Meeting Abstract
The internal phylogeny of Chelicerata and the attendant evolutionary scenario of arachnid terrestrialization have a long and contentious history. Datasets of morphologists and paleontologists typically recover scorpions close to the base of the arachnid tree of life, whereas recent phylogenomic analyses have recovered support for a clade comprised of scorpions and tetrapulmonates (i.e., Arachnopulmonata), implying a single origin of the arachnid book lung. To adjudicate between these competing hypotheses with an independent data class, I examined the composition of euchelicerate genomes, using mandibulate genomes for comparison. Here I show that a partial or whole genome duplication event is shared by arachnopulmonates, to the exclusion of apulmonate arachnids. These data imply a single, derived origin of the arachnid book lung at the base of Arachnopulmonata. To investigate the developmental origins of book lungs, I compare the developmental genetic basis for respiratory system development in insects and arachnids. I examined the development of respiratory primordia in a scorpion, a harvestman, and a spider. Here I show that the respiratory primordia of arachnids are not positionally homologous to those of insects, as they occur within the posterior region of the embryonic parasegment; in D. melanogaster, tracheal placodes occur anterior of this region. I further demonstrate that candidate genes critical to tracheal fate specification in D. melanogaster are expressed very differently in all three arachnid exemplars. Taken together, these data suggest that mechanisms of respiratory system development are not homologous in insects and arachnids, and that different terrestrial lineages have solved the challenge of aerial respiration using different developmental mechanisms.
