Male sterility in Drosophila mojavensis × D. arizonae hybrids is not caused by gene movements in the Y chromosome.

Y chromosomes play important roles in sex determination, male fertility, and, in some cases, hybrid male sterility. Nearly 40 years ago, Zouros and co-workers found that Drosophila mojavensis males carrying a Y chromosome from the sibling species D. arizonae are sterile, with fertility restored by chromosome 4 from D. arizonae. While these findings could result from gene interactions, they may also suggest the movement of an essential male fertility gene from the Y to chromosome 4 in the D. arizonae lineage. To test this, we analyzed the Y-linked gene content of D. mojavensis, D. arizonae, and the closely related D. navojoa. We identified 5 previously unreported Y-linked genes, bringing the total to 14. These genes fit the pattern observed in other Drosophila species: they originated from autosomal, testis-specific genes that duplicated onto the Y chromosome. All Y-linked genes are shared between D. arizonae and D. mojavensis, ruling out the movement of a protein-coding gene as the cause of sterility. However, we found a huge difference in the copy number of GI26128, an HP1 gene family member, with ∼1,400 Y-linked copies in D. arizonae but only 6 in D. mojavensis, making it a candidate to explain the hybrid male sterility.