In some lizard species, it's not just genetics that determines whether males or females will clamber out of hatching eggs. Some species are also governed by nest temperature, like the the three-lined skink lizard: Seven years ago, Australian biologist Rick Shine showed that low nest temperatures could overrule genetics, and cause embryos to develop into males. Now, Shine and his colleagues have taken their skink research a step farther, showing that the size of an egg's yolk also plays a mysterious role in the ultimate sex of the offspring. Physiologist Rachel Bowden, who was not involved in this research, says the study
"muddies the water" for everything researchers thought they knew about sex determination in lizards.... "It's clear that they have sex chromosomes. But it's also clear that those sex chromosomes can readily be overridden by some other factors. So, the process that leads to sex determination might be fairly plastic" [The Scientist].
In the study, published in the journal Current Biology, Shine's team first
weighed more than 800 eggs from 130 clutches and found that son-producing eggs were consistently a few fractions of a gram lighter than daughter-producing eggs [The Scientist].
Then the researchers began fiddling with the eggs' yolk levels, and found that when they removed yolk from eggs, more of the embryos developed into males. Shine notes that he had undertaken the experiment expecting to disprove the hypothesis that egg size had any influence on the offsprings' sex.
"We were confident that there would be no effect on hatchling sex whatsoever," Shine said. "When those baby boy lizards started hatching out, we were gob-smacked" [Eureka Alert].
The researchers also found that adding yolk to an egg caused the embryo to develop into a female, but noted that adding silicone instead had no effect--indicating that it's some mysterious substance in the yolk itself that's giving the sex signal. As for what's behind this fascinating control method, researchers say it's possible that this
size manipulation might allow moms to balance their babies’ sexes when a chilly nest would otherwise produce mostly sons [Scientific American].
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Image: flickr / Charles Haynes