Abstract
BackgroundFarmed Atlantic salmon are one of the most economically significant global aquaculture products. Early sexual maturation of farmed males represents a significant challenge to this industry and has been linked with the vgll3 genotype. However, tools to aid research of this topic, such as all-male and clonal fish, are still lacking. The present 6-year study examined if all-male production is possible in Atlantic salmon, a species with heteromorphic sex chromosomes (males being XY, females XX), and if all-male fish can be applied to further explore the vgll3 contribution on the likelihood of early maturation.ResultsEstrogen treatment of mixed sex yolk sac larvae gave rise to one sexually mature hermaphrodite with a male genotype (XY) that was used to produce both self-fertilized offspring and androgenetic double haploid (dh) offspring following egg activation with UV treated sperm and pressure shock to block the first mitotic division. There were YY supermales among both offspring types, which were crossed with dh females. Between 1 and 8% of the putative all-male offspring from the eight crosses with self-fertilized supermales were found to have ovaries, and 95% of these phenotypic females were also genetically female. None of the offspring from the one dh supermale cross had ovaries. When assessing the general contribution of the vgll3 locus on the likelihood of early post-smolt sexual maturation (jacking) in the all-male populations we found individuals that were homozygous for the early maturing genotype (97%) were more likely to enter puberty than individuals that were homozygous for the late maturing genotype (26%). However, the likelihood of jacking within individuals with an early/late heterozygous genotype was higher when the early allele came from the dam (94%) compared to the sire (45%).ConclusionsThe present results show that supermale Atlantic salmon are viable and fertile and can be used as a research tool to study important aspects of sexual maturation, such as to further explore the sex dependent parental genetic contribution to age at puberty in Atlantic salmon. In addition, we report the production of viable double haploid supermale fish.
Highlights
Farmed Atlantic salmon are one of the most economically significant global aquaculture products
When kept in stimulatory rearing environments, sexual maturity rates in domesticated male Atlantic salmon (Salmo salar) postsmolts may exceed 80% [8,9,10] during the early seawater phase when the fish are around 500 g, and the problem is primarily caused by the use of elevated rearing temperature together with photoperiod manipulation [9, 11, 12]
Neo-female and supermale production In the present study we managed to produce one hermaphroditic Atlantic salmon with male genotype and female phenotype, who produced viable sperm and eggs. It is unclear why the success of the sex-reversal was so low as Piferrer and Donaldson [30] reported 100% success following the same protocol in chinook salmon (Oncorhynchus tshawytscha) and a slightly better success than with use of Estradiol-17β, and Johnstone et al [31] reports 100% feminisation in Atlantic salmon after feeding Estradiol-17β during first feeding
Summary
Farmed Atlantic salmon are one of the most economically significant global aquaculture products. Aquaculture continues to expand rapidly on a global basis and is regarded as an important future source of protein production to feed the ever-growing human population Within this food-production sector, Atlantic salmon (Salmo salar L.) represents one of the most highly domesticated [1] and economically significant species [2], accounting for approximately 2.6 million tonnes of production in 2019. Artificial spring/ summer like conditions of warm environment combined with a short day to continuous light switch are used to enhance early life growth and/or induce the parr-smolt transformation, the process by which salmon alter their physiology before transition from freshwater to seawater. There is a need to develop strategies for reducing pre-harvest sexual maturation in Atlantic salmon
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