Harvestman genome helps explain how arachnids got grasping legs


A female harvestman (Mitopus morio) in a meadow, in the UK

Nick Upton / naturepl.com

Some spider-like animals grow long legs that wrap and grasp like a monkey’s tail – and a genetic study has helped establish how they develop.

Harvestmen are arachnids, but they aren’t spiders: they instead belong to a closely related group called the opiliones. They have 8 extraordinarily long legs that can measure up to 28 times their body length, and they can bend the tips of them to wrap around and grasp objects.

However, harvestmen – like spiders, ticks and scorpions – actually have 12 limb-like appendages in total. The four at the head end develop into short jaws or pincers, or short limbs called pedipalps, which are unique to arachnids and can often detect tastes.


Fascinated by the way these appendages develop differently, Guilherme Gainett at the University of Wisconsin-Madison and his colleagues teamed up with genome specialists at the Smithsonian Institution to draft a sequence of the genome of a lab-raised harvestman (Phalangium opilio).

Then they identified three genes they thought might affect how the animals’ legs develop, and they engineered dozens of harvestmen embryos with different combinations of modified ways of expressing these genes.

Some of the harvestmen developed deformed legs that more closely resembled the first 4 appendages, says Gainett. And when the team interfered with specific genetic pathways, the legs lacked the kind of segmentation – similar to joints in vertebrate– that normally allows harvestmen to curl their legs around objects.

“We’ve shown … how the combinations of these genes create a blueprint in the embryo to differentiate between what’s going to be a leg that is used for walking, and what is going to be a pedipalp, which can be used to manipulate food and assess the surroundings,” he says.

Unlike most other arachnids, harvestmen have changed little during their evolution and their genome architecture may resemble relatively closely that of the oldest arachnids that lived more than 400 million years ago. That makes them ideal models for studying arachnid genetics, says research team member Vanessa González at the Smithsonian Institution.

Journal reference: Proceedings of the Royal Society B, DOI: 10.1098/rspb.2021.1168

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