Testis mRNA expression in wild and hatchery-produced greater amberjack (Seriola dumerili).

INTRODUCTION The greater amberjack (Seriola dumerili) is a promising emergent aquaculture species. Greater amberjack males caught from the wild and reared in captivity exhibited small seminiferous tubules, a precocious arrest of spermatogenesis and high levels of apoptosis. In the present study, a comparative analysis of testis transcriptome of wild adult meagre versus hatchery-produced breeders is reported, as part of a wider research aiming at describing the effects of rearing in captivity on gene expression throughout the reproductive axis. METHODS Three wild and 6 hatchery-produced greater amberjack males were sampled on 31 May - 01 June 2021. Wild fish were caught around the Pelagic Islands (Sicily, Italy) and sampled immediately after death. Farmed fish belonged to a broodstock produced from fertilized eggs obtained through hormonal induction of spawning and reared in a sea cage of Argosaronikos Fishfarming S.A. (Salamina, Greece). Fish reproductive state was evaluated by gonado-somatic index (GSI) and gonad histological analysis. Total RNA was extracted from testis samples, checked for quantity and quality and then used to prepare the mRNA libraries. A pooled sample was then submitted to sequencing in Illumina NextSeq platform (Illumina Inc., U.S.A.) using paired-end 2x75 strategy. Sequencing raw data were quality-checked, cleaned and aligned onto the Seriola dumerilii reference genome using different bioinformatic tools. Read counts per gene and differential gene expression analysis was carried out and pathways involving differentially expressed genes (DEGs) were investigated using KEGG pathway (http://www.genome.jp/kegg/pathway.html). RESULTS & DISCUSSION Testes from wild greater amberjack (WIDL group) showed normal spermatogenic activity. Among the six hatchery-produced fish, four showed normal spermatogenesis (non-dysfunctional farmed fish, NF group) and two showed evident reproductive dysfunction, characterized by low GSI, smaller seminiferous tubules and reduced spermatogenic activity (dysfunctional farmed fish, DF group). The three groups underwent a comparative transcriptome analysis using the RNA-seq technology. A total of nine libraries were produced through Illumina platform, each resulting in the production of an average 25 million paired-end reads. Almost the 90% of produced reads were uniquely mapped to the reference genome. A high number (2157) of DEGs was found between WILD and DF groups and between DF and NF groups (1986), whereas a small number (74) of DEGs was observed between WILD and NF groups. The analysis of KEGG pathways evidenced that the observed reproductive dysfunction was associated to cell death, cell cycle and proliferation pathways. The present study improved our understanding of the molecular mechanisms underlying spermatogenesis impairment of greater amberjack reared in captivity. Ongoing analyses of pituitary and hypothalamus transcriptome will provide a comprehensive view of the effects of confinement in captivity on the reproductive axis of this species.