Thursday, February 25, 2016

Spider evolution

Spiders are an ancient and diverse group comprising over 45,000 described species with probably three times as many awaiting discovery. In addition to their remarkable diversity, ecology, and abundance, spiders are known for some extraordinary compounds, such as venoms and silks. Although few spider venoms are dangerous to humans, they potentially hold some medical promise as insecticides and therapeutics. And, no other animal can claim a more varied and elegant use of silk a super strong material we are started to explore more in order to use it to create biometic material such as artificial nerve constructs, implant coatings, and drug delivery systems.

Despite all that, our understanding of spider evolution was rather poor. The orb web, the spiral wheel shaped web made by many spider species, was once considered "the crowning achievement of aerial spiders" and consequently researchers believed it to have evolved independently at least twice across the group's evolutionary history. However, studies in the 1980s suggested the opposite and many colleagues concluded that the orb web and the taxa that spin it all shared a common ancestor. 

Well, this thought will likely change once again. Colleagues in the US conducted a massive phylogenetic study of spiders using next generation sequencing technologies. Perhaps I should start calling them HTS - High Throughput Sequencing which seems to become the new standard. The researchers generated a genomic data set composed of nearly 3,400 genes for 70 spider taxa that represented a wide breadth of the order's diversity. The resulting phylogenetic tree confirms earlier suspicions that all orb weaving spiders are, in fact, not closely related to one another. Although these data could be interpreted to mean that the orb web evolved more than once the team finds this hypothesis highly unlikely. 

Our results are relatively clear that while orb weaving taxa are not as closely related as previously thought, our data do support the notion that the orb web itself has evolved only once. What we are calling the "ancient origin hypothesis" for the web, simply means that the largest branch on the spider tree of life had an ancestor that constructed an orb web but most of its descendants abandoned it for other more seemingly more lucrative prey hunting strategies.

Further diversification analyses actually showed that the mostly ground-dwelling, web-less spiders diversified faster than orb weavers. Molecular dating estimates show a major increase in biomass of non-flying insects during the Cretaceous Terrestrial Revolution 125–90 million years ago favoring diversification of spiders that feed on running rather than flying prey. 

A very interesting read and as it has been noted by a couple of colleagues via Twitter that the new PeerJ graphical abstracts (image above) are pretty cool, too.

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