For many vertebrates, including humans, the lungs are crucial. Four extant amphibian clades, however, no longer breathe through their lungs and instead breathe predominantly through their moist skin. The development causes of lung loss in these clades are poorly understood.
Lungless Salamander (Plethodontidae)
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With more than 400 species, this is the largest family of living salamanders. The nasolabial grooves, which support chemoreception, are present in all members of this family, which does not have lungs and instead relies on cutaneous breathing, as per Amphibia Web.
The Appalachian Mountains, which are now a biodiversity hotspot for this family, were long supposed to have given rise to this group in the eastern United States.
This group's members can live in stream, arboreal, terrestrial, fossorial, and subterranean habitats. Presently, there are two subfamilies: the Plethodontinae and the Hemidactylinae.
The latter include species that can project their tongues in a ballistic manner to capture prey and have webbed feet. Plethodontids are salamanders that range in size and have a generalized body structure with four fingers and five toes. The majority of species directly evolve.
This family is only found in the New World, from southern Canada to Brazil and Bolivia, with a few outliers. Species of the genus Hydromantes found in the Middle Western Mediterranean in Europe (other members which also appear in California) and a single species known from Asia are the exceptions to the New World (Karsenia).
It contains the sole subset of tropical salamanders, which make up around two-thirds of the species and forty percent of all salamanders.
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 born without lungs, and discovered that they do, in fact, develop lungs as embryos. This finding sheds light on the evolution of lung loss over millions of years, as per Phys.org.
All adult plethondontids lack lungs and only breathe through nonpulmonary tissues, primarily the skin and mucous membranes in the mouth and throat.
Among frogs that are distantly related, lung loss has independently happened at least four times. Other amphibians and vertebrates have also experienced lung loss or reduction. This loss, though, is still unaccounted for in terms of development.
Lewis's doctoral work was conducted in Professor James Hanken's group, the senior author of this article. In order to evaluate the morphology of lung development in both lung and lungless salamanders, Lewis, Hanken, and co-author Associate Professor Ryan Kerney of Gettysburg College used histology and micro-CT.
They discovered that lung development in lungless salamanders begins much as it does in lung-bearing species. The structure that develops during lungless salamander embryonic development mimics a lung not only in regards to morphology but also in terms of the chemicals expressed, the researchers were able to demonstrate using in situ hybridization and RNA-sequencing.
The lack of cues to maintain lung growth, which is produced by the tissue called mesenchyme that covers the lung as it develops, is thought to be the reason why lung development stops in these species, according to the researchers' hypothesis.
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 gone by the time the egg hatched.
