Despite lung loss in adults for millions of years, lungless salamanders develop lungs as embryos.
For many vertebrates, including humans, their lungs are essential. Four living amphibian clades, however, no longer breathe via their lungs and instead breathe predominantly through their wet skin. Little is known about the developmental basis of lung loss in these clades.
Researchers at Harvard University’s Museum of Comparative Zoology and Department of Organismic and Evolutionary Biology examined the Plethodontidae, a dominant family 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. Their findings were recently published in the journal Science Advances.
Plethodontidae is the most species-rich salamander family, accounting for more than two-thirds of all current salamander diversity. All adult plethodontids lack lungs and breathe solely through nonpulmonary tissues, primarily the skin and mucus membranes of the mouth and throat. Lung loss has happened at least four times in distantly related amphibians independently, and there are more cases of lung reduction or loss in both amphibians as well as some vertebrates. However, the developmental explanation for this loss is unknown.
“Clearly lungless salamanders do fine without lungs given that they make up about two-thirds of all salamander species,” said lead author Zachary R. Lewis, former doctoral candidate (Ph.D.’16), “perhaps losing lungs enabled, rather than hindered, this remarkable evolutionary success.”
This research builds on Lewis’ doctoral work in the lab of senior author Professor James Hanken. Lewis, Hanken, and co-author Associate Professor Ryan Kerney of Gettysburg College examined the morphology of lung development in both lung and lungless salamanders using histology and micro-CT. They discovered that lungless salamanders develop lungs as embryos in the same manner as lungs develop in other species. The researchers subsequently employed in situ hybridization and RNA sequencing to demonstrate that the structure that arises during lungless salamander embryonic development is similar to a lung not just morphologically, but also in terms of the molecules expressed.
The researchers suggest that lung development stops in these species due to a lack of cues that maintain lung development, which emerge from the tissue, mesenchyme, that surrounds the developing lung.
“We put mesenchyme from a salamander with lungs into a lungless salamander embryo and allowed it to develop,” said Lewis, “it resulted in the formation of structures that resemble lungs, offering some evidence that lungless salamanders remain capable of continuing to develop lungs.”
The study also confirms Amy Grace Mekeel’s 1936 doctoral thesis that challenged the leading theory put forth by biologists that the slight fold in the adult pharynx is a vestigial lung which persisted since the initial lung loss of the plethodontids. Mekeel described a “lung rudiment” that formed in the embryo but was lost by the time it hatched.
“The lung precursor appears and disappears before the lungless salamander embryos hatch, just as Mekeel described,” said Kerney, “this work vindicates Mekeel’s earlier thesis and lays the initial adult vestige hypothesis to rest.”
The study reveals that lung developmental-genetic pathways are at least partially conserved despite the absence of functional adult lungs for at least 25 and possibly exceeding 60 million years. Understanding the evolution of lung loss in Plethondontidae could also shed light on organ loss in other vertebrates.
“In the future, if these genetic mechanisms are revealed, we will have a more complete understanding of how evolution acts to do away with an organ such as the lung, which was long thought critical to achieving life on land,” said Lewis who is currently a scientist with NanoString Technologies.
Reference: “Developmental basis of evolutionary lung loss in plethodontid salamanders” by Zachary R. Lewis, Ryan Kerney and James Hanken, 17 August 2022, Science Advances.
The study was funded by the National Science Foundation, the Museum of Comparative Zoology, the Wetmore-Colles Fund, and the Robert G. Goelet Summer Research Award.