# Limited inbreeding avoidance at the gamete level despite inbreeding depression in Atlantic salmon (Salmo salar)

## TL;DR

Domesticated Atlantic salmon show limited inbreeding avoidance at the gamete level, with reduced fertilization rates but no survival differences in offspring.

## Contribution

This study is the first to test post-mating inbreeding avoidance in full sibling crosses of externally fertilizing fish.

## Key findings

- Sperm motility and fertilization rates were reduced when activated in sibling females' ovarian fluid.
- Offspring from sibling crosses were smaller but showed no survival differences.
- Paternity outcomes were unaffected in sperm competition trials between sibling and non-sibling males.

## Abstract

Inbreeding and the associated increase in homozygosity and potential accumulation of deleterious alleles may reduce fitness in a process known as inbreeding depression. Mechanisms to mitigate reproduction between close relatives, ranging from pre‐mating mate choice to post‐mating gamete selection, have evolved across taxa. In external fertilisers like Atlantic salmon (Salmo salar), where females have limited control over paternity, mechanisms of inbreeding avoidance can be expected to evolve at the gamete level.Philopatric Atlantic salmon may run the risk of breeding between relatives, particularly in small populations, and frequent escapes from aquaculture settings are augmenting the chances of adults from the same sibling cohort overlapping in the wild, raising potential ecological and sustainability concerns. Moreover, the presence of inbreeding avoidance mechanisms between full siblings in externally fertilising fish is currently untested.This study tested post‐mating inbreeding avoidance mechanisms in domesticated Atlantic salmon. In a paired breeding design, we compared sperm motility parameters in sibling and non‐sibling ovarian fluid, and assessed fertilisation and hatching success, growth rate and paternity in sperm competition trials between sibling and non‐sibling males.Sperm activated in ovarian fluid of sibling females showed lower values of motility‐related parameters and led to an average of 18% reduction in fertilisation rates in the resulting crosses. Furthermore, offspring from sibling crosses were smaller before the onset of sexual maturation. However, we found no difference in survival rates between sibling and non‐sibling cross offspring. Besides, when sperm from sibling and non‐sibling males were competing simultaneously for the same egg batch, we found no influence of this on paternity.Our findings reveal the presence of post‐mating inbreeding avoidance at the gamete level in Atlantic salmon, but its effects are limited in competitive scenarios.Our results have implications for salmonid conservation and aquaculture, where small natural or closed domestic strains may both display a degree of inbreeding. Mating between escaped domestic and wild salmonids could favour admixed over wild or feral crosses if an inbreeding avoidance mechanism is present; although this remains to be tested in more outbred crosses.

Inbreeding and the associated increase in homozygosity and potential accumulation of deleterious alleles may reduce fitness in a process known as inbreeding depression. Mechanisms to mitigate reproduction between close relatives, ranging from pre‐mating mate choice to post‐mating gamete selection, have evolved across taxa. In external fertilisers like Atlantic salmon (Salmo salar), where females have limited control over paternity, mechanisms of inbreeding avoidance can be expected to evolve at the gamete level.

Philopatric Atlantic salmon may run the risk of breeding between relatives, particularly in small populations, and frequent escapes from aquaculture settings are augmenting the chances of adults from the same sibling cohort overlapping in the wild, raising potential ecological and sustainability concerns. Moreover, the presence of inbreeding avoidance mechanisms between full siblings in externally fertilising fish is currently untested.

This study tested post‐mating inbreeding avoidance mechanisms in domesticated Atlantic salmon. In a paired breeding design, we compared sperm motility parameters in sibling and non‐sibling ovarian fluid, and assessed fertilisation and hatching success, growth rate and paternity in sperm competition trials between sibling and non‐sibling males.

Sperm activated in ovarian fluid of sibling females showed lower values of motility‐related parameters and led to an average of 18% reduction in fertilisation rates in the resulting crosses. Furthermore, offspring from sibling crosses were smaller before the onset of sexual maturation. However, we found no difference in survival rates between sibling and non‐sibling cross offspring. Besides, when sperm from sibling and non‐sibling males were competing simultaneously for the same egg batch, we found no influence of this on paternity.

Our findings reveal the presence of post‐mating inbreeding avoidance at the gamete level in Atlantic salmon, but its effects are limited in competitive scenarios.

Our results have implications for salmonid conservation and aquaculture, where small natural or closed domestic strains may both display a degree of inbreeding. Mating between escaped domestic and wild salmonids could favour admixed over wild or feral crosses if an inbreeding avoidance mechanism is present; although this remains to be tested in more outbred crosses.

Our study investigates inbreeding avoidance mechanisms at the gamete level in domesticated Atlantic salmon using sperm motility traits assessments, fertilisation trials, and paternity analysis under competitive conditions. We demonstrate that ovarian fluid from sibling females reduces sperm motility traits and fertilisation rates but found no significant differences in offspring survival or paternity outcomes under sperm competition. These findings provide critical insights into the limited efficacy of post‐mating mechanisms for inbreeding avoidance in domesticated salmon, highlighting implications for aquaculture management and wild salmonid conservation. Moreover, the presence of inbreeding avoidance mechanisms between full siblings in externally fertilising fish is currently untested.