A tree frog found in Panama offers clues to an evolutionary link. JUSTIN TOUCHONJune 11 — The discovery of some amphibious sexual positions has the biology department at Boston University talking, as the land-and-water mating habits of tree frogs may reveal a missing link in evolutionary biology.
Biology professor Karen Warkentin and graduate student Justin Touchon discovered that a certain tree frog species is the first vertebrate animal to reproduce by laying eggs both on land and in water, bridging the gap between ancient vertebrates, which began life in water, and their distant terrestrial relatives, which had to develop eggs that could survive on land.
This research will shed light into a particular period of animal evolution, Touchon said of his report, published May 21.
‘No one even thought this was possible before,’ Touchon said. ‘Everyone viewed reproduction as this fixed thing. If we can study the ecological pressures that influence where the species lays its eggs, we can piece together the ecological pressures that drive evolution.’
The researchers, who were funded by the Animal Behavior Society, BU, the National Science Foundation and the Smithsonian Institute, said they discovered the species dendropsophus ebraccatus at a pond in Panama in 2006. Touchon had been studying the frogs there for five years, focusing on three different ponds. The researchers were puzzled when they could not find any frog eggs at one pond, while the land-laid eggs at the other two could be located.
The mystery was solved one night when the researchers came across a frog laying its eggs directly into the water, they said.
‘We were in the right place at the right time,’ Touchon said.
The researchers began looking at whether the anomaly was a reproductive difference induced by genetics or environment. The tree frogs that laid eggs in water were either a different species from the land-laying tree frogs or the same frogs acting differently in response to environmental difficulties.
Experiments proved the difference was environmental, Touchon said. The tree frogs could choose where to lay their eggs depending on environmental factors, such as the amount of oxygen eggs receive in water or sunlight. Researchers found the frogs turn to aquatic reproduction in sunny environments. The pond where the frogs were seen laying eggs in water had the least shade, explaining the unique behavior.
Touchon and Warkentin plan to look for other environmental factors that could influence the frogs’ reproduction decisions. In addition, they will look for other species that can lay their eggs both in water and on land. Other tree frog species in hot, dry places could also display this unique ability.
‘There could potentially be other species that do this,’ Warkentin said. ‘In fact, I’d be shocked if there aren’t.’
Geoffrey Cooper, chairman of the BU biology department, said this discovery is ‘extremely exciting.’
‘That sort of link really gives new insights into the way we think about the evolution of reproduction,’ Cooper said.
The Proceedings of the National Academy of Sciences published Touchon’s dissertation based on his and Warkentin’s research on May 21.
All papers submitted to PNAS are reviewed by ‘referees familiar with the scientific field of the paper,’ PNAS spokesman Jonathan Lifland said in an email.
‘The referees were positive about this paper’s findings and described it as ‘well-designed,’ ‘carefully analyzed’ and that it ‘had the potential to interest evolutionary biologists, ecologists, reproductive biologists and many other scientists,” Lifland said.
Only time will tell how big of an impact the paper’s findings have on the scientific community, Lifland said. With six to 10 news articles published about the discovery in the weeks following the report’s release, he said the research has already received significant attention.
‘It is remarkable to report any scientific discovery for the first time,’ Lifland said.
