On the left is a spider embryo expressing the original Dachshund gene; on the right, another spider embryo expresses the duplicated gene.

On the left is a spider embryo expressing the original Dachshund gene; on the right, another spider embryo expresses the duplicated gene.

Courtesy of: Nikola-Michael Prpic Göttinger Zentrum für Molekulare Biowissenschaften, Universität Göttingen, Germany

How spiders got their knees

A gene named after the stumpy legs of wiener dogs may give spiders and scorpions their knees. That’s the surprising result of a new study, which finds that an ancient duplication of the so-called Dachshund gene has provided arachnids the ability to swiftly scuttle across silken webs, piles of leaf litter, and even the kitchen floor. The gene, also present in fruit flies and mice, could provide insights into how DNA duplication gives rise to new body shapes.

“[This] takes us from just understanding how legs develop in arachnids to actually beginning to tease out how different appendages evolved,” says Alistair McGregor, an evolutionary developmental biologist at Oxford Brookes University in the United Kingdom who wasn’t involved in the study.

Until now, virtually nothing has been known about how arachnids got their knees. No other arthropods, which include insects and crustaceans, have kneecaps, which can aid in mobility.

Even in the current study, the researchers didn’t start out looking for the origin of spider knees. Instead, they wanted to know why some spider species had longer legs than others. Searching for an answer, evolutionary developmental biologist Natascha Turetzek and her colleagues at Georg-August-University Göttingen in Germany looked to the Dachshund gene, which plays a major role in arthropod limb development. Fruit flies missing the gene, for example, lack limb segments and have short legs.

They hit a dead end, however, when they discovered there was no difference in the gene’s expression between the two spider species. But they noticed something special in each spider’s genome: a copy of the Dachshund gene never before described in arachnids. Gene duplicates—the accidental product of DNA copying gone awry—are generally free to take on new roles and fuel the rapid evolution of novel body structures or functions.

Intrigued, the researchers conducted gene expression analyses in spider embryos, a technique that allowed them to see where and when the copied gene was activated. The duplicated gene was turned on in the patella region, a different part of the leg than was activated by the original Dachshund gene during embryonic development. To figure out what the copy was doing, the scientists used another technique called parental RNA interference to deactivate the duplicated gene in some of the spider’s offspring.

The result: baby spiders whose kneecaps had fused to the tibia, creating a single leg segment. The findings show that, over time, gene duplication gave rise to kneecaps in arachnids, granting them a new type of mobility, the researchers report online today in Molecular Biology and Evolution.

The research is a “beautiful illustration” of how gene duplication can lead to novel shapes and forms—a fundamental mechanism in evolution critical to explaining how the diversity of animal shapes came to be, McGregor says. “It’ll be interesting to move on from here and try to decipher the precise changes in the regulation of the gene that underlie this evolutionary innovation.”

Still, there are other arachnids like mites that have kneecaps but no duplicate copy of the Dachshund gene, says Prashant Sharma, an evolutionary biologist at the University of Wisconsin, Madison, who was not involved in the work. “Reconciling how that occurs is something the study needs to grapple with before it can claim that one particular gene copy explains how all arachnids have patellas.”

Whether the finding applies to other arachnids, the structure has very real implications for Turetzek’s spider subjects.  She speculates that the patellae may offer them greater degree of flexibility, creating more nimble creatures. In the study, the nymphs without kneecaps had a hard time walking properly, leaving them at the mercy of cannibalistic siblings who “really liked to eat them,” she says. What’s more, none of the malformed spiders survived past a second molt; their gangly legs trapped them in the exoskeletons. “It was a pity we couldn’t observe them as adults, too.”

*Correction, 7 October, 9:42 a.m.: This article has been corrected to clarify that mites do have the Dachshund gene but lack the duplicated copy.