Segregation Distorter mechanism

A selfish fruit-fly chromosome disables sperm carrying its competitor. How does it do that selectively?[1] [2]

A fruit fly’s competing chromosome copies need not have equal chances of reaching offspring. How can one copy cause sperm bearing the competing copy to fail while sparing sperm that carry it? In the Segregation Distorter system, the vulnerable sperm bear sensitive Responder repeats. Its main driver, Sd-RanGAP, is an altered copy of a gene involved in moving material into and out of the cell nucleus; Responder is a repeated-DNA target. Different inherited driver combinations, or haplotypes, share these components.[1] [2]

The driver gene and a repeated-DNA target are known. Changes in molecules copied from the target DNA, together with genetic experiments, link the target to sperm survival in one driver variant. Another variant responds differently, leaving the complete selective-failure mechanism open.[1]

The SD-Mad haplotype reduces short RNA molecules copied from Responder DNA, and increasing RNA production from the target weakens drive in the tested genetic backgrounds. SD-5 does not show the same responses. The observations do not identify the full chain from driver activity to selective sperm failure, nor establish one RNA mechanism for every haplotype.[1]

It reveals how genes can prosper while reducing their host’s fertility.[1] [2]

Selection can favor a chromosome because rival sperm fail, even if the fly loses fertility. Host suppression and chromosome rearrangements can then become part of the conflict. Studying the selective failure also tests whether repeat-derived RNAs help protect developing sperm, rather than treating repeat DNA as biologically inert.[1] [2]