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 opilionid draft genome for the species Phalangium
opilio, which bears elongate, prehensile appendages, made possible by
numerous distal articles called tarsomeres. Here, we show that the genome
of P. opilio 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 Deformed (Dfd) and Sex combs reduced (Scr), and a homolog of
Epidermal growth factor receptor (Egfr). Knockdown of Dfd 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 Dfd+Scr knockdown. Knockdown of Egfr incurred
shortened appendages and the loss of tarsomeres. The similarity of Egfr
loss-of-function phenotypic spectra in insects and this arachnid suggest
that repeated cooption of EGFR signaling underlies the independent gains
of supernumerary tarsomeres across the arthropod tree of life.