lungs. We can’t live without them, any more than most animals that carry their skeleton on the inside. However, some extraordinary amphibians are no longer concerned with these delicate, slimy organs.
Scientists have found that some animals without lungs first begin to grow these respiratory organs, and then their heart development process changes.
These animals, the plethodontid salamanders, have gotten away with being lungless for at least 25 million years by breathing through their slimy skin and mouth tissue. To do this, they have to cover themselves with mucus – as if they were wearing their lung mucus on their outside – because they can only absorb oxygen through their skin if it remains moist.
Lungless plethodontids are the largest group of salamanders with nearly 478 species, mainly found in the Americas, with a few in Europe and South Korea.
Little is known about how these amphibians lose their lungs, so Harvard University evolutionary biologist Zachary Lewis and colleagues took a closer look at these cold-blooded water enthusiasts.
“We confirm that a nascent lung forms in embryos of different species,” the researchers write in their paper. “The morphological features are very similar to those of the axolotl, ambystoma mexicanuma lunged salamander.”
These embryonic lungs develop and branch for about three weeks in one species, P. cinereusbefore something interrupts them. Well before the baby salamanders hatch, the early lung cells are programmed to undergo apoptosis, a form of cell death.
The researchers suspect that lung development stops because the cells don’t receive the regulatory signals necessary for them to multiply, which in other vertebrates comes from the tissues around the developing lungs called mesenchyme. So they decided to test this hypothesis.
“We put mesenchyme from a salamander with lungs into a lungless salamander embryo and let it develop,” says Lewis. “It resulted in the formation of structures that resemble lungs, which provides some evidence that lungless salamanders continue to be able to continue developing lungs.”
Although these slime-covered animals don’t have traces of lungs in them as adults, salamanders still have the genes and many of the developmental processes needed to grow them.
But some signals needed to complete the process, for example those from the mesenchymal tissues, seem to be missing.
So why are so many tools and ingredients for making lungs preserved in animals that haven’t used lungs for millions of years?
“For example, the lung principle may play an important role in the development of adjacent organs such as the heart,” the researchers speculate, explaining that some of the genes involved in initial lung formation are also used to make other organs. For example, a gene called Sonic hedgehog (yes, really) is used for the development of both the lungs and limbs.
These connections show how studying organ remains (rudimentary structures) – such as these or leg studs in snakes – can help us understand the twisting and turning mechanisms behind developmental and evolutionary processes and provide insight into how our organs work.
The scarce fossils of salamanders make it difficult for researchers to determine when exactly these animals lost their special respiratory organs, but genetic timelines estimate it to be somewhere between 25 and 110 million years ago, depending on whether it happened in one ancestral species or if lung loss was multiple. times evolved within this salamander family.
“Clearly, lungless salamanders do just fine without lungs, as they make up about two-thirds of all salamander species,” says Lewis. “Perhaps losing lungs has enabled rather than hindered this remarkable evolutionary success.”
This research was published in scientific progress.