A new study, led by Prof Minoru Tanaka of the National Institute for Basic Biology in Okazaki, Japan, shows that the foxl3 gene, which is expressed in reproductive precursor cells known as germ cells, is involved in sperm-egg fate decision in a tiny fish called medaka (Oryzias latipes).
Female medaka lacking foxl3 genes shows standard female body shape. In the ovaries of foxl3 mutant female medaka, a large number of sperm and a small number of eggs are formed. Image credit: National Institute for Basic Biology.
In vertebrates, gonadal somatic cells instruct germ cells to adopt their sexual fates. In medaka, the expression of a gene called DMY in the supporting somatic cells is critical for the fate decision of germ cells to enter spermatogenesis.
However, the molecular mechanism underlying the sexual fate decision in germ cells has remained unknown.
In a new study, Prof Tanaka and co-authors show that a gene called foxl3 controls whether germ cells eventually become sperm or eggs.
“While germ cells can become either sperm or eggs, nobody knew that in vertebrates the germ cells have a switch mechanism to decide their own sperm or egg fate,” said Prof Tanaka, senior author of the paper reporting the results in the journal Science this week.
“Our result indicates that once the decision is made the germ cells have the ability to go all the way to the end. I believe it is of very large significance that this mechanism has been found.”
The scientists revealed that foxl3 works in the germ cells of female medaka ‘to suppress differentiation into sperm.’
“In females lacking functional foxl3 genes, medaka’s body appearance is still totally female, however a large number of sperm are formed in the ovaries, and a small number of eggs are formed at the same time.”
It has also become clear that in these females lacking the function of foxl3 functional sperm can be obtained in a shorter period of time than in normal wild-type males.
“In spite of the environment surrounding the germ cells being female, the fact that functional sperm has been made surprised me greatly,” said co-author Dr Toshiya Nishimura, also from the National Institute for Basic Biology.
“That this sexual switch present in germ cells is independent of the body’s sex is an entirely new finding.”
Humans do not possess foxl3 gene, but the team suspects a similar mechanism may be at play in our species, too.
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Toshiya Nishimura et al. foxl3 is a germ cell-intrinsic factor involved in sperm-egg fate decision in medaka. Science, published online June 11, 2015; doi: 10.1126/science.aaa2657
