Once tadpoles lose lungs, they never get them back

ITHACA, N.Y. – Tadpole species that lost their lungs through evolution never re-evolve them, even when environmental change would make it advantageous – bucking long-standing assumptions about how lost traits can reemerge, according to a new Cornell University study.

Typical tadpoles have three main ways to get oxygen: from the air, with lungs; from the water, through gills; and from the air through their skin.

Curiously, all frogs have lungs, so tadpoles retain the developmental genetics to regain lungs when environmental pressures might favor having them but instead evolve alternate solutions for acquiring oxygen from the air. 

The study, published Oct. 27 in the journal Evolution, challenges long-standing assumptions that traits re-evolve more easily when the underlying developmental machinery of a lost trait remains intact. 

“The study highlights both the predictability of evolution on the loss side and the utter unpredictability of the solutions that evolution finds for problems,” said Jackson Phillips, the study’s first author and a doctoral student in the lab of senior author, Molly Womack, assistant professor of ecology and evolutionary biology.

Out of 530 species studied, including nearly every frog family and most genera, the researchers identified 28 instances of lung loss. A total of over 5,000 species of frogs exist globally, 95 of which are known to be lungless as tadpoles.

The researchers relied on relationships among existing frog species that have been determined with genetics and computer models to trace back in time when lung losses would have occurred and estimate the ecological condition or habitats that those tadpoles might have lived in.

In general, within Phillips’ sample, a few patterns emerged relating to the environments where lungs were lost. For some stream tadpoles, losing their lungs might have been advantageous, since water in flowing streams is already highly oxygenated and lungs create buoyancy, so tadpoles with lungs may easily be carried downstream.

“If a tadpole is in a fast-flowing stream, it may not want a life raft in its body,” Womack said. 

Stream tadpoles can occupy many different microhabitats. Phillips and colleagues found some tadpoles with sucker mouths, used to hold onto rocks in rapid water, were lungless. Another lungless group lives in sand, gravel or under leaf mats in streams. He also found, within the sample he collected, terrestrial tadpoles that lay eggs in nests on land, often near wet spray zones next to streams, also lost lungs. However, despite most currently existing frog species developing in ponds, it is likely tadpole lungs were only lost in past pond habitats a few times. 

The researchers suspect that there might be something about air breathing that affects survival, such as floating downstream, danger of predation when coming to the surface or moving out from hideaways.

“Maybe the adaptation is due to selection against air breathing instead of selection against lungs,” Phillips said.

For additional information, read this Cornell Chronicle story.

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Media note: Pictures and videos can be viewed and downloaded here: https://cornell.box.com/v/lunglesstadpoles

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