How the spider got his knees

Researchers have isolated the gene linked to knees in arachnids. 

Michael Reiche/dpa/AP/File
A spider sits on a net covered in dewdrops, near Doeringsdorf, central Germany, Friday, Oct. 3, 2014.

Spider torsos seem to hang off their eight legs, with their eight knees poking up above the rest of their bodies.

Arachnids have seven different leg segments, giving them a distinctive walk, but it’s those high knees that set them apart from their fellow arthropods, like lobsters, centipedes, and crayfish. 

Those all have multi-segmented legs, but none have the knee segment – the patella. 

How did spiders get this unique feature? Genetic duplication, say researchers. 

A gene involved in leg development, first identified in fruit flies, shows up twice in the spiders studied for a paper in Tuesday's issue of "Molecular Biology and Evolution." This second gene created the patella.

Courtesy of Nikola-Michael Prpic et al.
The drawings on the right show the leg segment composition in insects, isopod crustaceans, chilopod myriapods and arachnids (including spiders). The tree on the left depicts a simplified phylogenetic tree showing the interrelationships of these arthropod taxa (after Rota-Stabelli et al. 2011). All taxa possess a proximal short segment (coxa; shown in dark grey), and most taxa also have an additional short segment (trochanter; shown in light grey). But only arachnids have a third short leg segment (patella; shown in black) that is intercalated between two longer leg segments.

The authors outlined three possible scenarios that could result from gene duplication, if the second gene isn’t lost. In one, the double genes support functionality of the feature they produce. Another sees the second gene taking over an aspect of the original gene’s job, becoming more specialized. But it’s the third, called neofunctionalization, that produced spider knees.

In neofunctionalization, the duplicate gene finds a new function, which then becomes part of the essential functions of the organism.

"Species constantly adapt and evolve by inventing new body features," said study author Nikola-Michael Prpic in a news release. "Our work shows how a gene can be duplicated and then used during evolution to invent a new morphological feature."

The researchers used RNA interference experiments to test if this duplicated gene, dachshund, created the patella in arachnids. When the scientists deactivated dac2, the second dachshund gene, the patella and tibia fuse, forming a single leg segment. 

Courtesy of Nikola-Michael Prpic/Göttinger Zentrum für Molekulare Biowissenschaften/Universität Göttingen, Germany
On the left is a spider embryo expressing the "old" dac gene (dac1) and, on the right, another spider embryo expressing the "new" dac gene (dac2).

The researchers performed this test on the common house spider, Parasteatoda tepidariorum, and the cellar or skull spider, Pholcus phalangioides

The spiders are only distantly related, but both displayed the same results: the patella and tibia fused if dac2 was turned off. Thus, spiders evolved knees before the lineages split, say the authors.

Courtesy of Nikola-Michael Prpic et al.
Malformation of the patella after dac2 RNAi in P. tepidariorum. (A-D) Wildtype nymph. (A) Overview of the right half in ventral view. Boxed are those leg regions that are magnified in B-D. (B-D) Magnifications of the area between femur, patella, and tibia of the L1 leg (B), L2 leg (C) and L4 leg (D). (E-H) Nymph with malformations of the patella after dac2 RNAi. (E) Overview of the right half in ventral view. Boxed are those leg regions that 29 are magnified in F-H. (F-H) Magnifications of the area between femur, and the fused patella/tibia of the L1 leg (F), L2 leg (G) and L4 leg (H). (I) Summary of the dac2 RNAi experiment. The diagram in (a) shows the phenotype distribution after dac2 dsRNA injection. The diagram in (b) shows the phenotype distribution after GFP dsRNA injection as a control.
You've read  of  free articles. Subscribe to continue.

Dear Reader,

About a year ago, I happened upon this statement about the Monitor in the Harvard Business Review – under the charming heading of “do things that don’t interest you”:

“Many things that end up” being meaningful, writes social scientist Joseph Grenny, “have come from conference workshops, articles, or online videos that began as a chore and ended with an insight. My work in Kenya, for example, was heavily influenced by a Christian Science Monitor article I had forced myself to read 10 years earlier. Sometimes, we call things ‘boring’ simply because they lie outside the box we are currently in.”

If you were to come up with a punchline to a joke about the Monitor, that would probably be it. We’re seen as being global, fair, insightful, and perhaps a bit too earnest. We’re the bran muffin of journalism.

But you know what? We change lives. And I’m going to argue that we change lives precisely because we force open that too-small box that most human beings think they live in.

The Monitor is a peculiar little publication that’s hard for the world to figure out. We’re run by a church, but we’re not only for church members and we’re not about converting people. We’re known as being fair even as the world becomes as polarized as at any time since the newspaper’s founding in 1908.

We have a mission beyond circulation, we want to bridge divides. We’re about kicking down the door of thought everywhere and saying, “You are bigger and more capable than you realize. And we can prove it.”

If you’re looking for bran muffin journalism, you can subscribe to the Monitor for $15. You’ll get the Monitor Weekly magazine, the Monitor Daily email, and unlimited access to CSMonitor.com.