Selfish DNA

Dividing female germ cells have asymmetric spindles (white) that can be exploited by “selfish” DNA to increase its chances of being passed on to offspring.

Takashi Akera

When a female germ cell divides during meiosis, it does so unevenly. As the cellular material is pulled apart from opposite ends, a set of chromosomes on the larger side produces the egg. Elements on the other side are expelled into a tiny cell called a polar body. Though each chromosome should have the same chance of entering the egg, previous research has identified a selfish (or parasitic) DNA element that has more than a random, fifty-fifty chance of being found in the egg. A new study has uncovered how such selfish DNA is able to increase its odds.

During female meiosis, the cell’s spindle machinery binds to the chromosomes at their centromere (a non-coding region of DNA) and tugs the chromosomes towards opposite ends of the dividing cell. A team led by Takashi Akera and Michael Lampson at the University of Pennsylvania showed that this spindle is asymmetric, with greater levels of the amino acid tyrosine at key points on the spindle on the future polar body side.

The researchers showed that chromosomes with selfish centromeres more often detach from the spindle side with greater tyrosination. Consequently, when selfish centromeres find themselves attached to the future polar body side of the spindle, they are more likely to detach themselves and get another chance at finding more stable reattachment on the future egg side. In other words, instead of going directly to the egg side, the chromosomes with selfish DNA randomly connect with the spindle on one side or the other, disconnect when they are on the polar body side, and stay connected when they are on the egg side, where they have the chance of being passed on to offspring.

When the researchers blocked the difference in tyrosination, the chromosomes with selfish DNA were just as likely to end up in the egg as in the polar body, showing that the amino acid is crucial for “selfish” centromere asymmetry. Future work is needed to explain the mechanics of how differences in tyrosination allows the “selfish” centromeres to detach. (Science)