Analysis of a Strong Suppressor of Segregation Distorter in Drosophila melanogaster.

Segregation Di st orter (SD) is a naturally occurring male meiotic drive system in Drosophila melanogaster, characterized by almost exclusive transmission of the SD chromosome owing to dysfunction of sperm receiving the SD+ homolog. Previous studies identified at least three closely linked loci on chromosome 2 required for distortion: Sd, the primary distorting gene; E(SD) (Enhancer of SD), which increases the strength of distortion; and Rsp (Responder), the apparent target of Sd Strength of distortion is also influenced by linked upward modifiers including M(SD) (Modifier of SD) and St(SD) (Stabilizer of SD), and by various unlinked suppressors. Although Sd is known to encode a mutant RanGAP protein, none of the modifiers have been molecularly identified. This work focuses on the genetic and cytological characterization of a strong X-linked suppressor, Su(SD), capable of restoring Mendelian transmission in SD/SD+ males. Sd and its cohort of positive modifiers appear to act semiquantitatively in opposition to Su(SD) with distortion strength depending primarily on the total number of distorting elements rather than which particular elements are present. Su(SD) can also suppress male sterility observed in certain SD genotypes. To facilitate its eventual molecular identification, Su(SD) was localized by deletion mapping to polytene region 13C7-13E4 These studies highlight the polygenic nature of distortion and its dependence on a constellation of positive and negative modifiers, provide insight into the stability of Mendelian transmission in natural populations even when a drive system arises, and pave the way for molecular characterization of Su(SD) whose identity should reveal new information about the mechanism of distortion.