Dorsal-ventral inversion in the air-filled organ (lungs, gas bladder) of vertebrates: RNA-sequencing of laser capture microdissected embryonic tissue
How modification of gene expression generates novel traits is key to understanding the evolutionary process.  Here we investigated the genetic basis for the origin of the piscine gas bladder from lungs of ancestral bony vertebrates.  Distinguishing these homologous organs is the direction of budding from the foregut during development; lungs bud ventrally and the gas bladder buds dorsally.   We investigated whether this morphological inversion is associated with the molecular inversion of conserved genes regulating lung and gas bladder development. Using laser-capture microdissection and RNA-seq, we assayed transcript abundance and compared expression patterns between dorsal and ventral foregut tissues at three developmental stages spanning gasbladder development.  Our focal taxon, bowfin (Amia calva), representing the sistergroup to teleosts, is an early diverging ray-finned fish with a gas bladder. We discovered a number of genes with unknown function during lung development that are differentially expressed during gas bladder development and annotated to functions relevant for organ budding. We also identified several known lung-regulatory genes that exhibit inverted dorsoventral expression during gasbladder development relative to lung development. In particular, we found Tbx5 is strongly expressed in the dorsal mesoderm surrounding the gas bladder during bowfin development, and several interacting genes are co-expressed dorsally with Tbx5. In contrast, in mouse and bichir (Polypterus senegalus), the only ray-finned fish that have lungs, Tbx5 is expressed in the ventral lung mesoderm during lung development. Our data demonstrating dorsoventral inversion of conserved genes suggest that these genes may have contributed to the evolutionary transition between ventral lungs and a dorsal gas bladder in ray-finned fishes. Overall design: mRNA profiles of dorsal and ventral regions of the foregut and gas bladder at three stages of bowfin, Amia calva, development