Fine Mapping of Dominant <i>X</i>-Linked Incompatibility Alleles in <i>Drosophila</i> Hybrids

<div><p>Sex chromosomes have a large effect on reproductive isolation and play an important role in hybrid inviability. In <i>Drosophila</i> hybrids, <i>X</i>-linked genes have pronounced deleterious effects on fitness in male hybrids, which have only one <i>X</i> chromosome. Several studies have succeeded at locating and identifying recessive <i>X</i>-linked alleles involved in hybrid inviability. Nonetheless, the density of dominant <i>X</i>-linked alleles involved in interspecific hybrid viability remains largely unknown. In this report, we study the effects of a panel of small fragments of the <i>D. melanogaster X</i>-chromosome carried on the <i>D. melanogaster Y</i>-chromosome in three kinds of hybrid males: <i>D. melanogaster/D. santomea, D. melanogaster/D. simulans</i> and <i>D. melanogaster/D. mauritiana</i>. <i>D. santomea</i> and <i>D. melanogaster</i> diverged over 10 million years ago, while <i>D. simulans</i> (and <i>D. mauritiana</i>) diverged from <i>D. melanogaster</i> over 3 million years ago. We find that the <i>X</i>-chromosome from <i>D. melanogaster</i> carries dominant alleles that are lethal in <i>mel/san</i>, <i>mel/sim</i>, and <i>mel/mau</i> hybrids, and more of these alleles are revealed in the most divergent cross. We then compare these effects on hybrid viability with two <i>D. melanogaster</i> intraspecific crosses. Unlike the interspecific crosses, we found no <i>X</i>-linked alleles that cause lethality in intraspecific crosses. Our results reveal the existence of dominant alleles on the <i>X</i>-chromosome of <i>D. melanogaster</i> which cause lethality in three different interspecific hybrids. These alleles only cause inviability in hybrid males, yet have little effect in hybrid females. This suggests that <i>X</i>-linked elements that cause hybrid inviability in males might not do so in hybrid females due to differing sex chromosome interactions.</p>