Hemidactylium scutatum larvae, a lungless salamander native to eastern North America. Credit: Zachary R. Lewismorpho
The lungs are vital for many vertebrates, including humans. However, four living amphibian clades no longer breathe via their lungs, instead breathing mostly through their wet skin. Little is known about the developmental causes of lung loss in these clades.
Researchers at Harvard University's Museum of Comparative Zoology and Department of Organismic and Evolutionary Biology studied the Plethodontidae, a dominant species of salamanders that are all lungless as adults, and discovered that they do develop lungs as embryos, providing insight into the evolution of lung loss over millions of years.
Plethodontidae are the most species-rich salamander family, accounting for over two-thirds of all current salamander diversity. All adult plethodontids lack lungs and breathe solely through nonpulmonary tissues, mostly the skin and mucus membranes of the mouth and throat, and there are more instances of lung reduction or loss in both amphibians and vertebrates.
According to lead author Zachary R. Lewis, who is a former doctoral candidate (Ph.D.’16), "perhaps losing lungs has facilitated, rather than hindered, this remarkable evolutionary achievement."
Lewis and Hanken, who directed the study at Gettysburg College, examined the morphology of lung development in both lung and lungless salamanders using histology and micro-CT. The authors then used in situ hybridization and RNA sequencing to demonstrate that the structure that arises during lungless salamander embryonic development is similar to a lung, both morphologically and in terms of the molecules expressed.
The authors argue that the development of the lung in these animals is halted due to a lack of cues that maintain lung development, which arise from the tissue, the mesenchyme, that surrounds the developing lung.
Lewis said the study of a salamander with lungs into a lungless salamander embryo resulted in the formation of structures that resemble lungs, providing some evidence that lungless salamanders are still capable of developing lungs.
Amy Grace Mekeel's 1936 doctoral dissertation challenged biologists' theories that the minor fold in the adult pharynx is a vestigial lung, consistent with the initial lung loss of the plethodontids. Mekeel described a "lung rudiment" that formed in the embryo but was dissected by the time it was born.
According to Kerney, "the lung precursor appears and disappears before the lungless salamander embryos hatch, just as Mekeel described." "This work supports Mekeel's earlier thesis and puts the initial adult vestige hypothesis to rest."
Despite the absence of functional adult lungs for at least 25 and possibly exceeding 60 million years, the study finds that lung development-genetic pathways are at least partially conserved. Understanding the evolution of lung loss in Plethondontidae might also reveal organ loss in other vertebrates.
“If these genetic mechanisms are discovered, we will have a greater understanding of how evolution works to eradicate an organ such as the lung, which was previously believed to be essential to obtaining life on land,” said Lewis, who is currently a scientist with NanoString Technologies.
Zachary R. Lewis, Ryan Kerney, and James Hanken, "Developmental basis of evolutionary lung loss in plethodontid salamanders," Science Advances, 17 August 2022. DOI: 10.1126/sciadv.abo6108.
The National Science Foundation, the Museum of Comparative Zoology, the Wetmore-Colles Fund, and the Robert G. Goelet Summer Research Award funded the study.