Arthropods arose during the Cambrian period and radiated into millions of species over 500 million years. They represent the most diverse group of life forms. The evolution and adaptation of arthropods have been the subject of intense interest. Scorpions represent a special type of arthropods. They are considered “living fossils” as they maintain the primary features of Paleozoic scorpions, and are well adapted to have survived extreme climate changes. As a basal lineage of Arachnida (Arachnida being a major branch of Chelicerata), scorpions occupy a key phylogenetic position in the evolution of arthropods.
The genome sequence of the Asian scorpion, Mesobuthus martensii has 32,016 protein-coding genes, the most among sequenced arthropods. Although M. martensii appears to evolve more conservatively, surprisingly it has a greater gene family turn-over than the insects having undergone diverse morphological and physiological changes, suggesting decoupling of the molecular and morphological evolution in scorpions, a phenomenon documented for the first time in any arthropod. Underlying the long-term adaptation of scorpions, the expansions of the common and the scorpion lineage-specific gene families show distinct dynamics through evolution history, and are enriched in the primary metabolic pathways, signaling and stress response pathways, neurotoxins, and cytochrome P450. M. martensii represents a unique adaptation model distinctive to other arthropods.