Transcriptome of Sterile Testes in <em>dnd</em>-Depleted Atlantic Salmon (<em>Salmo salar</em> L.) Highlights Genes Involved in Gonadal and Brain Development

Inactivation of the dnd gene, which is essential for the development of primordial germ cells (PGCs), leads to the loss of gametes and halts further reproductive development in vertebrates. Studies on the gonads of sterile fish are useful for identifying GC-related genes, as well as the structures and processes dependent on this lineage. We identified genes affected in GC-ablated testes of Atlantic salmon following temporal silencing of dnd and tracked their expression during normal gonadal development. In sterile testes, transcripts of several GC markers were detected at low levels, suggesting the presence of cells with a GC-related expression profile that failed to initiate spermatogenesis. We found 260 genes silenced in the gonads of sterile males and females, and 61.5% of these were also inactivated during first maturation of fertile testes. This group was enriched with genes highly expressed in the brain, including those involved in endocrine and paracrine regulation, synaptic transmission, and numerous genes critical for brain development, among them, 45 genes encoding homeobox proteins. Another group of 229 genes showed increased expression in developing testes and included genes involved in neurosecretion and brain development regulation. Genes upregulated in GC-ablated testes included reproductive regulators such as amh and sdy, as well as numerous genes related to the innate and adaptive immune systems, suggesting a reprogramming of germ cell-depleted testes. Temporal silencing of dnd revealed the presence of a complex neural structure in the Atlantic salmon testis that becomes inactive at first maturation. Potential roles for this structure may include PGC homing, the creation of a specific environment required for spermatogenesis, or facilitating communication between the gonads and brain to signal readiness for reproduction, among other possibilities.