Rapid protamine evolution suppresses meiotic drive in Drosophila

Many animal species replace histones with protamines during spermatogenesis. Despite their importance for sperm function, protamines rapidly evolve in many species; the biological causes behind their rapid evolution remain unknown. Here, using in vivo gene replacement, we investigated the causes and consequences underlying the rapid evolution of protamine Mst77F, which is essential for male fertility in D. melanogaster. Mst77F ortholog replacements led to defects in DNA compaction of X-chromosome-bearing sperm compared to Y-chromosome-bearing sperm during spermatogenesis, resulting in fewer X-bearing mature sperm and male-biased progeny. Unlike D. melanogaster, Mst77F is not essential for male fertility in D. yakuba but is still required to suppress sex-ratio distortion. Our results suggest that relentless pressure to suppress sex chromosomal meiotic drive drives the rapid evolution of protamines.