The segmented heads of spiders and scorpions arise from the actions of a gene that in other arthropod species is responsible for creating legs.
That’s the somewhat surprising finding made by two scientists, Emily Setton and Prashant Sharma from the University of Wisconsin-Madison, US, during an investigation into the evolutionary origin of spider silk-spinning.
The discovery came about as a by-product of work designed to reveal whether spiders’ main silk-spinning organs, known as spinnerets, are, in effect, legs that have evolved a different function over millions of years.
To test the idea, Setton and Prashant focussed on a pair of genes known as Sp6-9 and Dll. In insects, these genes are known to effectively tell embryos where to sprout legs. It was assumed that they fulfil the same role in spiders and scorpions, so the scientists decided to see whether blocking them affected the development of spinnerets.
In an unexpected development, blocking the two genes instead resulted in head segments disappearing.
Double-checking, Setton and Prashant looked at spider genomes to search for another gene, known as Sp5, which is known to regulate head-segmentation in other insects, including fruit flies.
They found that the gene was missing. Sharma calls the absence “the plot twist that gives away the story”.
“It’s a very arachnid-specific loss,” adds Setton.
The discovery that arachnids have co-opted leg-governing genes to take over head-segmentation, ditching another gene in the process, is an example of what evolutionary biologists and geneticists call parsimony. There are many cases in which certain genes develop over time to control several features – and many cases of identical genes performing very different functions in only distantly related species.
“Evolution doesn’t want to reinvent the wheel,” says Setton.
“A big question of the lab is how is diversity built genetically, evolutionarily?” says Sharma.
“How are ancient lineages related, and what are the genetic mechanisms that underlie the differences between them?”
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