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Lungless salamanders develop lungs as embryos despite lung loss in adults over millions of years

Hemidactylium scutatum larvae, lungless salamander native to eastern North America. Credit: Zachary R. Lewis

Lungs are essential for many vertebrates, including humans. However, four living amphibians have independently eliminated lung respiration and have no lungs, breathing primarily through their wet skin. Little is known about the developmental basis of lung loss in these clades.

In a new study in scientific progress Researchers from the Department of Organic and Evolutionary Biology and the Museum of Comparative Zoology at Harvard University examined the Plethodontidae, a dominant family of salamanders, all of which are lungless as adults, and find that they actually develop lungs as embryos that shed light on evolution. of lung loss over millions of years.

The lungless salamander family Plethodontidae is the most species-rich family of salamanders, accounting for more than two-thirds of the existing salamander diversity. All adult flathounds have no lungs and breathe completely through non-pulmonary tissues, primarily the skin and mucous membranes in the mouth and throat. Lung loss has occurred independently at least four times in distantly related amphibians, and there have been other cases of lung reduction or loss in both amphibians and some vertebrates. However, the developmental reason for this loss remains a mystery.

“It’s clear that lungless salamanders do just fine without lungs, as they make up about two-thirds of all salamander species,” said lead author Zachary R. Lewis, a former PhD student (Ph.D. ’16), “maybe they can lose lungs, in rather than hindered this remarkable evolutionary success.”

This study builds on Lewis’ doctoral work in the lab of senior author Professor James Hanken. Lewis, Hanken and co-author Associate Professor Ryan Kerney, Gettysburg College, used histology and micro-CT to examine the morphology of lung development in both lung and lungless salamanders. They found that lungless salamanders begin to develop lungs as embryos in much the same way that species with lungs develop them. The researchers then used in situ hybridization and RNA sequencing to show that the structure that forms during the embryonic development of the lungless salamander not only resembles a lung morphologically, but also in terms of the molecules expressed.

The researchers hypothesize that lung development stops in these species due to a lack of signals that sustain lung development arising from the tissue, mesenchyme, that surrounds the lung as it develops.

“We put mesenchyme from a lung salamander into a lungless salamander embryo and let it develop,” Lewis said, “resulting in the formation of lung-like structures, providing some evidence that lungless salamanders continue to be able to remain lungs.” develop. “

The study also corroborates the 1936 thesis of Amy Grace Mekeel that challenged the leading theory advanced by biologists that the slight fold in the adult pharynx is a vestigial lung that has persisted since the initial lung loss of the plethodontids. Mekeel described a “longrudiment” 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,” Kerney said. “This work confirms Mekeel’s previous thesis and settles the initial hypothesis of adult remains.”

The study shows that the developmental genetic pathways of the lungs are at least partially preserved despite the absence of functional adult lungs for at least 25 and possibly more than 60 million years. Understanding the evolution of lung loss in Plethondontidae could also shed light on organ loss in other vertebrates.

“In the future, as these genetic mechanisms are revealed, we will have a more complete understanding of how evolution works to abolish an organ like the lung, which has long been considered crucial to achieving life on land,” said Lewis, who is currently a scientist at NanoString Technologies.

Research shows that the skin of lungless salamanders expresses proteins critical to lung function

More information:
Zachary R. Lewis et al, Developmental basis of evolutionary lung loss in plethodontid salamanders, scientific progress (2022). DOI: 10.1126/sciaadv.abo6108. www.science.org/doi/10.1126/sciaadv.abo6108

Provided by Harvard University

Quote: Lungless salamanders develop lungs as embryos despite lung loss in adults over millions of years (2022, August 17) retrieved August 17, 2022 from https://phys.org/news/2022-08-lungless-salamanders-lungs-embryos-lung. html

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