Non-Mendelian segregation of sex chromosomes in heterospecific Drosophila males

Interspecific hybrids and backcrossed organisms generally suffer from reduc ed viability and/or fertility. To identify and genetically map these defect s, we introgressed regions of the Drosophila sechellia genome into the D. s imulans genome. A female-biased sex ratio was observed in 24 of the 221 rec ombinant inbred lines, and subsequent tests attributed the skew to failure of Y-bearing sperm to fertilize the eggs. Apparently these introgressed lin es fail to suppl ess a normally silent meiotic drive system. Using molecula r markers we mapped two regions of the Drosophila genome that appear to exh ibit differences between D. simulans and D. sechellia in their regulation o f sex chromosome segregation distortion. The data indicate that the sex rat io phenotype results from an epistatic interaction between at least two fac tors. We discuss whether this observation is relevant to the meiotic drive theory of hybrid male sterility.