Reproductive performance of alternative male phenotypes of growth hormone transgenic Atlantic salmon (Salmo salar)

Growth hormone (GH) transgenic Atlantic salmon (Salmo salar) is one of the first transgenic animals being considered for commercial farming, yet ecological and genetic concerns remain should they enter the wild and interact reproductively with wild fish. Here, we provide the first empirical data reporting on the breeding performance of GH transgenic Atlantic salmon males, including that of an alternative male reproductive phenotype (i.e. small, precocially mature parr), in pair-wise competitive trials within a naturalised stream mesocosm. Wild anadromous (i.e. large, migratory) males outperformed captively reared transgenic counterparts in terms of nest fidelity, quivering frequency and spawn participation. Similarly, despite displaying less aggression, captively reared nontransgenic mature parr were superior competitors to their transgenic counterparts in terms of nest fidelity and spawn participation. Moreover, nontransgenic parr had higher overall fertilisation success than transgenic parr, and their offspring were represented in more spawning trials. Although transgenic males displayed reduced breeding performance relative to nontransgenics, both male reproductive phenotypes demonstrated the ability to participate in natural spawning events and thus have the potential to contribute genes to subsequent generations.

Neuroendocrine control of growth hormone in fish

The biological actions of growth hormone (GH) are pleiotropic, including growth promotion, energy mobilization, gonadal development, appetite, and social behavior. Accordingly, the regulatory network for GH is complex and includes many endocrine and environmental factors. In fish, the neuroendocrine control of GH is multifactorial with multiple inhibitors and stimulators of pituitary GH secretion. In fish, GH release is under a tonic negative control exerted mainly by somatostatin. Sex steroid hormones and nutritional status influence the level of brain expression and effectiveness of some of these GH neuroendocrine regulatory factors, suggesting that their relative importance differs under different physiological conditions. At the pituitary level, some, if not all, somatotropes can respond to multiple regulators. Therefore, ligand- and function-specificity, as well as the integrative responses to multiple signals must be achieved at the level of signal transduction mechanisms. Results from investigations on a limited number of stimulatory and inhibitory GH-release regulators indicate that activation of different but convergent intracellular pathways and the utilization of specific intracellular Ca(2+) stores are some of the strategies utilized. However, more work remains to be done in order to better understand the integrative mechanisms of signal transduction at the somatotrope level and the relevance of various GH regulators in different physiological circumstances.

The effects of N-methyl-D,L-aspartate and gonadotropin-releasing hormone on in vitro growth hormone release in steroid-primed immature rainbow trout, Oncorhynchus mykiss

In the present study we investigated the effects of 17 beta-estradiol (E2) and 5 alpha-dihydrotestosterone (5 alpha DHT) on the ability of the glutamate agonist, N-methyl-D,L-aspartate (NMA), to stimulate growth hormone (GH) release from perifused pituitary glands of sexually immature rainbow trout (Oncorhynchus mykiss). Two weeks after steroid hormone implantation, pituitary glands were removed from the fish and challenged with NMA (10(-8) M) in a perifusion unit. NMA rapidly and significantly elevated GH release from perifused pituitary fragments taken from all treatment groups, and there was a main effect of in vivo steroid hormone treatment on the in vitro GH response to NMA. To examine the relationship between NMA and gonadotropin-releasing hormone on GH release, pituitaries from E2- and 5 alpha DHT-primed and control fish were exposed to a single pulse of salmon gonadotropin-releasing hormone (sGnRH) which also elicited a significant elevation in GH release from perifused pituitary fragments, although the response in the E2- and 5 alpha DHT-primed fish was significantly smaller (P < 0.05) than that for the NMA challenge. Administration of a specific GnRH antagonist, D-pGlu1,D-Phe2,D-Trp3,6-LHRH, did not affect the GH response to NMA, whereas administration of the NMDA receptor antagonist DL-2-amino-5-phosphonovaleric acid blocked the GH response to NMA. These data suggest that NMA acts to stimulate GH release directly at the level of the somatotroph, likely through the NMDA receptor and not through increased release of GnRH.