Intrinsically weak sex chromosome drive through sequential asymmetric meiosis.

Meiotic drivers are selfish genetic elements that bias their own transmission, violating Mendel's Law of Equal Segregation. It has long been recognized that sex chromosome-linked drivers present a paradox: Their success in transmission can severely distort populations' sex ratio and lead to extinction. This paradox is typically solved by the presence of suppressors or fitness costs associated with the driver, limiting the propagation of the driver. Here, we show that Stellate (Ste) in Drosophila melanogaster represents a novel class of X chromosome-linked driver that operates with an inherent mechanism that weakens its drive strength. Ste protein asymmetrically segregates into Y-bearing cells during meiosis I, subsequently causing their death. Unexpectedly, Ste segregates asymmetrically again during meiosis II, sparing half of the Y-bearing spermatids from Ste-induced defects, thereby weakening the drive strength. Our findings reveal a mechanism by which sex chromosome drivers avoid suicidal success.