Scientists shorten daddy long-legs' iconic limbs to figure out how they got so gangly

Through a process known as RNA interference (RNAi), scientists have been able to modify the genetic make-up of the daddy long-legs arachnid so that its distinctive spindly limbs become twice as short.

This process – which uses a gene's own DNA sequence and small fragments of RNA to turn the gene off – was applied to the Phalangium opilio species, one of the most common species of daddy long-legs in the world.

The result is effectively a daddy short-legs instead of a daddy long-legs. The team behind the work is hoping that the experiments can teach us more about how these elongated limbs evolved in the first place.

"We anticipate that the genome of P. opilio will facilitate the development of more sophisticated tools for functional genetics, toward refining the understanding of how daddy long-legs make their long legs," write the researchers in their published paper.

After sequencing the genome of P. opilio, researchers identified three genes that act as a map for various body parts. Two of those genes were then found to be turned on in the legs of the arachnid embryos.

Turning off or silencing the genes in other embryos produced daddy long-legs that were different: two or more of their legs were shorter than normal and had transformed into pedipalps, which are limbs used specifically for handling food.

The team then turned off the third gene thought to be linked to building legs in embryos. The legs didn't turn into pedipalps, but they did get shorter and lose their tarsomeres, the knuckle-like joints used for grip. Similar experiments have been run on fruit flies.

"Looking forward, we are interested in understanding how genes give rise to novel features of arachnids, such as spider fangs and scorpion pinchers, and also leveraging the genome to develop the first transgenic harvestmen," geneticist Guilherme Gainett, from the University of Wisconsin-Madison, told CNET.

Technically, the P. opilio species isn't a spider but rather a close relative, which makes these creatures useful for figuring out how the multitude of different arachnids on our planet evolved. Not all daddy long-legs have legs that can wrap around twigs and other objects like these bugs can, for example.

The thinking is that the spider genome duplicated in the distant past, giving these creatures a wide choice of genes to use for their own unique evolutionary journeys – there could be a link between more complex genomes and greater organism variation.

This latest research should prove useful in future work to trace back the development of P. opilio and other arachnids, to establish if long and spindly legs were indeed something that evolved separately in each group of arthropods.

"The effectiveness of single and double RNAi in this system makes P. opilio an opportune point of comparison for future investigations of arachnid body plan evolution," write the researchers in their published paper.

The research is published in Proceedings of the Royal Society B: Biological Sciences.

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