Identification and genetic analysis of a pervasive ‘needle-eye’ sperm phenotype in Drosophila sterile hybrid males

Interspecies hybrid sterility has been extensively studied, especially in the genus <em>Drosophila</em>. Hybrid sterility is more often found in the heterogametic (XY or ZW) sex, a trend called Haldane's rule. Although this phenomenon is pervasive, identification of a common genetic mechanism remains elusive, with modest support found for a range of potential theories. Here, we identify a single precise morphological phenotype, which we call 'needle-eye sperm', that is associated with hybrid sterility in three separate species pairs that span the <em>Drosophila</em> genus. The nature of the phenotype indicates a common point of meiotic failure in sterile hybrid males. We used 10 generations of backcross selection paired with whole-genome pooled sequencing to genetically map the regions underlying the needle-eye (NE) sperm phenotype. Surprisingly, the sterility phenotype was present in ~50% of males even after 10 generations of backcrossing, and only a single region of the X chromosome was associated with sterility in one direction of backcross. Owing to the common phenotype among sterile male hybrids<em>,</em> and the strong effect of individual loci, further exploration of these findings may identify a universal mechanism for the evolution of hybrid sterility.