Exploring genetic variation in the sex determination signal in Drosophila melanogaster
Authors: Frederick Xu; Andrew Clark; Daniel Barbash
Affiliation: Department of Molecular Biology and Genetics, Cornell University, Ithaca, NY
Keywords: Genetic interactions
Sex determination mechanisms vary extensively across taxa, but the extent of variation within a sex determination pathway is not fully known. In Drosophila, sex is determined by counting X-Signal Elements (XSEs). The five XSEs—sisterlessA (sisA), sisterlessB (sisB), runt, unpaired (sisC), and diminutive (myc)—activate in females Sex lethal (Sxl), the primary sex determination switch. Activation occurs by sufficient XSE dosage when two X chromosomes are present to overcome maternally deposited repressors in the embryo. Sxl proteins subsequently control the somatic sex determination and dosage compensation pathways. In 1988, Thomas Cline found wide variation among wildtype D. melanogaster strains for sensitivity to perturbations in sex determination: some strains skewed heavily female-biased in their sex ratios when sisA and sisB were duplicated, while others skewed heavily male-biased when heterozygous for sisA and Sxl mutations. Interestingly, strains with sex ratios more sensitive to XSE duplications were more resilient to XSE mutations—and vice versa, suggesting that wildtype strains contain hidden variation that places them along a continuum of sensitivity to Sxl-dependent sex ratio perturbations. This begs the question: what is the source of this variation in Drosophila sex determination, and how does it shape the evolutionary dynamics of sex determination, sex ratios, and reproduction?
The Drosophila Synthetic Population Resource (DSPR) is a collection of ~1700 Recombinant Inbred Lines (RILs) generated from 15 founder lines that can be used for fine-scale mapping to a resolution of ~1-2cM. We mated D. melanogaster lines containing sisA, sisB, and/or Sxl mutations or duplicated sisA and sisB elements to the DSPR founder lines and scored progeny sex ratios to determine which founders exhibit the strongest F1 sex ratio skew, either female- or male-biased. After determining the optimal set of mapping RILs, we will mate the same mutant lines from the founder crosses with each of the selected RILs to find the genomic region(s) that harbor variation in progeny sex ratio. Since the DSPR has extensive SNP data for the founders and RILs, we can determine the founder identity of the region(s) driving sex ratio bias and compare it to the same region(s) in other founders in order to identify candidate genes underlying variation in sex ratio skew. Functional analyses will then elucidate mechanisms behind sex determination bias.