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Scientists have isolated the gene responsible for temperature-controlled sex determination in turtles.
Red eared slider turtles, a common household pet, develop into male or female embryos according to their egg incubation temperature.
This little understood process is also at work in the eggs of crocodiles, alligators and some lizards.
Researchers are now one step closer to solving a mystery which has persisted for over 50 years.
Genetic 'knockout'
An international team from China and the United States used a recently refined process to "knock out" the gene they suspected to be responsible for sex determination in the turtles - known as Kdm6b.
"Knockouts come in several flavours," explained Prof Blanche Capel from Duke University, an author on the study. "It usually means a genetic manipulation that deletes a gene from the genome or blocks its function."
With Kdm6b blocked, over 80% of turtles incubated at the (usually male-producing) temperature of 26C shifted their development to female.
Females usually only develop when eggs are incubated at 32C.
Dr Nicole Valenzuela from Iowa State University, who was not involved in the study, noted that the findings confirmed earlier predictions that such genes "are themselves turned on at a temperature that produces one sex and turned off at a temperature that produces the opposite sex".
Uncertain future
Other recent studies have suggested that rising temperatures due to climate change could be causing a female skew in turtle populations in the wild.
Hatchling mortality could also increase if nests remain at high temperatures for long periods of time.
Prof Capel points out that "if you're incubating at low temperature, males take almost twice as long to incubate as females."
Next, the research team hopes to figure out how embryos are measuring and reacting to the nest temperature while in the egg.
"It's clear that this gene is responding to temperature. There's something activating it and that's the next question," Prof Capel told BBC News.
Dr Valenzuela agrees, citing this work as "an important contribution to our understanding of sexual differentiation," but for her, fully deciphering the process of temperature-dependent sex determination remains "the magic bullet that has eluded scientists since its discovery over half a century ago."
The findings were published in Science.