Introduction
Rainbow trout (Oncorhynchus mykiss) is the most important branch of the freshwater aquaculture sector in Europe, responsible for over 75% of the production. Despite of that, the sector suffers from specific problems such as low disease resistance (Tobback et al. 2007) and variability of reproductive performance (Bonnet, Fostier, and Bobe 2007) that are contributing to the stagnation of the production in Europe (FEAP 2020). Lack of genetic information on the strains produced nowadays also put at risk the production and species management (FAO 2020). The sector thus calls for improvement of the productive chain in order to provide food security in a sustainable manner. The selective breeding performed nowadays can be highly enhanced by gathering objective data from various production phases where linkage between the reproductive performance and future performance of the offspring is largely ignored (Janssen et al. 2015). Non-genetic inheritance (NGI) mechanisms are responsible for all heritable features that breeders transmit to offspring that are not directly part of the DNA sequence but still interact or derive from it (Adrian-Kalchhauser et al. 2020). Fish eggs content, in this context, constitutes maternal NGI with major impact in early embryonic development and also later consequences. Investigating these mechanisms can help to shed light on maternal impact on offspring performance and they could also, in a long term run, be considered during selective breeding programs. This project aims to characterize three different molecules responsible for NGI in females and to which extent they are implicated in offspring performance by studying breeders, embryos and juveniles from three farmed strains of rainbow trout.
Materials and methods
Experiments were conducted to evaluate (1) egg quality and thus, maternal investment for reproduction, offspring performance regarding (2) growth and (3) disease resistance and (4) NGI potentially implicated in (1)-(3). More specifically, proteins, mRNA and miRNA profiles in eggs. Additionally, immune response related genes in liver, spleen and gills of fish from (3) will also be assessed.
To assess egg quality, eggs from six females from each strain were fertilized using a pool of sperm of males from the same strain. Fertilization, eyed stage, hatching and mortality rates were recorded for (1). Hatched embryos from all females from one strain were pooled and once larvae reached swimming-up stage, total biomass from each strain was calculated. Larvae were placed into glass tanks for (2) in triplicate. Weekly, larvae were weighted and measured (n=30) for food offer correction and growth evaluation. Mortality during this period was also recorded. When fish reached average 6g individual weight, 162 fish were sampled for (3). They were divided in control and infected group in triplicate (n=27). Both fish groups were subjected to a bath for 1h. Infected group bath contained Yersinia ruckeri at a concentration of 4.5x108 while control group contained only tank water. After, all fish were placed back into their original tanks. Fish tissues that will be used for molecular analysis were sampled and snap frozen before, at three, five and seven days post-infection in both control and infected group (n=6). Mortality and major infection signs were recorded up to 14 days post-infection.
For (4), a portion of unfertilized freshly stripped eggs from same females as the above mentioned experiments were snap frozen for molecular analysis. These samples will be used for RNA and protein extraction and subsequent mRNA, miRNA and proteins profiling that will be compared between and within strains.
Results
High quality eggs were obtained from the two strains already evaluated. Fertilization (93.4±3.8 and 85.8±3.1%), eyed stage (84.7±5.2 and 90.5±3.6%), hatching (97.2±1.0 and 97.5±2.2%), egg mortality (2.8±1.0 and 2.6±2.2%) and larvae mortality rates (1.1±0.5 and 0%) did not present differences between strains A and B. In growth performance, however, difference was observed between both strains from the fourth week. No mortality difference during this period was observed between strains. Concerning bacterial challenge, mortality in strain B started earlier and is already double of the one observed in strain A. Trial is still in progress. Trials for third strain have started and molecular analysis of fish tissues will be performed in May/June. Thus, zootechnical and molecular data will be available for presentation at the conference.
Discussion
Commonly used in farmed mammals, progeny tests are largely applied to select the best breeders and thus improve overall stock quality (Rosa 2013). Selection based on offspring performance is a valid option also for fish selective programs once enough information on progeny is available. Our results will contribute in providing this knowledge in terms of early survival, growth, late mortality and disease resistance. NGI has the potential for changing offspring gene expression and, as a consequence, change offspring phenotype and their interaction with environmental challenges. From this perspective, it is of highest importance to identify mechanisms responsible for expression and/or heritability of particular traits in relation to the genetic background and experience of the population studied. Therefore, the knowledge on maternal NGI implicated in the production of high quality offspring in different fish populations may contribute to improve selective breeders programs and hence aquaculture sustainability.
Acknowledgments
This project is funded by the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 847639 and by the Polish Ministry of Education and Science. Authors thank Dabie Hatchery (Bytów, Poland) for providing biological material and information on the studied strains.
References
Adrian-Kalchhauser, Irene, Sonia E. Sultan, Lisa N. S. Shama, Helen Spence-Jones, Stefano Tiso, Claudia Isabelle Keller Valsecchi, and Franz J. Weissing. 2020. “Understanding ‘Non-Genetic’ Inheritance: Insights from Molecular-Evolutionary Crosstalk.” Trends in Ecology & Evolution 35 (12): 1078–89.
Bonnet, Emilie, Alexis Fostier, and Julien Bobe. 2007. “Characterization of Rainbow Trout Egg Quality: A Case Study Using Four Different Breeding Protocols, with Emphasis on the Incidence of Embryonic Malformations.” Theriogenology 67 (4): 786–94.
FAO, SOFIA. 2020. The State of World Fisheries and Aquaculture 2020. FAO.
FEAP, Federation of European Aquaculture Producers. 2020. “European Aquaculture Production Report.”
Janssen, Kasper, Hervé Chavanne, Paul Berentsen, and Hans Komen. 2015. “Rainbow Trout (Oncorhynchus Mykiss) – Current Status of Selective Breeding in Europe,” 12.
Rosa, G. J. M. 2013. “Progeny Testing.” In Brenner’s Encyclopedia of Genetics (Second Edition), edited by Stanley Maloy and Kelly Hughes, 466–67. San Diego: Academic Press.
Tobback, E., A. Decostere, K. Hermans, F. Haesebrouck, and K. Chiers. 2007. “Yersinia Ruckeri Infections in Salmonid Fish.” Journal of Fish Diseases 30 (5): 257–68.