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By Mary Halton
Science reporter, BBC News
10 May, 2018
Getty Images. 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.
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".
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."
Loggerhead turtle hatchlings may be affected by climate change
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.
[doublepost=1526103144,1526018806][/doublepost]May 11, 2018 | Original Press Release from Duke University
For a turtle called the red-eared slider, a hatchling’s sex depends on the environment. Cooler egg incubation temperatures produce mostly male hatchlings; warmer incubation temperatures mean more females. Credit: Pixnio.
Boy or girl? For those who want to influence their baby’s sex, superstition and folk wisdom offer no shortage of advice whose effectiveness is questionable at best -- from what to eat to when to make love. But some animals have a technique backed by scientific proof: In turtles and other reptiles, whether an egg hatches male or female depends on the temperature of its nest.
The phenomenon was first discovered in reptiles more than 50 years ago, but until now the molecular details were a mystery.
In a study published May 11 in the journal Science, researchers say they have finally identified a critical part of the biological “thermometer” that turns a developing turtle male or female.
According to a team at Duke University and Zhejiang Wanli University in China, the explanation lies not in the DNA sequence itself -- the A’s, T’s, C’s and G’s -- but in a molecule that affects how genes are expressed without altering the underlying genetic code.
“Temperature-dependent sex determination has been a puzzle for a really long time,” said Blanche Capel, a cell biology professor at Duke who led the research. “This is the first functional evidence of a molecular link that connects temperature with sexual development.”
Unlike humans and most other mammals, the sex of many turtles, lizards and alligators isn’t determined by the chromosomes they inherit, but by ambient temperatures during a sensitive stage of development.
For a common pond and pet turtle called the red-eared slider, for example, eggs incubated at 32 degrees Celsius (nearly 88 Fahrenheit) produce all female hatchlings, while those kept at 26 degrees Celsius (79 Fahrenheit) hatch as males.
In the study, the researchers show that cooler egg incubation temperatures turn up a key gene called Kdm6b in the turtle’s immature sex organs, or gonads. This in turn acts as a biological “on” switch, activating other genes that allow testes to develop.
To home in on the critical Kdm6b gene, the researchers took a group of freshly laid turtle eggs, incubated them at either 26 or 32 degrees Celsius, and looked for differences in the way genes were turned on in the turtles’ gonads early in development -- before their fate as ovaries or testes has been decided.
In a previous study, researchers examined all the gene readouts, or transcripts, produced in the turtles’ gonads over this critical time window.
They found several genes that were turned up or down at one temperature but not the other. But one of the first genes to shift was one called Kdm6b, which became much more active at the cooler incubation temperatures that produce males, and was almost silent at warmer, female-producing temperatures.
In the new study, the team used a technique developed by collaborators at Zhejiang Wanli University to suppress the Kdm6b gene in turtle gonads and see how it affects their sexual development.
Silencing the Kdm6b gene, they found, transforms a growing turtle embryo kept at temperatures that would otherwise produce a male with testes into a baby female with ovaries instead.
Further experiments showed that the protein encoded by the Kdm6b gene in turn interacts with a region of the genome called Dmrt1, which acts as a master switch to turn on testis development.
They found that Kdm6b activates the Dmrt1 master switch by modifying histones, the ball-like proteins that DNA is wrapped around inside the cell nucleus, like thread wound around a spool.
In many species, the tail of histone proteins is decorated with special chemical markers, or methyl tags, that keep genes along the DNA molecule inactive.
Kdm6b gene activity turns on the Dmrt1 master switch by removing these repressive tags and “loosening” the histone tails, which makes the DNA wound around the histones easier to access and read.
“It’s like removing the brakes off the male pathway,” said co-author Ceri Weber, a PhD candidate in the Capel lab at Duke.
Researchers have found temperature-related shifts in Kdm6b gene activity in other species whose sex depends on incubation temperature, such as alligators and bearded dragons. This suggests that similar molecular mechanisms may be at work in other reptiles too, Capel said.
The researchers think that Kdm6b and the protein it encodes don’t sense heat or changes in temperature inherently, since cooler incubation temperatures increased gene activity in the turtle’s future testes but not in other developing organs such as the heart or the liver.
The next step is to find the temperature-sensing trigger, Weber said. “We’re trying to narrow down the possibilities.”
This article has been republished from materials provided by Duke University. Note: material may have been edited for length and content. For further information, please contact the cited source.