Abstract
BackgroundAtlantic halibut (Hippoglossus hippoglossus) is a high-value, niche market species for cold-water marine aquaculture. Production of monosex female stocks is desirable in commercial production since females grow faster and mature later than males. Understanding the sex determination mechanism and developing sex-associated markers will shorten the time for the development of monosex female production, thus decreasing the costs of farming.ResultsHalibut juveniles were masculinised with 17 α-methyldihydrotestosterone (MDHT) and grown to maturity. Progeny groups from four treated males were reared and sexed. Two of these groups (n = 26 and 70) consisted of only females, while the other two (n = 30 and 71) contained balanced sex ratios (50% and 48% females respectively). DNA from parents and offspring from the two mixed-sex families were used as a template for Restriction-site Associated DNA (RAD) sequencing. The 648 million raw reads produced 90,105 unique RAD-tags. A linkage map was constructed based on 5703 Single Nucleotide Polymorphism (SNP) markers and 7 microsatellites consisting of 24 linkage groups, which corresponds to the number of chromosome pairs in this species. A major sex determining locus was mapped to linkage group 13 in both families. Assays for 10 SNPs with significant association with phenotypic sex were tested in both population data and in 3 additional families. Using a variety of machine-learning algorithms 97% correct classification could be obtained with the 3% of errors being phenotypic males predicted to be females.ConclusionAltogether our findings support the hypothesis that the Atlantic halibut has an XX/XY sex determination system. Assays are described for sex-associated DNA markers developed from the RAD sequencing analysis to fast track progeny testing and implement monosex female halibut production for an immediate improvement in productivity. These should also help to speed up the inclusion of neomales derived from many families to maintain a larger effective population size and ensure long-term improvement through selective breeding.
Highlights
Atlantic halibut (Hippoglossus hippoglossus) is a high-value, niche market species for cold-water marine aquaculture
Synteny analysis showed that DNA sequences containing Atlantic halibut sexassociated Single Nucleotide Polymorphism (SNP) were consistently clustered in several other fish genomes. These results suggested that sex determination in H. hippoglossus is likely to be monogenic (XX/XY) and localised within a 3.2 cM window on linkage group 13
Hormonal sex reversal and neomale verification The control group exhibited a sex ratio not significantly different from 1:1 (52% ♂; 48% ♀), whereas 97% of the group treated for six weeks (5 ppm) and 70% of the group treated for three weeks (10 ppm) were confirmed as phenotypic males (Table 1)
Summary
Atlantic halibut (Hippoglossus hippoglossus) is a high-value, niche market species for cold-water marine aquaculture. Gonochoristic animals show genetic and/ or environmental sex-determining mechanisms. In environmental sex-determining systems, the environment plays a decisive role, such as temperature in turtles, alligators and fish [1,7,8]. Both systems can interact in some species such as in O. latipes, which has an XX/XY genetic system, where high temperatures can cause female-tomale sex reversal [9,10,11]. The understanding of sex determination systems in fish has direct commercial applications, given the strong sexual dimorphism exhibited in a wide variety of aquaculture fish species for a range of commercially important traits like growth or age at maturity
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