Introduction
Sexual maturation before farmed Atlantic cod reaches the harvest size is coupled with economic losses and potential ecological concerns . Reduced appetite and allocation of energy from somatic growth into gonads starts prior to and continues throughout the spawning season causing a significant, up to 50%, loss in body weight. Investment in gonadal growth and significant loss in body weight translate into resource wastage in terms of feed costs. Additionally, spawning and spread of fertilized eggs from sea-cages poses a threat of genetic introgression from domesticated cod into the wild. Earlier genetic studies on early sexual maturation in cod have been either conducted in the infancy of cod aquaculture (Kolstad et al., 2006) or struggled with maturation frequencies suitable for genetic studies ((Drangsholt et al., 2014). Kolstad et al. (2006) estimated low to moderate heritability for sexual maturation registered at 2 years in two geographical locations: 0.16-0.29. Drangsholt et al. (2014) reported non-significant heritability for sexual maturation at 2 years from the same genetic material as presented here.
The assessment of selection potential against early sexual maturation in the National Cod Breeding Program after 5 generations of growth selection was prompted by the re-vitalisation of the Norwegian cod aquaculture.
Material and methods
I ndividually tagged Atlantic cod representing 104 full-sib families of the National Cod Breeding Program were tested for early sexual maturation at LetSea Research facilities (Dønna, Norway) . Experiment was run on a semi-commercial scale from 29th of January 2020 until 23rd of March 2021 following standard production protocols , except from light manipulation regimes to avoid sexual maturation . At the end of the experiment, fish were euthanized and individually registered for round weight (WT), length (LENGTH), liver and gonads weight, and sex (male/female) and the status of sexual maturation ( MAT, mature/immature) were determined by visual inspection after dissection . Gutted weight (GWT) was calculated by subtracting weights of liver and gonads from WT. Tissue samples were collected and 1272 individuals representing 67 families were genotyped with a newly developed 21K SNP- array (Illumina Infinium™ Array) . Additionally, h arvest weight (HWT) was recorded for 2011 sibs from 173 families in November 2021. Heritability for MAT was estimated with univariate sire-dam model both on the observed and underlying liability scale using ASReml (pedigree relationships)
, and with GCTA (genomic relationships, GWAS)
. Bivariate animal model was used for estimation of phenotypic and genetic correlations between growth traits (WT, GWT, LENGTH, HWT ) and MAT.
Results and discussion
Out of the 1680 registered individuals, 814 were females and 866 males, and 87.4% of the fish were classified as sexually mature The frequency of sexually mature females was 83.7% whereas maturation frequency in males was 90.9%. Mature (and female) fish were significantly heavier than immature ( and male) fish. These differences in round body weight were mainly due to differences in gonad and liver weight; there was only 30 g and 20g difference in GWT between mature and immature fish and between females and males, respectively.
Estimated heritability for MAT using pedigree relationships was moderate to high: 0.33±0.06 on the observed scale and 0.51± 0.10 on the underlying liability scale. Estimated heritability utilizing genomic relationships from a subsample of the material was somewhat lower: 0.23±0.04. Genetic correlations between MAT and growth traits were negative (favourable) and connected with high standard errors (Table 1) , indicating that selection for rapid growth has not affected the frequency of sexually mature fish at 2-years of age. GWAS detected a suggestive QTL for early sexual maturation at chromosome 23 where a single SNP crossed chromosome-wide threshold. Additionally, earlier detected QTL for sex was validated in our breeding population.
The significant additive genetic variation , and insignificant genetic interrelationships with growth traits, indicate that selective breeding has potential as a part of a solution to solve problems connected with sexual maturation in Atlantic cod without adversely affect ing the genetic gain in growth. The suggestive QTL for MAT should be confirmed from a larger data set.
References
Drangsholt, T. M. K., Nielsen, H. M., Bangera, R., and Puvanendran, V. 2014. Sexual maturation of 1 and 2 year old farmed Atlantic cod -- prevalence and heritability estimates. 10th World Congress on Genetics Applied to Livestock Production, Vancouver, BC, Canada.
Gilmour, A. R., Gogel, B. J., Cullis, B. R., Welham, S. J., and Thomson, R. 2015. ASReml User Guide Release 4.1. Structural Specification.
Kolstad, K., Thorland, I., Refstie, T., and Gjerde, B. (2006). Body weight, sexual maturity, and spinal deformity in strains and families of Atlantic cod (Gadus morhua) at two years of age at different locations along the Norwegian coast. ICES Journal of Marine Science 63 (2): 246-252. .https://doi.org/10.1016/j.icesjms.2005.11.007
Yang, J., Lee, S. H., Goddard, M. E., and Visscher, P. M. (2011). GCTA: a tool for genome-wide complex trait analysis. Am J Hum Genet 88 (1): 76-82. https://doi.org/10.1016/j.ajhg.2010.11.011.
Acknowledgements
We wish to acknowledge LetSea AS for running the experiment .