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Nodavirosis is a viral disease that can cause mass die-offs in sea bass farms. Preferring warm waters (around 25°C), this virus is more prevalent in the Mediterranean, particularly in Greece, Italy and Turkey, where Europe's largest sea bass farms are concentrated. As it replicates within the infected organism, it causes brain lesions - similar to Parkinson's disease in humans - which impair swimming behaviour and, more often than not, lead to lethal cerebral haemorrhage.

Yet some sea bass seem to resist this disease. Why is this? What is the difference between resistant and susceptible individuals? Is it something to do with their genome? These questions have been motivating the work of Ifremer and INRAE teams since 2013. Their findings* have just been published in the journal Genetics Selection Evolution.

Scientists from Ifremer and INRAE, in collaboration with Anses, SYSAAF (Syndicat des sélectionneurs avicoles et aquacoles français) and professional hatcheries, searched the genomes of 7,000 sea bass for differences that could explain their ability to resist this disease. After several years of research, they finally pinpointed two key genes, called ZDHHC14 and IFI6, capable of synthesizing proteins involved in viral resistance.

Survival rates rise from 40% to 80

They showed that doubly resistant sea bass populations (homozygous - carrying the 2 "resistant" alleles) had survival rates of 80% when brought into contact with the virus. These figures drop to 60% for heterozygous sea bass populations carrying one "resistant" and one "susceptible" allele, and to 40% for doubly-susceptible populations.

We think that these genes act by reducing virus replication in sea bass, as was shown by a Japanese team in grouper," explains François Allal, researcher in aquaculture genetics and genomics at Ifremer. We have already launched a new project to use these markers to select resistant individuals and enable hatcheries to supply breeders with more resistant juveniles".

The discovery of these resistance markers is an important step for sea bass aquaculture in the Mediterranean and beyond. Although the virus is currently confined to the warm waters of the Mediterranean, its prevalence and range could increase on European coasts as a result of climate change.

* These results were obtained as part of the GeneSea and MedMax projects supported by the European Maritime Affairs and Fisheries Fund (FEAMP).