Male mice made sans y chromosomes: scientists engineer female embryos to develop as opposite sex.
Reproductive biologist Monika Ward of the University of Hawaii in Honolulu and colleagues started with mice that have only one X chromosome (and no second sex chromosome). Normally, those animals would develop as females. Rut when the researchers manipulated genes found on the X and on another chromosome, the mice became males that could produce immature sperm. Those engineered males fathered offspring with reproductive assistance from the researchers, who injected the sperm into eggs, Ward and colleagues report in the Jan. 29 Science.
The experiments show that there are multiple ways to make males, says Richard Behringer, a developmental geneticist at MD Anderson Cancer Center in Houston. "They've done it without any Y chromosome gene information."
At first glance, the experiments would seem to suggest Y chromosomes aren't necessary for reproduction, which hints that evolution may eventually show Y's the door. "To me, it is a paradigm of the decline and fall of the Y chromosome," says reproductive biologist Jennifer Marshall Graves of La Trobe University in Melbourne, Australia.
But Ward and others say the Y isn't going anywhere. Because the Y-less males needed help to reproduce, "clearly we need the Y chromosome for full natural male reproduction," says reproductive biologist Mary Ann Handel of Jackson Laboratory in Bar Harbor, Maine.
Ward and colleagues had previously shown that two Y chromosome genes--Sry and Eif2s3y--are crucial for male mouse development. Sry is a master gene that turns on male developmental programming in early embryos. It turns on a gene called Sox9, which then sets off a biochemical chain reaction that leads to male development.
But in the new experiments, the researchers turned on Sox9 through other means. Activating Sox9 in a genetically female embryo causes it to develop as a male, Ward and colleagues found. But those males didn't make sperm.
To produce sperm, mice need the Eif2s3y gene, the researchers had previously discovered. In the new experiments, the mice lacked the gene because they didn't have Y chromosomes. So the team substituted a similar gene from the X chromosome called Eif2s3x. Only one copy of the Y version is needed to make immature, tailless sperm, but it takes at least five copies of the X version to do so. "This indicates that the Y chromosome gene is the strong one," Ward says.
The research suggests that the Y chromosome has optimized production from genes that are necessary for making males. Making just the right dose of male development factors is how the Y protects itself from evolutionary erasure, Ward says. "Our work does not support that the Y chromosome will disappear."
Graves disagrees. The work is "a lovely example of how you can lose even a really important gene," she says. At least two species of rodents have already jettisoned their Y chromosomes entirely.
Primates, including humans, don't have Eif2s3y genes on their Y chromosomes. No one knows whether Eif2s3ys function in primates was taken over by other genes on the Y, its X counterpart or genes elsewhere in the genome. The new work may help explain how primates get along without the gene, Graves suggests, and may "give us useful information about what happens at the end of the life of the Y chromosome."
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|Title Annotation:||GENES & CELLS|
|Author:||Saey, Tina Hesman|
|Date:||Mar 5, 2016|
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