Effects of triploidy on genetic gains in a rainbow trout (Oncorhynchus mykiss) population selectively bred for diploid growth performance

Authors: Leeds, Timothy - Tim; Weber, Gregory - Greg

Submitted to: Aquaculture
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/2/2019
Publication Date: 3/4/2019
Citation: Leeds, T.D., Weber, G.M. 2019. Effects of triploidization on genetic gains in a rainbow trout (Oncorhynchus mykiss) population selectively bred for diploid growth performance. Aquaculture. 505:481-487. https://doi.org/10.1016/j.aquaculture.2019.03.003.
DOI: https://doi.org/10.1016/j.aquaculture.2019.03.003

Interpretive Summary: Domestic rainbow trout are marketed at approximately 500 grams. There is interest in marketing the fish at larger sizes (equal to or greater than 1 kilogram), but at sexual maturation growth and fillet quality traits are adversely affected. Selective breeding and triploidization - a process that exposes newly-fertilized eggs to hydrostatic pressure or heat to produce sterile fish with three sets of chromosomes instead of two - are complimentary strategies to produce faster-growing rainbow trout that can be marketed at larger sizes while avoiding negative effects of sexual maturation. In addition, triploid fish are important to the industry because their sterility prevents genetic contamination with native populations and provides intellectual property protection for breeding programs. Whereas the genetic improvement of diploid rainbow trout growth performance is straightforward, genetic improvement of triploid rainbow trout is complicated by the fact that they are sterile and thus cannot be used for breeding. Some studies have shown limited correlation of familial growth performance between the diploid and triploid fish, leading authors to speculate that selective breeding based on diploid performance may not be effective for improving triploid performance. To test this, we compared growth performance of a selectively-bred line to a randomly-mated control line as diploids and triploids up to 21 months post-hatch (approximate body weight of 3 kilograms). Body weights of the selectively-bred line averaged 20.6% larger as diploids, and 21.5% larger as triploids, compared to the randomly-mated control line. Thus, this study demonstrates that selective breeding based on diploid growth performance is also effective for improving triploid growth performance to and beyond traditional market size in rainbow trout.

Technical Abstract: The aim of this study was to evaluate the effects of triploidization on genetic gains made via selective breeding of a rainbow trout population for diploid growth performance to the standard ~500-gram US market weight and beyond (greater than 1 kg). Selection was applied to estimated breeding values for 10-month body weight (i.e., approximate market weight) and thermal growth coefficient between 10 and 13 months of age. Growth traits of diploid and triploid fish from the selected line (10 families) were compared to diploid and triploid fish derived from a contemporary, randomly-mated control line (7 families) between 5 and 21 months of age (approximately 50 grams and 3.2 kg, respectively). Both lines were pedigreed, monosex female, and derived from the same base population, and at the time of this study were differentiated by two generations of selective breeding. Genetic line did not interact with ploidy type at any age during the study to affect body weight (P greater than or equal to 0.15), fork length (P greater than or equial to 0.40), or condition factor (0.36), thus growth superiority of the selected line compared to the control line was similar regardless of ploidy type. For body weight, superiority of the selected line compared to the control line increased each month between 5 and 11 months of age, at which time the selected line was 28.6% larger as diploids and 28.3% larger as triploids. Body weight superiority of the selected line decreased gradually thereafter in both ploidy types, and at the end of the study (21 months of age) the selected line was 15.2% larger as diploids and 22.2% larger as triploids. Despite this maintenance of genetic gain as triploids, family was found to interact with ploidy type (P less than or equal to 0.04) to affect body weight between 5 and 13 months of age. This suggests that further improvement in triploid growth performance may be possible if selection is practiced on triploid instead of diploid performance, but the amount of phenotypic variation explained by this interaction generally decreased over time from 16.4% (5 months of age) to 7.5% (13 months of age) when select-line triploids approached 1 kg. Genetic gains in growth performance in this study were maintained in triploid fish, thus demonstrating that direct selection on diploid performance is also effective in improving triploid performance.