The Segregation Distorter (SD) complex and the accumulation of deleterious genes in laboratory strains of Drosophila melanogaster

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.

Inversion polymorphism and accumulation of lethals in selected lines of Drosophila melanogaster

Five long-term selected lines of Drosophila melanogaster were monitored for the presence of lethals and inversions on the third chromosome for a period of nearly 100 generations after the cessation of selection. The results provide an example of the action of inversions as a trap for genes with homozygous detrimental effects in small populations. Lines polymorphic for cosmopolitan inversions In(3R)C and In(3L)P maintain linked lethal and detrimental genes in a near-balanced system. Consideration of the high selection response attained despite inversion polymorphism leads to the conclusion that extreme responses to selection can be obtained without exploiting the potential from large genomic regions.