My first publication, “The homology of lungs and gas bladders: insights from arterial vasculature” came out in print this month, which is very exciting. You can find it here with Journal of Morphology. Even cooler, is that we also got the cover!
In summary, my coauthors Amy McCune, Mark Riccio, and I used micro-CT scans of barium-injected fishes to redescribe the arterial vascularization associated with the gills and air-filled organs (lungs and gas-bladders) of early bony fish lineages: oddballs like sturgeon, paddlefish, lungfish, bowfin, gar and African bichirs. We then used ancestral state reconstructions to look at the evolutionary history of the arterial blood supply to air-filled organs. In particular we were interested in pulmonary arteries. These arteries bring deoxygenated blood to the lungs of every organism with functioning lungs, including the African bichirs, which are sister to all other ray-finned fishes. Interestingly, pulmonary arteries also supply blood to the gas bladder of the bowfin, Amia. This fish does not have a lung but uses its gas bladder to breathe air in low oxygen environments or to supplement stressful activity. The possession of pulmonary arteries in Amia has been seen as evidence that gas bladders and lungs are homologous–derived from the same structure in the common ancestor of bony fishes. However, the picture is complicated by the phylogenetic position of bowfin; they may have pulmonary arteries, but other early-branching ray-finned fishes like paddlefish and sturgeon were not thought to have pulmonary arteries.
By looking at micro-CT scans of the arterial systems of paddlefish and sturgeon, I came to an exciting realization; while their gas bladders were indeed supplied with blood in a manner similar to teleost fishes that also don’t use their gas bladder to breathe air, they did have a set of paired arteries originating from the fourth efferent branchial artery–where pulmonary arteries always originate!* We realized that these were probably vestigial pulmonary arteries which had been co-opted for new functions.
The discovery of vestigial pulmonary arteries in paddlefish and sturgeon lends support to the homology of pulmonary arteries in Amia with pulmonary arteries of organisms with lungs. A homologous blood supply for the gas bladder in Amia and organisms with lungs also lends further support to the homology of lungs and gas bladders as air-filled organs.
One of the interesting things about our ancestral state reconstructions was that they highlighted the fact that the blood supplies to the air-filled organ become diverse with the dorsal shift of the air-filled organ, what we call a gas bladder. It may have been this dorsal shift, as well as the loss of a respiratory function, that resulted in the loss or co-option of pulmonary arteries in taxa other than bowfin. Indeed, a literature search of tetrapods that have lost or reduced lungs showed that pulmonary arteries are lost, reduced, or co-copted in organisms as diverse as aquatic snakes, plethodontid salamanders, and caecillians.
*Most adult tetrapods obviously don’t have gills, but developmental studies have shown that pulmonary arteries always originate from the sixth aortic arch (homologous to the fourth efferent branchial artery of fishes).