Puberty in male atlantic cod (Gadus morhua)

Abstract

Puberty is the process by which a juvenile acquires for the first time the capacity to reproduce sexually, a critical basis for the conservation of a species. In general, the biological processes constituting the first gonadal maturation and the mechanisms involved in this process are similar among vertebrates. Internal and/or external signals trigger the secretion of gonadotropin-releasing hormone (Gnrh) by neurosecretory neurons in the ventral forebrain, and Gnrh stimulates the synthesis and secretion of two pituitary hormones, the gonadotropins known as follicle-stimulating hormone (Fsh) and luteinizing hormone (Lh). These two glycoproteins are the major hormones regulating the remarkable structural and functional changes occurring in the pubertal gonads during the activation of the two principal gonad tasks, the production of gametes and sex steroids. In context with the male-biased problems of precocious testis maturation in farmed cod, the present thesis focuses on male puberty. Pituitary fsh gene expression starts to increase at the beginning of testis development (proliferation of spermatogonia), followed by a slow elevation in testosterone plasma levels. With rapidly continuing spermatogenesis, which takes place in a peculiar spatio-temporal organization in the testicular lobes, high gonado-somatic indexes of up to 16 % are attained. Close to spawning, testosterone and 11-ketotestosterone, two potent ligands for the cod androgen receptor (Ar), peak in parallel with pituitary lh and testicular lhr (Lh receptor) gene expression, while fshr seems to be expressed in a constitutively manner in the cod testis. It is possible that attaining high plasma levels of potent androgens involves an auto-protective manner of androgen production, where weak Ar ligands, such as androstenedione and 11-ketoandrostenedione, are produced by the testis, thereby avoiding androgen-induced down-regulation of steroidogenic enzymes, while conversion to 17߭hydroxylated, potent androgens would occur in extra-gonadal tissues. After spawning, pituitary gonadotropin expression and androgens plasma levels decrease to basal levels, and only non-proliferating type A spermatogonia are present as germ cell population in the testis, conditions characterizing the spent condition. Like in many other teleost fish living at high latitudes, seasonally changing parameters are crucial environmental cues for cod. In this context, the present thesis also provides important information on the effects of constant light, a commonly used tool to prevent gonad maturation in cod farms, on the activity of the pituitary-testis axis. For instance, we found that even under constant light conditions, some males can escape the light-mediated inhibition and develop, associated with elevated pituitary lh and testicular ar expressions, spermatogenic activity including sperm production, albeit in a qualitatively and quantitatively compromised manner