Why dog and cat can´t marry

or in more scientific terms: Why are F1 hybrids so often sterile or lethal? The Dobzhansky-Muller theory says that there is an incompatibility between genes with reduced fitness that have diverged between species. So far nobody has ever observed a D-M gene but a new Science paper describes two genes that separate D. simulans and D. melanogaster: lhr (lethal hybrid rescue) and hmr (hybrid male rescue). Yea, Yea.