A Protamine Knockdown Mimics the Function of Sd in Drosophila melanogaster

Segregation Distorter (SD) is an autosomal meiotic drive system found worldwide in natural populations of Drosophila melanogaster This gene complex induces the preferential and nearly exclusive transmission of the SD chromosome in SD/SD males. This selfish propagation occurs through the interplay of the Sd locus, its enhancers and the Rsp locus during spermatid development. The key distorter locus, Sd, encodes a truncated but enzymatically active RanGAP (RanGTPase-activating protein), a key nuclear transport factor in the Ran signaling pathway. When encoded by Sd, RanGAP is mislocalized to the nucleus interior, which then traps Ran inside the nucleus and disrupts nuclear import. As a result of this aberrant nuclear transport, a process known as the histone-to-protamine transition that is required for proper spermatid condensation fails to occur in SD/SD⁺ males. In this process, sperm-specific protamine proteins enter the spermatid nucleus and replace the formerly chromatin-complexed histones. Previously, we have shown that mutations affecting nuclear import and export can enhance distortion in an SD background, thus verifying that a defect in nuclear transport is responsible for the unequal transmission of chromosomes. Herein, we show that specifically reducing protamines induces distortion in an SD background, verifying that protamines are transported via the RanGAP/GEF pathway and indicating that E(SD) plays a significant and unique role in the process of distortion.