German aquarium stock in 1995, as well as ones caught in the wild in Madagascar.

They found that the marbled crayfish has three sets of 92 chromosomes, unlike the typical two sets seen in most animals. This unique trio of chromosome sets may be the key to the animal’s adaptability in various ecosystems. Usually clones lack the genetic variations to thrive in new environments, but the marbled crayfish’s three sets of chromosomes may contain just enough variety to put it at more of an advantage.

The researchers also counted 3.5 billion base pairs in the crayfish genome, making it about seven percent larger than the human genome.

“We could detect only a few hundred variants in a genome that is larger than the human genome. That is an incredibly small number," said Lyko.
Although they are considered a “potent” pest, the cloning ability of the marbled crayfish is of particular interest to Lyko and colleagues at the German Cancer Researcher Center.

Lyko’s focus of study is in epigenetics – or the process of how genes are turned on and off without changing the underlying genetic code.

"Epigenetic variants are often influenced by genetic variants. In Marmorkrebs, however, epigenetic variation is independent, because there is virtually no genetic variation," Lyko explained. "Marmorkrebs is an animal that reproduces clonally and therefore represents a model of a central aspect in tumor development."

Tumors can also successfully adapt to their environments by developing resistance against anticancer drugs and therapies. Previous research has shown that epigenetic mechanisms can play a strong role in these processes, and can influence cancer risk and the course of the disease.

“The analysis of mutations in marbled crayfish populations provides an opportunity to detect the generation, fixation and elimination of genetic changes with particularly high sensitivity and robustness and could therefore disentangle the specific contributions of individual factors. As such, it will be interesting to further explore marbled crayfish as a model system for clonal genome evolution in cancer,” wrote the study authors.

The study was published this week in Nature Ecology & Evolution.