Pre-fertilisation gamete thermal environment influences reproductive success, unmasking opposing sex-specific responses in Atlantic Salmon

The environment gametes perform in just before fertilisation is
increasingly recognised to affect offspring fitness, yet the contributions
of male and female gametes and their adaptive significance remain largely
unexplored. Here, we investigated gametic thermal plasticity and its
effects on hatching success and embryo performance in Atlantic salmon
(Salmo salar). Eggs and sperm were incubated overnight at 2°C or 8°C,
temperatures within the optimal thermal range of this species. Crosses
between warm- and cold-incubated gametes were compared using a
fully-factorial design, with half of each clutch reared in cold
temperatures and the other in warm temperatures. This allowed
disentangling single-sex interaction effects when pre-fertilisation
temperature of gametes mismatched embryonic conditions. Pre-fertilisation
temperature influenced the age at hatch, and matching sperm and embryo
temperatures resulted in earlier hatching. Warm incubation benefited eggs
but harmed sperm, reducing the hatching success and, overall, gametic
thermal plasticity did not enhance offspring fitness, indicating
vulnerability to thermal changes. We highlight the sensitivity of male
gametes to higher temperatures, and that gamete acclimation may not
effectively buffer against deleterious effects of thermal fluctuations.
From an applied angle, we propose the differential storage of male and
female gametes as a tool to enhance sustainability within the hatcheries.