19 December, 2013

The evolution of scorpion venom

An artcile on the origin and diversification of scorpion venom has been recently published by Kartik Sunagar and co-workers.
All scorpions possess venom and and a few species have venom which can cause death or serious symptoms in humans. The venom of one species is a cocktail of different toxins and there is such a cocktail for each scorpion species. But how did these different venoms evolve?

Kartik Sunagar and co-workers have now published a study on scorpion venom evolution. Lacking biochemistry and molecular biology in my education this article is a little over my head, so I just refer to the abstract for the conclusions :)

The episodic nature of natural selection and the accumulation of extreme sequence divergence in venom-encoding genes over long periods of evolutionary time can obscure the signature of positive Darwinian selection. Recognition of the true biocomplexity is further hampered by the limited taxon selection, with easy to obtain or medically important species typically being the subject of intense venom research, relative to the actual taxonomical diversity in nature. This holds true for scorpions, which are one of the most ancient terrestrial venomous animal lineages. The family Buthidae that includes all the medically significant species has been intensely investigated around the globe, while almost completely ignoring the remaining non-buthid families. Australian scorpion lineages, for instance, have been completely neglected, with only a single scorpion species (Urodacus yaschenkoi) having its venom transcriptome sequenced. Hence, the lack of venom composition and toxin sequence information from an entire continent’s worth of scorpions has impeded our understanding of the molecular evolution of scorpion venom. The molecular origin, phylogenetic relationships and evolutionary histories of most scorpion toxin scaffolds remain enigmatic. In this study, we have sequenced venom gland transcriptomes of a wide taxonomical diversity of scorpions from Australia, including buthid and non-buthid representatives. Using state-of-art molecular evolutionary analyses, we show that a majority of CSα/β toxin scaffolds have experienced episodic influence of positive selection, while most non-CSα/β linear toxins evolve under the extreme influence of negative selection. For the first time, we have unraveled the molecular origin of the major scorpion toxin scaffolds, such as scorpion venom single von Willebrand factor C-domain peptides (SV-SVC), inhibitor cystine knot (ICK), disulphide-directed beta-hairpin (DDH), bradykinin potentiating peptides (BPP), linear non-disulphide bridged peptides and antimicrobial peptides (AMP). We have thus demonstrated that even neglected lineages of scorpions are a rich pool of novel biochemical components, which have evolved over millions of years to target specific ion channels in prey animals, and as a result, possess tremendous implications in therapeutics.

Sunagar K, Undheim E, Chan A, Koludarov I, Muñoz-Gómez S, Antunes A, et al. Evolution Stings: The Origin and Diversification of Scorpion Toxin Peptide Scaffolds. Toxins. 2013;5(12):2456-87. [Free full text]

Thanks to Bryan Fry for sending me this article!

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