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Attack of the (crustacean) clones

Abbey Bigler, Staff Writer

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A completely new species of crayfish appeared in the German aquarium trade in 1995. The mysterious crustacean multiplied by producing clones of itself, was spread by the pet trade and eventually invaded Europe’s freshwater ecosystems before spreading to Africa. Now, researchers have sequenced and studied its genome and published their results in Nature Ecology & Evolution.

The marbled crayfish is the first decapod (10 legged) crustacean to have its full genome sequenced. Previously, the only crustacean genomes sequenced were those of the water flee and the sand flee. The researchers fully sequenced the genomes of 11 marbled crayfish from various locations and partially sequenced those of 24 from Madagascar. The results suggest the proposed origin story of the marbled crayfish is accurate.

Researchers believe the species came into being due to an evolutionary macromutation, a genetic accident, when two slough crayfish from different regions of the world mated. It is thought the slough crayfish were most likely imported to Germany for the aquarium trade and kept in the same tank. What is certain is that their offspring, the marbled crayfish, received three sets of chromosomes, rather than the standard two (one from each parent.)

Slough crayfish typically have two sets of 92 chromosomes for a total of 184. However, the marbled crayfish have 276 chromosomes, exactly triple the number in one set. Researchers believe this resulted from either the sperm or egg cell that combined to form the first marbled crayfish not dividing correctly and containing two sets of chromosomes instead of one. This theory is supported by genome sequencing, which found two of the chromosome sets in the marbled crayfish are similar to each other, while one is markedly different. This suggests two of the sets came from the same creature.

Due to its genetic anomaly, the marbled crayfish cannot reproduce sexually. Instead, the species consists entirely of female animals producing clones of themselves, all genetically identical to the original, by laying eggs that do not require fertilization.

Typically, clonal species are not hearty because they cannot adapt to changes in environmental conditions via genetic variation and selection. However, the marbled crayfish has successfully invaded a range of freshwater habitats of different temperatures, salinities and acidities. Researchers believe the crustacean’s extra set of chromosomes may play a large role in this because it adds variety to the genome. Previous research involving other organisms with an extra set of chromosomes supports this hypothesis.

However, the marbled crayfish’s success is a problem. It is threatening native species in the habitats it has invaded and has been banned from being sold, possessed or released by the European Union. It is especially a problem in Madagascar, where the population of marbled crayfish is estimated to be in the millions and threatens the seven freshwater crayfish species native to the island.

Researchers believe studying the marbled crayfish and its unusual success could improve our understanding of an even bigger clonal threat: cancer. Frank Lyko, a molecular geneticist at the German Cancer Research Center in Heidelberg and a co-author of the study, believes the sudden emergence of the clonal marbled crayfish from the sexually reproducing slough crayfish parallels a normal human cell becoming cancerous and uncontrollably producing clones of itself.

Jean-François Flot, an evolutionary genomicist at the Free University of Brussels, agrees with Lyko’s assessment. In commenting on the research, he stated, “In many ways, the invasive expansion of [the marbled crayfish] is analogous to a cancerous lineage spreading asexually at the expense of its host.” While this evolutionary anomaly is being combated as a serious pest in freshwater ecosystems, it can redeem itself somewhat by shedding light on a serious threat to human health.

Abbey Bigler is a fourth-year student majoring in English with minors in business and technical writing, communications studies and biology. ✉ [email protected].

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Attack of the (crustacean) clones