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 homologues 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 RNA interference. 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 homologues 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.