A Puzzling Mystery: Why Do Salamanders Lose Their Lungs?



Salamanders without lungs produce lungs as embryos despite having lost their lungs as adults over millions of years.

The lungs of many vertebrates, including humans, are crucial. However, four extant amphibian clades now mostly breathe through their wet skin rather than their lungs. The developmental causes of lung loss in these clades are poorly understood.

When the Plethodontidae, a dominant family of salamanders that are all lungless as adults, were examined, researchers from Harvard University's Museum of Comparative Zoology and Department of Organismic and Evolutionary Biology found that they do develop lungs as embryos. This finding sheds light on the evolution of lung loss over millions of years. The journal Science Advances just published their findings.

With about two-thirds of the diversity in the current salamander population, Plethodontidae is the salamander family with the greatest number of species. The skin and mucous membranes of the mouth and throat serve as the primary nonpulmonary tissues through which all adult plethodontids breathe. There have been numerous instances of lung shrinkage or loss in both amphibians and certain vertebrates, and it has occurred independently at least four times in distantly related amphibians. However, it is unknown what developmental mechanism accounts for this loss.

Since nearly two-thirds of all salamander species are lungless, lead author Zachary R. Lewis, a former doctoral candidate (Ph.D.'16), suggested that perhaps removing lungs allowed, rather than hampered, this amazing evolutionary success.

Lewis' PhD work in Professor James Hanken's lab, the senior author, served as the foundation for this study. Lewis, Hanken, and co-author Associate Professor Ryan Kerney of Gettysburg College used histology and micro-CT to analyze the lung development's morphology in both lung and lungless salamanders. They found that lungless salamanders develop their lungs as embryos in a manner similar to how other species' lungs do. Later, the scientists used in situ hybridization and RNA sequencing to show that the structure that forms throughout the development of lungless salamander embryos is comparable to a lung both morphologically and in terms of the chemicals produced.

The lack of cues that support lung development, which arise from the tissue, mesenchyme, that surrounds the growing lung, is thought to be the reason why lung development stops in these animals, according to the researchers.

Lewis explained that after implanting lung-containing salamander mesenchyme into a lungless salamander embryo and allowing it to grow, "it resulted in the creation of structures that mimic lungs, affording some evidence that lungless salamanders remain capable of developing lungs."

The discovery also supports Amy Grace Mekeel's 1936 PhD dissertation, which refuted the prevalent hypothesis advanced by biologists that the small fold in the adult pharynx represents a vestige of a lung that has lasted since the plethodontids' initial lung loss. Mekeel spoke of an embryonic "lung rudiment" that developed but was lost by the time the egg hatched.

According to Mekeel's description, the lung precursor appears and vanishes before the lungless salamander embryos hatch. This work supports Mekeel's earlier thesis and disproves the original adult vestige idea, according to Kerney.

The research shows that despite the absence of adult lungs for at least 25 and possibly over 60 million years, lung developmental-genetic processes are at least largely intact. It may be possible to learn more about organ loss in other vertebrates by studying the evolution of lung loss in Plethondontidae.

Lewis, who is presently a scientist with NanoString Technologies, said that if these genetic mechanisms are discovered in the future, "we will have a more full understanding of how evolution acts to do away with an organ like the lung, which was long thought crucial to achieving life on land."

The National Science Foundation, the Museum of Comparative Zoology, the Wetmore-Colles Fund, and the Robert G. Goelet Summer Research Award provided funding for the investigation.

By HARVARD UNIVERSITY, DEPARTMENT OF ORGANISMIC AND EVOLUTIONARY BIOLOGY

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