Developmental gene expression as a phylogenetic data class: support for the monophyly of Arachnopulmonata.

Despite application of genome-scale datasets, the phylogenetic placement of scorpions within arachnids remains contentious between two different phylogenetic data classes. Paleontologists continue to recover scorpions in a basally branching position, partly owing to their morphological similarity to extinct marine orders like Eurypterida (sea scorpions). Phylogenomic datasets consistently recover scorpions in a derived position, as the sister group of Tetrapulmonata (a clade of arachnids that includes spiders). To adjudicate between these hypotheses using a rare genomic change (RGC), we leveraged the recent discovery of ancient paralogy in spiders and scorpions to assess phylogenetic placement. We identified homologs of four transcription factors required for appendage patterning (dachshund, homothorax, extradenticle, and optomotor blind) in arthropods that are known to be duplicated in spiders. Using genomic resources for a spider, a scorpion, and a harvestman, we conducted gene tree analyses and assayed expression patterns of scorpion gene duplicates. Here we show that scorpions, like spiders, retain two copies of all four transcription factors, whereas arachnid orders like mites and harvestmen bear a single copy. A survey of embryonic expression patterns of the scorpion paralogs closely matches those of their spider counterparts, with one paralog consistently retaining the putatively ancestral pattern found in the harvestman, as well as the mite, and/or other outgroups. These data comprise a rare genomic change in chelicerate phylogeny supporting the inference of a distal placement of scorpions. Beyond demonstrating the diagnostic power of developmental genetic data as a phylogenetic data class, a derived placement of scorpions within the arachnids, together with an array of stem-group Paleozoic scorpions that occupied marine habitats, effectively rules out a scenario of a single colonization of terrestrial habitat within Chelicerata, even in tree topologies contrived to recover the monophyly of Arachnida.

Expression and function of spineless orthologs correlate with distal deutocerebral appendage morphology across Arthropoda.

The deutocerebral (second) head segment is putatively homologous across Arthropoda, in spite of remarkable disparity of form and function of deutocerebral appendages. In Mandibulata this segment bears a pair of sensory antennae, whereas in Chelicerata the same segment bears a pair of feeding appendages called chelicerae. Part of the evidence for the homology of deutocerebral appendages is the conserved function of homothorax (hth), which has been shown to specify antennal or cheliceral fate in the absence of Hox signaling, in both mandibulate and chelicerate exemplars. However, the genetic basis for the morphological disparity of antenna and chelicera is not understood. To test whether downstream targets of hth have diverged in a lineage-specific manner, we examined the evolution of the function and expression of spineless (ss), which in two holometabolous insects is known to act as a hth target and distal antennal determinant. Toward expanding phylogenetic representation of gene expression data, here we show that strong expression of ss is observed in developing antennae of a hemimetabolous insect, a centipede, and an amphipod crustacean. By contrast, ss orthologs are not expressed throughout the cheliceral limb buds of spiders or harvestmen during developmental stages when appendage fate is specified. RNA interference-mediated knockdown of ss in Oncopeltus fasciatus, which bears a simple plesiomorphic antenna, resulted in homeotic distal antenna-to-leg transformation, comparable to data from holometabolous insect counterparts. Knockdown of hth in Oncopeltus fasciatus abrogated ss expression, suggesting conservation of upstream regulation. These data suggest that ss may be a flagellar (distal antennal) determinant more broadly, and that this function was acquired at the base of Mandibulata.