(STUP×B6)F1 (PB, sample size n = 51), (B6×STUP)F1 (BP, n = 72), STUP (n = 38), and B6 (n = 55). (A), epididymal sperm count (SCe) versus age (days); (B), relative testis weight (rTW, mg TW per g BW) versus age (days). The letters symbolize values for the time point of the four genotypes named in ABC order from the left (value “a” for B6); the same letter used for the value of another genotype (e.g., “aa”) means that there is no significant difference between the two values (P>0.05, ANOVA) and a different letter used for another genotype (e.g., “b” for STUP) indicates a significant difference (P<0.05, ANOVA) between the values at these time points; log(SCe +1) was used for statistical analysis instead of SCe. SCe was significantly lower in young adult (60 to 85 days of age) versus aged adult F1 hybrids (100 to 130 days of age; see Table 6 for statistics), but even the aged hybrids did not reach the parameters of parental controls.