The genome of a daddy-long-legs (Opiliones) illuminates the evolution of arachnid appendages.

Chelicerate arthropods exhibit dynamic genome evolution, with ancient whole-genome duplication (WGD) events affecting several orders. Yet, genomes remain unavailable for a number of poorly studied orders, such as Opiliones (daddy-long-legs), which has hindered comparative study. We assembled the first harvestman draft genome for the species , which bears elongate, prehensile appendages, made possible by numerous distal articles called tarsomeres. Here, we show that the genome of exhibits a single Hox cluster and no evidence of WGD. To investigate the developmental genetic basis for the quintessential trait of this group-the elongate legs-we interrogated the function of the Hox genes () and (), and a homologue of (). Knockdown of incurred homeotic transformation of two pairs of legs into pedipalps, with dramatic shortening of leg segments in the longest leg pair, whereas homeosis in L3 is only achieved upon double knockdown. Knockdown of incurred shortened appendages and the loss of tarsomeres. The similarity of loss-of-function phenotypic spectra in insects and this arachnid suggest that repeated cooption of EGFR signalling underlies the independent gains of supernumerary tarsomeres across the arthropod tree of life.