Dissecting the genetic basis of selfish sex chromosomes

性别比例（SR）染色体是破坏Y染色体传播的X染色体变体，

Sex-Ratio (SR) chromosomes are X chromosome variants that subvert the transmission of Y chromosomes, distort the sex ratio of progeny and gain an evolutionary advantage at a cost to organismal fitness. SR chromosomes initiate intense genetic conflict between X-linked distorters, Y-linked targets, and an array of autosomal suppressors. This evolutionary arms race provides a powerful explanation of meiotic sex chromosome inactivation, turnover of sex chromosomes, and evolution of sex determination systems. SR chromosomes are thought to target satellite repeats and heterochromatic regions found on degenerate Y chromosomes, yet very little is known about the specific genetic basis or molecular mechanisms of distortion. This is because SR systems are primarily known from non-model organisms and are often bound up in complex genomic rearrangements. Without characterizing the underlying genetic basis and molecular mechanisms it remains impossible to directly connect the arms race initiated by selfish elements to broader phenomena in the evolution of meiosis and sex chromosome systems. I develop two independent methods to circumvent the confounding effects of chromosomal inversions and dissect the genetic basis underlying this selfish behavior in closely related Drosophila species. Specific Aim 1: Developing a mutagenesis approach to identify the genes causing Sex-Ratio distortion in D. pseudoobscura. Through saturation mutagenesis of SR chromosomes, I identify X-linked of genes contributing to distortion by screening for reduced transmission. To date, >5,000 lines have been screened (95% saturation), with 33 lines exhibiting reduced distortion. Using CRISPR-Cas9 genome editing, I propose to test the necessary and sufficient conditions of resulting candidate loci and place them into a functional pathway for SR distortion. Specific Aim 2: Engineering a synthetic chromosomal inversion to allow recombination mapping of Sex-Ratio distortion in D. persimilis. This approach adapts the Flp/FRT site-specific recombination tools to generate a perfectly collinear non-driving chromosome to allow free recombination in the region containing all necessary and sufficient genes for SR distortion. Once candidate genes are mapped, validated, and organized into a functional pathway for D. persimilis, a comparative analysis of these two systems will test whether SR mechanisms are unique or shared. Together, these two methods will provide the most complete genetic architecture of sex-linked segregation distorters to date, open the door to understanding the molecular mechanisms of distortion in two species, and for the first time explicitly test both the evolutionary conservation hypothesis and the population genetic hypothesis that Sex-Ratio distortion is determined by epistatically interacting X-linked genes bound together by chromosomal inversions. The proposed research provides training opportunities in both genomic analyses and genetic engineering, and establishes a strong basis for my continuing research program dissecting the evolutionary and cellular basis of selfish sex chromosomes.