Adipose tissue mitochondrial respiration in Atlantic salmon: implications for sex-dependent life-history variation

Adipose tissue is essential for energy homeostasis, with mitochondria having a central role in its function. Mitochondria-mediated adipose tissue dysfunction has been linked to several metabolic disorders in humans but surprisingly little is known about whether variation in adipose tissue processes, such as mitochondrial function, is maintained in wild animal populations, how it is related to reproductive success, and its evolutionary significance. Early sexual maturation (age-at-maturity) in Atlantic salmon ( Salmo salar ) is promoted by higher adiposity and has a strong genetic association with the vestigial like 3 ( vgll3 ) locus. Vgll3 is also linked to the control of adipose tissue growth and pubertal timing in mammals. This makes Atlantic salmon ideal for studying whether functional variation in adipose tissue processes is linked to reproductive timing. Here, using 16 Atlantic salmon individuals of different sexes reared in common-garden conditions, we conducted a proof-of-principle study and tested whether mitochondrial respiration in adipose tissue, mitochondrial DNA (mtDNA) amount, and adipocyte size are associated with sex and vgll3 genotype. Mitochondrial respiration was quantified from freshly collected white adipose tissue using high-resolution respirometry, and mtDNA amounts were quantified relative to nuclear DNA amounts. We found differences in mtDNA amount between sexes, plausibly caused by a confounding effect of maturation status, but without corresponding differences in mitochondrial respiration. Mitochondrial respiration, leak respiration, and coupling capacity (P/E ratio) were marginally lower in immature females carrying the vgll3 early maturation genotype compared to those with the late maturation genotype, although larger sample sizes are required to validate this difference. Our results generate new hypotheses on how coupling capacity of oxidative phosphorylation could be linked with the timing of maturation via adiposity as well as pave the way to study the mechanistic relationships between life-history variation and mitochondrial bioenergetics in wild populations. ### Competing Interest Statement The authors have declared no competing interest.