Data from: Developmental system drift in the patterning of the arthropod tarsus

The current understanding of proximodistal axis patterning in arthropod
legs is grounded in insect models. The paradigm for appendage evolution in
this phylum is that the gene regulatory network responsible for leg
subdivision and patterning is broadly conserved. Recent surveys of these
genes have suggested that chelicerate exemplars exhibit divergent
appendage patterning dynamics, though functional data remain limited. One
salient mismatch in expression occurs in homologs of the homeobox gene
clawless. In insects, clawless is expressed in the distalmost leg
territory, specifying the claw-bearing pretarsus. In the harvestman
Phalangium opilio, clawless occupies a broad tarsal domain early in
development, localizing later to the metatarsus-tarsus boundary,
suggestive of a tarsal patterning function. Here, we tested the function
of harvestman clawless using RNAi. Unlike insects, we show that clawless
knockdown results in disrupted tarsal growth and patterning of its
proximal segmental boundary, with no effect on the claw. Truncation of the
tarsus is associated with defective tarsomere formation. We additionally
surveyed the expression of clawless homologs in exemplars of chelicerate
diversity, which suggests that the tarsal-patterning function for clawless
was likely present in the most recent common ancestor of Chelicerata.
These results join a suite of other comparative works suggesting that
panarthropod appendage patterning exhibits numerous cases of developmental
system drift.