How the spider got his knees

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.