Sequence-capture phylogenomics of true spiders reveals convergent evolution of respiratory systems

The common ancestor of spiders likely used silk to line burrows or make
simple webs, with specialized spinning organs and aerial webs originating
with the evolution of the megadiverse “true spiders” (Araneomorphae). The
base of the araneomorph tree also concentrates the greatest number of
changes in respiratory structures, a character system whose evolution is
still poorly understood, and that might be related to the evolution of
silk glands. Emphasizing a dense sampling of multiple araneomorph lineages
where tracheal systems likely originated, we gathered genomic-scale data
and reconstructed a phylogeny of true spiders. This robust phylogenomic
framework was used to conduct maximum likelihood and Bayesian character
evolution analyses for respiratory systems, silk glands, and aerial webs,
based on a combination of original and published data. Our results
indicate that in true spiders, posterior book lungs were transformed into
morphologically similar tracheal systems six times independently, after
the evolution of novel silk gland systems and the origin of aerial webs.
From these comparative data we put forth a novel hypothesis that
early-diverging web building spiders were faced with new energetic demands
for spinning, which prompted the evolution of similar tracheal systems via
convergence; we also propose tests of predictions derived from this
hypothesis.