This may sting a little: Scorpions have about 10,000 more genes than humans do. The Chinese golden scorpion, Mesobuthus martensii, has at least 32,016 genes, Zhijian Cao of Wuhan University in China and colleagues report October 15 in Nature Communications. Humans have just over 22,000 genes.
The researchers found 116 genes that encode neurotoxins, including 45 previously unknown ones. Many of the neurotoxins paralyze proteins in cell membranes that open and close to generate electrical signals, which nerve cells use to communicate. Mutations in the scorpion’s own membrane protein genes make the arachnid immune to its own venom.
Scorpions also have 160 enzymes that help them digest fats and detoxify plant chemicals from the herbivorous insects they eat. Some of these enzymes transform a chemical called coumarin into fluorescent compounds that make the animals glow blue under UV light, the team found.
The animals also make a type of light-sensing protein called Mmopsin3 in their tails. The protein senses ultraviolet to blue light. At least 20 other proteins made in the scorpion’s tail help transmit the light signal from the skin to the brain, the researchers discovered.
As the first complete scorpion genome and the second in Chelicerata, M. martensii is found to have the most protein-coding genes among sequenced arthropods. The M. martensii genome expands the genetic repertoire of arthropods into a previously unknown territory, which will aid further studies on the comparative genomics and evolution of arthropods. Considered a special type of arthropods, extant scorpions have preserved the primary features of Paleozoic ancestors from the Cambrian age. However, the Mesobuthus lineage is found to have a gene family turnover at a level significantly greater than the insects, challenging the common wisdom that scorpions apparently evolved more conservatively as ‘living fossils’. The data reveal the decoupling of the molecular and morphological evolution in scorpions, a phenomenon documented for the first time in an arthropod.
Underlying the molecular evolution of the M. martensii genome are the expansion of the gene families enriched in the basic metabolic pathways, signalling and stress response pathways, neurotoxins, and cytochrome P450 families of enzymes, and the different dynamics of expansion between the shared and the scorpion lineage-specific gene families. Genomic and transcriptomic analyses further illustrate the genetic features in M. martensii associated with the prey, nocturnal behaviour, feeding and detoxification, which are believed to be important to its long-term adaptation. These include the diversification of neurotoxins and their receptor genes, the expression of light-signal transduction genes enabling photosensor in the tail and the expansion of P450 families involved in detoxification and hormone biosynthesis. Taken together, these analyses on the scorpion genome reveal a unique adaptation model distinctive to other sequenced arthropods. The genomics study on M. martensii yields new insights into the evolution of arthropods, and raises some new questions as well, for example, the cause of the accelerating expansion of the scorpion lineage-specific gene families, and the general roles of the non-visual photosensor in the evolution of arthropods. This work builds a foundation for future exploration of these intriguing creatures, and also provides a valuable resource for addressing those fundamentally important questions.
Accession codes: The genome assembly has been deposited in GenBank under BioProject PRJNA171479. The genome and related annotation files are accessible at http://lifecenter.sgst.cn/main/en/scorpion.jsp.