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

Chelicerates exhibit dynamic evolution of genome architecture, with multiple whole genome duplication events affecting groups like spiders, scorpions, and horseshoe crabs. Yet, genomes remain unavailable for several chelicerate orders, such as Opiliones (harvestmen), which has hindered comparative genomics and developmental genetics across arachnids. We assembled a draft genome of the daddy-long-legs Phalangium opilio, which revealed no signal of whole genome duplication. To test the hypothesis that single-copy Hox genes of the harvestman exhibit broader functions than subfunctionalized spider paralogs, we performed RNA interference against Deformed in P. opilio. Knockdown of Deformed incurred homeotic transformation of the two anterior pairs of walking legs into pedipalpal identity; by comparison, knockdown of the spatially restricted paralog Deformed-A in the spider affects only the first walking leg. To investigate the genetic basis for leg elongation and tarsomere patterning, we identified and interrogated the function of an Epidermal growth factor receptor (Egfr) homolog. Knockdown of Egfr incurred shortened appendages and the loss of distal leg structures. The overlapping phenotypic spectra of Egfr knockdown experiments in the harvestman and multiple insect models are striking because tarsomeres have evolved independently in these groups. Our results suggest a conserved role for Egfr in patterning distal leg structures across arthropods, as well as cooption of EGFR signaling in tarsomere patterning in both insects and arachnids. The establishment of genomic resources for P. opilio, together with functional investigations of appendage fate specification and distal patterning mechanisms, are key steps in understanding how daddy-long-legs make their long legs.