Introduction
Mussel is one of the most cultured and high-value species in France. French mussel culture entirely depends on the wild spat collection. In the last decade, Abnormal Mass Mortality (AMM) has been observed in mussel farms along the Atlantic coasts, and the causes are not clearly established. Recent studies suggest that genetic factors are involved in mortality outbreaks, and poor cytogenetic quality is one among them (Degremont et al., 2019; Benabdelmouna et al., 2018). Abnormal mass mortality was shown to be strongly correlated to genomic abnormalities, in particular with a poor cytogenetic quality which is defined as a lower proportion of diploid to non-diploid cells in their haemolymph (Benabdelmouna and Ledu, 2016). The purpose of our study is to estimate the response to selection/realized heritability of cytogenetic quality using a divergent selection in the two main mussel species cultivated in France and to explore the relationship between the cytogenetic quality and the resistance to mortality using a cohabitation protocol with wild mussels sampled in site regularly impacted by AMM.
Materials and methods
Wild adult mussels were collected for M. edulis in Agnas and for M. galloprovincialis in Biarritz in January 2022. For each species, 420 mussels were phenotyped for cytogenetic quality after collecting haemolymph using flow cytometer analyses (FCM). For each species, a divergent selection was applied using an intensity of selection of 1.40 (i.e. 20% of the population selected for each group) producing a low and a high cytogenetic quality groups, respectively LCQ and HCQ, as well as a control group. In May 2022, five replicate spawns per group were produced for each species using two to five females and three to six males per spawning. Spawning, larval rearing and grow-out culture were carried out in our experimental hatchery at ASIM, Ifremer, La Tremblade, France. The cytogenetic quality of each replicate spawn will occur in summer 2023. Survival evaluation started on March 2023, using a cohabitation protocol between groups and wild mussels sampled in a mussel farm impacted by AMM. Thirty mussels per replicate spawns are tested in triplicate in a flow-through tanks, and mortality is recorded weekly until the summer 2023.
Results
The mean proportions of non-diploid cells were significantly higher for M. edulis (12%) than M. galloprovincialis (9%) (p <0.0001). The previous studies on non-diploid cells percentage of French mussels stocks varies between 2.44% to 21.37% (Sample size = 74-117) before mortality events (Benabdelmouna and Ledu, 2016). The mean proportions of non-diploid cells of LCQ, control and HCQ mussels were 28%, 12%, and 3% for M. edulis, and 26%, 9%, and 2% for M. galloprovincialis. HCQ has been found significant between the species (p <0.0001). However, LCQ have not showed any significant difference (p = 0.36) between species. The mean of non-diploid percentage of selected parents is shown in figure 1, and those of their progenies will be known during the summer, as well as their survival examined in the cohabitation experiment will be identified in summer. The mean mortality of the mussel species after 45 days was showed less than 1%.
Expected outcomes
It will be the first report on the possibility of selection for cytogenetic quality in shellfish. The previous results suggest that mortality level is significantly negatively correlated with the cytogenetic quality of the mussel. Selection for cytogenetic quality would not only improve the survival against the specific pathogen, but it should also improve the overall health status/fitness of the animal. If the cytogenetic character is heritable, it will give an important insight into the selection approach on the cytogenetic quality of the shellfish and will lead to new selection criteria for disease/survival/welfare traits of the shellfish.
References
Benabdelmouna, A., Ledu, C., 2016. The mass mortality of blue mussels (Mytilus spp.) from the Atlantic coast of France is associated with heavy genomic abnormalities as evidenced by flow cytometry. J Invertebr Pathol 138, 30-38. https://doi.org/10.1016/j.jip.2016.06.001
Benadelmouna, A., Saunier, A., Ledu, C., Travers, M.-A., Morga, B., 2018. Genomic abnormalities affecting mussels (Mytilus edulis-galloprovincialis) in France are related to ongoing neoplastic processes, evidenced by dual flow cytometry and cell monolayer analyses. J Invertebr Pathol 157, 45-52. https://doi.org/10.1016/j.jip.2018.08.003
Degremont, L., Maurouard, E., Rabiller, M., Glize, P., 2019. Response to selection for increasing resistance to the spring mortality outbreaks in Mytilus edulis occurring in France since 2014. Aquaculture 511. https://doi.org/ARTN 73426910.1016/j.aquaculture.2019.734269