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

Background/Objectives: Inactivation of the dnd gene involved in the development of primordial germ cells (PGCs) leads to the loss of gametes and halts reproductive development. Studies on sterile fish allow for the identification of genes and processes associated with GC differentiation. Methods: Atlantic salmon with GC-ablated testes were produced by temporal silencing of dnd. Gene expression was analyzed in sterile and fertile testes using 44k microarray and qPCR. Results: 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. Expression of 260 genes was undetectable 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. GC-ablated testes showed increased expression of reproductive regulators such as amh and sdy and numerous immune genes, suggesting a reprogramming of GC-depleted testes. Temporal silencing of dnd indicated common developmental processes in the brains and gonads of Atlantic salmon testis that become inactive in testes at first maturation. These processes may play roles in PGC homing, the creation of a specific environment required for spermatogenesis, or facilitating communication between the gonads.