# Diagnosis of Betanodavirus Infection in the Gonad of Greater Amberjack Broodstocks Shows a Sex-Biased Infection and Immune Responses

## TL;DR

This study shows that a fish virus infects both male and female fish, but their immune responses differ, with males showing stronger reactions in their testes.

## Contribution

A non-lethal method for diagnosing NNV in fish gonads and revealing sex-biased immune responses in infected greater amberjack.

## Key findings

- NNV is present in similar proportions in the blood and gonads of both male and female greater amberjack.
- Infected males showed up-regulated immune genes in the testes, while infected females showed no immune changes in the ovaries.
- High estradiol levels in infected females may help combat the virus.

## Abstract

Knowledge about fish-pathogen interaction is essential for optimizing technical and biological aspect of culture techniques to guarantee the economic viability of the aquaculture production. Nodavirus (NNV) is one of the most prevalent viruses worldwide causing disease in more than 170 species. In this study, we aimed to develop a non-lethal method to diagnose NNV in the gonad of asymptomatic broodstock specimens of greater amberjack (Seriola dumerili) using a RTG cell line expressing the luciferase reporter gene under the control of the Mx promoter (RTG-pmx-luciferase system) and real-time PCR. We also characterize the immune response and reproductive health of both sexes by means of gene expression and functional parameters analysis. Our data showed that NNV is present in both male and female blood and gonads in similar proportions demonstrating that both sexes might develop persistent infection and transfer the virus to their progeny by vertical transmission. Regarding immune response, the levels observed in the testis were higher in infected males. Specifically, we observed an up-regulation of genes related to the interferon pathway, antimicrobial peptides and leucocyte-markers molecules in infected testis, which might lead to a functional alteration. Otherwise, there were no alterations of immune parameters in the infected ovaries suggesting a latent infection. Serum estradiol levels were high in infected females constituting a potential mechanism to fight against the virus. In conclusion, we optimized a non-lethal method for NNV diagnosis involving gonadal biopsies. Moreover, our study revealed sex-biased host-NNV interactions.

The online version contains supplementary material available at 10.1007/s00248-026-02733-2.