Test of the genetic basis of Haldane's rule

The chromosomes of one species (e.g., D. simulans) are shown in white and those of the other species (e.g., D. teissieri) in black. Sex chromosomes are shown at the left (X on top; Y, with a "hook", on the bottom). Haploid sets of autosomes shown at the right. All genotypes carry cytoplasm from the "white" species. The top genotype represents F1 male hybrids and the middle genotype represents F1 female hybrids. The bottom genotype depicts hybrid females who carry an attached X chromosome from the white species. The traditional explanation of Haldane's rule predicts that this "unbalanced" genotype will be inviable. (Population genetics models show that this theory requires an additional assumption: the alleles causing reproductive isolation act as loss-of-function mutations; that is, more recessive alleles must have greater homozygous effects on hybrid fitness.)