Meeting Abstract
Studying the gene networks that dictate morphogenesis during development can provide important insight into evolutionary origins of morphological novelties. The gas bladder, an air-filled sac that affords buoyancy control, is a major morphological innovation within the vertebrate clade. It is unique to ray-finned fishes; all other bony vertebrates have ventral lungs. These structures are homologous, but the key difference to understanding the evolutionary transition from lungs to the gas bladder is that lungs bud ventrally from the foregut endoderm during development while the gas bladder buds dorsally. We investigated the genetic basis of this dorsoventral inversion by examining the expression patterns of Bmp4, ventrally expressed during lung budding in mouse, and Bmp16, a paralog expressed during gas bladder budding in zebrafish, in three species: bichir, which retains ventral lungs; bowfin, which diverged soon after gas bladder evolution; and zebrafish, which diverged more recently. Bmp4 is expressed in the lung buds in bichir, as it is in mouse, but is not expressed in the gas bladder in bowfin or zebrafish. Bmp16 is expressed in the gas bladder bud and associated foregut endoderm in bowfin. Bmp16 was lost in numerous bony vertebrate lineages, including mammals, but it is not known whether it is retained in bichir. If Bmp16 is retained in the genome and expressed during lung budding in bichir, it is possible that Bmps 4 and 16 both functioned in lung development in the bony vertebrate ancestor, Bmp4 taking over the functionality of Bmp16 in lineages in which the latter was lost. In contrast, if Bmp16 is retained in bichir but not expressed during lung development, it is more likely that Bmp16 only became involved in air-filled organ development when the gas bladder evolved.
