Chromosomal aberrations and early mortality in a non-mammalian vertebrate: example from triploid-induced Atlantic salmon

In commercial aquaculture, the production of triploid fish is currently the most practical approach to prevent maturation and farm-to-wild introgression following escapes. However, triploids often exhibit poor welfare, and the underlying mechanisms remain unclear. Inheritance issues associated with (sub-optimal) hydrostatic pressure treatments used to induce triploidy, or the genetic background of parental fish, have been speculated to contribute. Here, we quantified the frequency and type of chromosomal aberrations in Atlantic salmon subjected to a gradient of sub-optimal pressure treatments (Experiment 1) and from multiple mothers (Experiment 2). From these experiments, we genotyped a subsample of ~900 eyed eggs and all ~3,300 surviving parr across ~20 microsatellites. Chromosomal aberrations included aneuploidy and uniparental disomy, which are evidence of genetic nondisjunction. Although a low frequency of chromosomal aberrations were identified in eyed eggs from the diploid and triploid controls, our results demonstrated that sub-optimal pressure treatments resulted in lower triploidization rates, increased incidence of chromosomal aberrations, and higher egg mortality rates. We also observed maternal effects on triploidization success and incidence of chromosomal aberrations. In contrast to the results in eggs, chromosomal aberrations were rare among surviving parr, suggesting a purge of maladapted individuals during early development. This is the first study to document a link between chromosomal aberrations and early mortality in a non-mammalian vertebrate, and has implications for aquaculture breeding programs.