Gene that determines sex in birds identified

August 27th, 2009

LONDON - Ending a long debate about what determines the sex of birds, scientists have now found that the expression of just one gene in chickens controls whether they will grow up to become a hen or a cockerel.

While sex is decided via genes on one of two sex chromosomes (Y in mammals and X in fruit flies) in most organisms, but in birds the picture is less clear.

The distribution of the sex chromosomes is reversed - males have two Z chromosomes, and females have one Z and one W - and it has been hotly contested whether the sex of avian embryos is decided by a female-specific gene on the W chromosome, or whether it’s the number of Z chromosomes that matters.

The researchers proposed two genes that could unravel the mystery- on one hand, the W-linked gene HINTW has been touted as a potential female-only factor, while on the other, the Z-linked gene DMRT1, which is involved in sexual differentiation in many organisms, provides a potential dose-sensitive sex determinant.

According to new data, the latter gene has been touted as the master sex regulator.

Researchers led by Craig Smith of the Murdoch Childrens Research Institute in Melbourne, Australia, reduced the expression of DMRT1 in young chick embryos using RNA interference (RNAi).

They found that male chicks - those with two Zs - with abnormally low levels of the gene developed partially feminized gonads instead of proper testes.

In an earlier research, Smith’s group showed that overexpressing HINTW in male embryos did not alter the course of male development2.

Taken together, the studies support the theory that the Z-linked double dose mitigates avian gender.

“These experiments are giving us the ultimate proof. This system of sex determination is valid for all 500 other birds,” Nature quoted Michael Schmid, a geneticist at the University of Würzburg, Germany, as saying.

However, Graves noted that DMRT1 might work in unison with a gene on the W chromosome that enhances the dose signal coming from the master regulator.

“A two-to-one ratio is a bit dicey. With something as important as sex you’d want to be a bit more robust than that,” she said.

Hans Ellegren, an evolutionary biologist at Uppsala University in Sweden, said that the new findings are consistent with the dose hypothesis, but they don’t definitively show that it is the number of gene copies - rather than the expression levels of those genes - that is the key factor.

“The experiments cannot formally exclude the dominant-W hypothesis,” he added.

He explained that there could be a W-linked gene in females that suppresses DMRT1 to prevent male development.

The findings have been published in Nature. (ANI)

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