Dual functions of labial resolve the Hox logic of chelicerate head segments

Despite an abundance of gene expression surveys, comparatively little is known about Hox gene function in Chelicerata, with emphasis on the Hox logic of the anterior prosomal segments, which bear the mouthparts. Previous investigations of individual paralogs of labial (lab) and Deformed (Dfd) in the spider Parasteatoda tepidariorum have shown that these play a role in tissue maintenance of the pedipalpal segment (labial-1) and in patterning the first walking leg identity (Deformed-1), respectively. However, broader extrapolations of these data points across chelicerates are hindered by the existence of duplicated copies of Hox genes in arachnopulmonates (e.g., spiders and scorpions), which have resulted from an ancient whole genome duplication event. Here, we investigated the function of single-copy orthologs of lab in the harvestman Phalangium opilio, an exemplar of a lineage that was not subject of this whole genome duplication. Embryonic RNAi against lab resulted in homeotic transformations of pedipalps to chelicerae, as well as reduction and fusion of the pedipalpal segment with adjacent segments. To test for combinatorial function, we performed double knockdown of lab and Dfd, which results in homeotic transformation of both pedipalps and first walking legs into cheliceral identity, whereas the second walking leg is transformed into a pedipalpal identity. Taken together, these results elucidate a model for the Hox logic of head segments in Chelicerata. To substantiate the validity of this model, we additionally performed expression surveys for duplicated copies of lab and Dfd in scorpions and horseshoe crabs, toward understanding the genetic basis of a heteronomous prosoma. We show that repetition of morphologically similar appendages is correlated with uniform expression levels of the Hox genes lab and Dfd, irrespective of the number of gene copies.