Male reproductive failure rates between French and Zambian Drosophila melanogaster crosses deviate significantly from within-population crosses, and have a variable and sometimes cross-direction-specific pattern. (A) Histogram depicting percentage of male reproductive failure for between-population (blue) and within-population (yellow) cross directions. Values on the x-axis are the percentage of males that failed in all four reproductive assays, each involving one female from each parental population (binned by rounding percentages up to nearest whole number). Within-population cross averages are displayed for France (purple line) and Zambia (orange line), as well as between-population average (light blue line). In total, 36 of the 50 between-population crosses have reproductive failure rates higher than both within-population averages (bins right of both lines), and the rate of reproductive failure between these groups is significantly different (bootstrap p < 1.0 × 10⁻⁶). Individual cross counts are supplied in Supplementary Tables S4 and S5. (B) Male F2 reproductive failure rates among all possible crosses between five France and five Zambia strains, for both cross directions. Each cross is represented by a separate male failure rate for each cross direction (FR maternal top-left, ZI maternal bottom right). Individual cross directions with significant failure rates compared to within-population controls are bolded. Crosses whose direction-combined cross failure rates have significant raw p-values (p < .05) are highlighted pink, and those with .05 < p < .1 are highlighted yellow. These highlighted crosses were tested for differences between cross directions in failure rate, and crosses with significant (p < .05) differences are denoted by a blue solid line.