Polyploidy - the duplication of chromosome sets - is relatively common in plants, and can result in the development of new species. (Many modern food crops are polyploids.) It's much less common in animals, although found in some frogs & salamanders (amphibians) & leeches (annelids).
So it was with a mix of excitement, surprise, & alarm that I read about a triploid crayfish species: excitement, because I hadn't heard about a polyploid crustacean; surprise, because it's a triploid organism; & alarm, because it's an invasive pest across its range.
Image: Wikimedia commons; photo by Zfaulkes
Procambarus virginalis, the marbled crayfish, was first found in Germany in the mid-1990s but is now widespread in Europe & Africa, including Madagascar. In a paper published this month, Frank Lyko & his colleagues reported on their study of the species' genome (Gutekunst, Andriantsoa, Falckenhayn, Hanna et al., 2017). They found that it has 3 copies of each of its 92 chromosomes (276 chromosomes in total), and that all the chromosomes come from the slough crayfish (Procambarus fallax), but from two individuals that weren't closely related. The team suggested that the marbled crayfish originated from a mating between 2 slough crayfish, where one parent contributed a normal, haploid, gamete (one copy of each chromosome) & the other, a diploid gamete with 2 copies of each chromosome, produced by non-disjunction during meiosis. Their genomic analysis pointed to the aquarium trade in Germany as the source of the new species.
Now, triploid organisms are usually sterile, because they're not able to produce viable gametes via meiosis. (The same would be true of a pentaploid, with 5 copies of every chromosome.) Yet this crayfish has rapidly become an invasive species, & that means it makes lots of baby crayfish. How does it do this?
By parthenogenesis. That is, this is a clonal species. (The researchers describe the Madagascan population of P.virginalis as "genetically homogeneous and extremely similar to the oldest known stock of marbled crayfish founded in Germany in 1995.)
Every marbled crayfish is female, producing 'apomictic' eggs by mitosis. No sperm necessary. And because every individual is capable of producing eggs and - in this species, a lot of them - in ideal conditions the species' population can grow much faster than that of a sexually-reproducing species. This gives the marbled crayfish quite an advantage over other, competing, species when it's introduced into a new ecosystem, which is why it has been able to expand quickly across Europe & Africa - having likely arrived in these countries via the aquarium trade. And again, because they are parthenogenetic, you need just a single individual to begin a new invasive population. In Madagascar their spread was enhanced by human activity in terms of moving animals around to establish new food populations, the warmer temperatures (compared to those in Europe), and the ready availability of suitable freshwater habitats, and there's concern that endemic crayfish species, and their unique ecosystems, are threatened by the exotic invader.
But there's much more to this story than a tale of an unusual crayfish. I found it fascinating that that understanding how the marbled crayfish genome evolves over time may have applications to cancer research:
The generation of genetic diversity will be shaped by a complex set of factors, including the intrinsic mutability of the genome, environmental mutagens, genetic drift and selective pressure. All these factors are known to play an important role in the evolution of tumour genomes. 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.
J.Gutekunst, R.Andriantsoa, C.Falckenhayn, K.Hanna, W.Stein, J.Rasamy & F.Lyko (2017) Clonal genome evolution and rapid invasive spread of the marbled crayfish. Nature Ecology & Evolution doi:10.1038/s41559-018-0467-9
Interested readers will also enjoy this summary of the paper, with commentary from other scientists.