Yukiko Yamashita

One question about reproduction that we’ve been exploring is what influences whether offspring are male or female. In humans and flies, each sperm carries either an X or a Y chromosome. If a sperm with an X chromosome fertilizes an egg, the offspring is female. If a sperm with a Y chromosome fertilizes an egg, the offspring is male. The odds for either outcome appear to be fifty-fifty.

However, we’ve all seen a family that has only boys or only girls. Is this a coincidence? In fruit flies, biases toward one sex are a real biological phenomenon: some male flies produce many more X-carrying sperm than Y-carrying sperm. 

We’ve now discovered why this happens. A certain “selfish” version of the X chromosome can generate a toxin that kills sperm carrying the Y chromosome during sperm production. But the dilemma here is that if all the Y-carrying sperm die, there would be no males, and the species could go extinct. 

To prevent this, the cells that produce sperm divide asymmetrically, sacrificing half of the Y-carrying sperm to the toxin to rescue the other half. In this way, this X chromosome can spread to offspring without threatening the survival of the species. These discoveries raise some fascinating questions about sperm production.