Loss of Increases Germ Cell Apoptosis but Is Still Compatible With Sperm Production in Atlantic Salmon ().

Entering meiosis strictly depends on () gene function in mammals. This gene is missing in a number of fish species, including medaka and zebrafish, but is present in the majority of fishes, including Atlantic salmon. Here, we have examined the effects of removing on male fertility in Atlantic salmon. As in mammals, expression was restricted to germ cells in the testis, transcript levels increased during the start of puberty, and decreased when blocking the production of retinoic acid. We targeted the salmon gene with two gRNAs one of these were highly effective and produced numerous mutations in , which led to a loss of wild-type (WT) expression in F0 salmon testis. In maturing crispants, the spermatogenetic tubuli were partially disorganized and displayed a sevenfold increase in germ cell apoptosis, in particular among type B spermatogonia and spermatocytes. The production of spermatogenic cysts, on the other hand, increased in maturing crispants. Gene expression analysis revealed unchanged () or reduced levels () of transcripts associated with undifferentiated spermatogonia. Decreased expression was recorded for some genes expressed in differentiating spermatogonia including and or in spermatocytes, such as . Different from -deficient mammals, a large number of germ cells completed spermatogenesis, sperm was produced and fertilization rates were similar in WT and crispant males. While loss of increased germ cell apoptosis during salmon spermatogenesis, crispants compensated this cell loss by an elevated production of spermatogenic cysts, and were able to produce functional sperm. It appears that also in a fish species with a gene in the genome, the critical relevance this gene has attained for mammalian spermatogenesis is not yet given, although detrimental effects of the loss of were clearly visible during maturation.