Cloned crayfish conquers the world
A bizarre aquarium escapee has extra chromosomes, clones itself, and is spreading around the world. Stephen Fleischfresser reports.
It may sound far-fetched, a but a real-life super-macromutant has been born, creating a new species with bizarre biology in the process.
And it’s hell-bent on cloning itself in a bid for world domination.
A team of scientists led by epigeneticist Frank Lyko of the German Cancer Research Centre in Heidelberg, Germany, is on the case of the remarkable story of the small freshwater crustacean known as the “marbled crayfish”.
The start of this tale can all be traced back to an insect trade fair in Frankfurt, Germany, in 1995, where an American trader gave a German aquarium enthusiast a bag of unidentified crayfish. These multiplied rapidly, and the aquarist decided to distribute for other hobbyists. Soon, they spread through the German pet trade.
Somewhere in this string of events, the mutant marbled crayfish came into existence, almost certainly to parents who were genetically related to each other, and belonging to the species Procambarus fallax, the American slough crayfish, a popular aquarium pet. Their mutant prodigy was very different from its parents, so much so that Lyko determined it to be a whole new species, which he named Procambarus virginalis.
Most sexually reproducing organisms have two copies of each chromosome – the strings of DNA that contain all an individual’s genetic information – one from each parent. P. virginalis, however, has three – one from each parent plus an extra duplicate copy of one of the parental chromosomes, a condition called “autopolyploidy”.
This triplicate structure gives the marbled crayfish some remarkable abilities. To begin with, it does not reproduce sexually. Instead, it has the capacity for parthenogenesis, which means that it can produce young from unfertilised eggs, something it can do at a much higher rate than other crayfish. Lyko’s most recent paper, published in Nature Ecology & Evolution, further reveals that the parthenogenesis is “apomictic”, meaning that the young are actually clones of the original individual mutant. All members of P. virginalis are genetically identical.
These changes have made P. virginalis reproductively incompatible with P. fallax, part of the reason that Lyko determined them to be separate species. Like everything about the marbled crayfish, this too is fascinatingly different.
The evolutionary formation of species, certainly according to Darwin and many others, normally proceeds gradually, with the accumulation of mutations gently pushing populations away from each other to become separate species. With P. virginalis, Lyko and his team argue that speciation has occurred in a single large and sharp jump, from P. fallax to P. virginalis in one generation, a process known as saltational speciation. The marbled crayfish then is a macromutation, or what geneticist Richard Goldschmidt famously referred to in 1940 as a ‘hopeful monster’.
If all this wasn’t strange enough, this self-cloning super macromutant escaped the aquarium and leapt into the wild. Now spread around the world, it is endlessly cloning itself and taking up residence in habitat after habitat. Interestingly, Madagascar seems particularly hospitable, and P. virginalis is now emerging as a determined invasive species in its waterways. Lyko and his team have shown that the Madagascan population of has increased 100-fold in the past 10 years.
So, what about the future of this hopeful monster?
Despite its relentless spread, conventional wisdom suggests that the species will suffer at some point due to its incredibly restricted genepool. Such genetic bottlenecks normally reduce the robustness of a species. P. virginalis will be particularly vulnerable to environmental change.
Lyko is philosophical: “The situation is what it is – obviously these animals are quite successful, even if they don’t have any genetic variation,” he says.
He add that the fact they carry three, rather than two, copies of each chromosome might protect them from the slings and arrows of environmental change.
The future of Lyko’s research is clearer. Given that all P. virginalis individuals are identical clones, any evolutionary adaptation can be observed very clearly and Lyko thinks this will take place epigenetically. Epigenetics is the study of the way in which various molecular mechanisms from the environment interact with and effect the way genes are expressed, and this is Lyko’s primary interest.
P. virginalis, it transpires, might well help us to more clearly understand the role epigenetics play in evolution.
“This is actually how I got drawn into the project,” he says. “I’m an epigeneticist and we believe that marbled crayfish represent the perfect model for investigating the role of epigenetics in phenotypic adaptation and variation.”
While there is still some controversy surrounding the idea that Procambarus virginalis is actually a new species (with some suggesting it is just a parthenogenetic lineage), the incredible story of the marbled crayfish is, no doubt, one to follow closely, as is the work of Lyko and his team.