Meeting Abstract
The phylogenetic position of Scorpiones and the attendant evolutionary scenario of arachnid terrestrialization remain an unresolved puzzle for arthropod biologists. Datasets of morphologists and paleontologists typically recover scorpions at or close to the base of the arachnid tree of life, whereas molecular sequence data have recovered support for a clade comprised of scorpions and tetrapulmonates (groups united by a respiratory organ called the book lung). To adjudicate between these competing hypotheses using an independent data class, we conducted bioinformatic surveys of arachnid genomic resources and identified genes duplicated in both spiders and tetrapulmonates, to the exclusion of arachnids like harvestmen and mites. Focusing on a subset of genes that pattern appendages, we surveyed expression patterns of each paralog in developing embryos of the Arizona bark scorpion Centruroides sculpturatus, and compared these to spider and harvestman homologs. Here we show that multiple pairs of paralogs share distinct expression patterns in spiders and scorpions, whereas the harvestman and mite single-copy orthologs reflect the ancestral pattern observed in mandibulate and/or onychophoran counterparts. Together with analyses of gene tree topologies and Bayesian branch length ratios of the paralogs, our results show that independent sub-/neofunctionalization events unite scorpions and tetrapulmonates to the exclusion of the apulmonate arachnids.
