To Be A Sperm Or An Egg?

Female fish without the foxl3 gene produce fully functional sperm instead of eggs in their ovaries.

AsianScientist (Jun. 22, 2015) – Researchers in Japan have found, for the first time in vertebrates, a genetic switch that determines whether germ cells become sperm or eggs. The gene is named foxl3, and has been identified using a small fish called medaka (Oryzias latipes). The finding, published in Science, could help researchers learn more about how the sexual fate of germ cells is determined during vertebrate development.

In medaka without foxl3 function, sperm are produced in the ovaries of females. The sperm that are produced function normally, and have been confirmed to produce normal offspring.

Dr. Toshiya Nishimura, Associate Professor Minoru Tanaka and their colleagues of the National Institute for Basic Biology, National Institutes of Natural Sciences, in collaboration with Dr. Satoru Kobayashi of the Okazaki Institute for Integrative Bioscience and through joint research with Kyushu University’s Dr. Mikita Suyama and Dr. Yasuyuki Ohkawa, revealed that the foxl3 gene works in the germ cells of females “to suppress differentiation into sperm.”

The researchers found that foxl3, which is expressed in germ cells but not in the surrounding cells of the fish’s reproductive organs, provides a molecular cue that prevents the start of sperm formation. When the researchers disrupted foxl3 in adult fish with two X chromosomes (the female state), sperm formed in the female ovary. These sperm were functional and could fertilize eggs normally.

In females lacking functional foxl3 genes, the small fish’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. Applied research into uses for aquaculture is already underway.

“In spite of the environment surrounding the germ cells being female, the fact that functional sperm has been made surprised me greatly. That this sexual switch present in the germ cells is independent of the body’s sex is an entirely new finding,” Nishimura said.

The results indicate that germ cells in these fish—and potentially other vertebrates—do not need to be in the environment of the male reproductive organ to begin their switch into sperm.

“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. 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,” commented Tanaka.

The article can be found at: Nishimura et al. (2015) foxl3 Is A Germ Cell-Intrinsic Factor Involved In Sperm-Egg Fate Decision In Medaka .


Source: National Institutes of Natural Sciences.
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