Diploid sexual reproduction involves segregation of allelic pairs, ensuring equal representation of genotypes in the gamete pool. Some genes, however, are able to "cheat" the system by promoting their own transmission. The Segregation distorter (Sd) locus in Drosophila melanogaster males is one of the best-studied examples of this type of phenomenon. In this system the presence of Sd on one copy of chromosome 2 results in dysfunction of the non-Sd-bearing (Sd + ) sperm and almost exclusive transmission of Sd to the next generation. The mechanism by which Sd wreaks such selective havoc has remained elusive. However, its effect requires a target locus on chromosome 2 known as Responder (Rsp). The Rsp locus comprises repeated copies of a satellite DNA sequence and Rsp copy number correlates with sensitivity to Sd. Under distorting conditions during spermatogenesis, nuclei with chromosomes containing greater than several hundred Rsp repeats fail to condense chromatin and are eliminated. Recently, Rsp sequences were found as small RNAs in association with Argonaute family proteins Aubergine (Aub) and Argonaute3 (AGO3). These proteins are involved in a germline-specific RNAi mechanism known as the Piwi-interacting RNA (piRNA) pathway, which specifically suppresses transposon activation in the germline. Here, we evaluate the role of piRNAs in segregation distortion by testing the effects of mutations to piRNA pathway components on distortion. Further, we specifically targeted mutations to the aub locus of a Segregation Distorter (SD) chromosome, using ends-out homologous recombination. The data herein demonstrate that mutations to piRNA pathway components act as enhancers of SD. E VOLUTION of sexual organisms relies on the faithful segregation and transmission of alleles from one generation to the next, allowing unbiased exposure of these alleles to natural selection. Nevertheless, nature contains multiple examples of genes that violate this basic tenet of Mendelian inheritance and act selfishly to ensure their own propagation (Lyttle 1991). One such phenomenon, known as meiotic drive, occurs when one of two alleles alters the gametic ratio to enhance its own representation in the next generation, violating Mendel's first law (Sandler and Novitski 1957). First discovered more than 50 years ago, Segregation Distorter (SD) in Drosophila melanogaster is one of the beststudied examples of this type of "selfish" genetic behavior Temin et al. 1991;Kusano et al. 2003).Segregation Distorter chromosomes contain a dominant gain-of-function mutation that strongly favors the transmission of the SD chromosome from [SD/SD + ] heterozygous males by causing dysfunction of wild-type (SD + ) sperm Sandler and Hiraizumi 1960b;Hartl et al. 1967;Tokuyasu et al. 1977;Temin et al. 1991). While the SD/SD + male transmits the SD chromosome to as many as 99% of his progeny, the gametes of heterozygous females show normal Mendelian segregation Burt and Trivers 2008;Larracuente and Presgraves 2012). The Segregation distorter (Sd) locus w...

Evidence for the chemical conversion of adenosine-to-inosine (A-to-I) in messenger RNA (mRNA) has been detected in numerous metazoans, especially those "most successful" phyla: Arthropoda, Mollusca, and Chordata. The requisite enzymes for A-to-I editing, ADARs (adenosine deaminases acting on RNA) are highly conserved and are present in every higher metazoan genome sequenced to date. The fruit fly, Drosophila melanogaster, represents an ideal model organism for studying A-to-I editing, both in terms of fundamental biochemistry and in relation to determining adaptive downstream effects on physiology and behavior. The Drosophila genome contains a single structural gene for ADAR (dAdar), yet the fruit fly transcriptome has the widest range of conserved and validated ADAR targets in coding mRNAs of any known organism. In addition, many of the genes targeted by dADAR have been genetically identified as playing a role in nervous system function, providing a rich source of material to investigate the biological relevance of this intriguing process. Here, we discuss how recent advances in the use of ends-out homologous recombination (HR) in Drosophila make possible both the precise control of the editing status for defined adenosine residues and the engineering of flies with globally altered RNA editing of the fly transcriptome. These new approaches promise to significantly improve our understanding of how mRNA modification contributes to insect physiology and ethology.