No evidence for paternal age effects on sons or daughters, when accounting for paternal sperm storage

A father’s age at conception is predicted to affect not only his own
fertility but also his offspring’s fitness. Offspring born to old fathers
are assumed to be less fit than those of young fathers. However, under low
mating rates, paternal age might be confounded with the duration for which
mature sperm are stored in fathers prior to ejaculation. Studies that
disentangle the confounding paternal effects of sperm storage duration
from those of age, on offspring, are lacking. We use Drosophila
melanogaster to test the separate and interactive effects of paternal age
and sexual rest on offspring fitness. As expected, old fathers produce
fewer offspring than young fathers, however, paternal age does not
influence the survival or age-dependent reproductive success of sons or
daughters. Instead, a long duration of paternal sexual rest negatively
impacts the reproductive success of the conceived sons. Furthermore,
daughters of low reproductive quality selectively disappear with age, but
sons do not, highlighting that demographic processes can further modulate
paternal age effects. Overall, we highlight that paternal age effects
might not be as pervasive as previously assumed, and suggest that paternal
sexual rest might be more important in influencing offspring phenotypes.