Hormone still rules no-tadpole frogs

Skipping most of the tadpole business, a coqui frog hops out of the egg as a miniature adult, smaller than a pea. Even so, it doesn’t escape the king of tadpole chemistry, thyroid hormone, say Canadian researchers.

In most mainland U.S. frogs, a surge of thyroid hormone tells a tadpole to grow up into a frog shape. However, hundreds of other frog species, mostly in the tropics, have lost the tadpole stage. “How do you get rid of a whole life stage?” wonders Richard P. Elinson of the University of Toronto.

One scenario would be to escape thyroid hormone control. Previous work had shown that coqui frogs still need the hormone to form proper rumps, but biologists had speculated that it had little other function.

Not so, report Elinson and Elizabeth M. Callery of the State University of New York at Stony Brook. While still in their eggs, coqui frogs undergo a metamorphosis ruled by thyroid hormone, the researchers argue in a paper scheduled for the March 14 Proceedings of the National Academy of Sciences. “We propose a new model for the evolution” of frogs that develop directly, they say.

The apricot-size coqui frogs set the Puerto Rican dusk vibrating with the “co-key, co-key” call of males. “It’s very high-pitched, very loud, very bright—it’s wonderful,” Elinson recalls. Females that agree lay clear eggs on a dry spot and hop away, leaving the males to sit on the clutch until the eggs hatch in about 3 weeks. Eggs can dry out fast, so coqui males leave their clutch only to find a quick meal and to defend the eggs from foraging males.

When Elinson sees curled-up miniature adults in mature eggs in the laboratory, he jiggles their dish to trigger hatching. “They’ll all hatch at once, popping out of their eggs in just a few minutes,” he says. “The babies are fabulous.” They’re hard to raise, however, because they need tiny, live food. “Even a fruit fly is a bit too big,” Elinson notes.

He and Callery discovered that the genes for two thyroid hormone receptors switch on about 2 weeks after the eggs are laid. When researchers treated younger eggs with the drug methimazole to block hormone formation, development stalled for a range of features, including limb length, jaw shape, skin texture, and color. Giving thyroid hormone to the treated eggs restarted the maturation. Blocking hormone formation later didn’t stop development.

The study brings welcome clarity to the role of thyroid hormone, comments Donald D. Brown of the Carnegie Institution of Washington’s embryology department in Baltimore (SN: 7/17/99, p. 43). He says that he’s especially interested in the list of organs affected by the hormone.  “It’s a nice paper,” he says.

Some sea urchins, mollusks, and insects have, like coqui frogs, lost a larval stage, notes Rudolf A. Raff of Indiana University in Bloomington. The unusual life plans mix radical cuts in some processes with perfect conservation of others. As he puts it, “What you’re seeing are the vagaries of evolution.”

Susan Milius is the life sciences writer, covering organismal biology and evolution, and has a special passion for plants, fungi and invertebrates. She studied biology and English literature.

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