When did scorpions develop venoms dangerous to humans and other mammals?

 

Scorpions are an old animal groups that dates back to the Siluarian period appr. 400 million years ago. In the family Buthidae we find the species that have the most potent venoms when it comes to humans and other mammals. But how and when did the evolution create the venom composition that turned out to make many scorpions effective against mammalian predators?

Carlos Santibáñez-López and a group of international cooperators have recently published an impressive study where they assembled a phylogenomic data set of 100 scorpion venom gland
transcriptomes and genomes, emphasizing the sampling of highly toxic buthid genera.

The analysis show that toxins effective against mammals developed independently several times within the family Buthidae and that the evolutionary steps for this was quite recent in evolutionary times. And the latter is not suprising, as mammal predators were not present until late in the scorpion's evolutionary time scale.

This study and its phylogenomic results will be very useful for future studies of scorpions both in taxonomy, phylogeny and toxicology.

Abstract:
Scorpions constitute a charismatic lineage of arthropods and comprise more than 2500 described species. Found throughout various tropical and temperate habitats, these predatory arachnids have a long evolutionary history, with a fossil record that began in the Silurian. While all scorpions are venomous, the asymmetrically diverse family Buthidae harbors nearly half the diversity of extant scorpions, and all but one of the 58 species that are medically significant to humans. However, the lack of a densely sampled scorpion phylogeny has hindered broader inferences of the diversification dynamics of scorpion toxins. To redress this gap, we assembled a phylogenomic data set of 100 scorpion venom gland transcriptomes and genomes, emphasizing the sampling of highly toxic buthid genera. To infer divergence times of venom gene families, we applied a phylogenomic node dating approach for the species tree in tandem with phylostratigraphic bracketing to estimate the minimum ages of mammal-specific toxins. Our analyses establish a robustly supported phylogeny of scorpions, particularly with regard to relationships between medically significant taxa. Analysis of venom gene families shows that mammal-active sodium channel toxins (NaTx) have independently evolved in five lineages within Buthidae. Temporal windows of mammal-targeting toxin origins are correlated with the basal diversification of major scorpion mammal predators such as shrews, bats, and rodents. These results suggest an evolutionary model of relatively recent diversification of buthid NaTx homologs in response to the diversification of scorpion predators.

Reference:
Santibáñez-López CE, Aharon S, Ballesteros JA, Gainett G, Baker CM, González-Santillán E, et al. Phylogenomics of Scorpions Reveal Contemporaneous Diversification of Scorpion Mammalian Predators and Mammal-Active Sodium Channel Toxins. Syst Biol. 2022. [Sunscription required for full text]

Thanks to Carlos for sending me their article and for Matt Simon for recommending it to me.