Felix G. Ayson

Growth Regulation by Insulin-like Growth Factor-I in Fish

Insulin-like growth factor-I (IGF-I) is a mitogenic polypeptide that plays an essential role in the regulation of development and somatic growth of vertebrates, mainly by mediating growth hormone actions. It has clearly been established that the structure of IGF-I and its biological function has been highly conserved among vertebrates. In this paper, we review the recent developments in the molecular, biochemical, and physiological properties of IGF-I in fish.

Thyroxine injection of female rabbitfish (Siganus guttatus) broodstock: changes in thyroid hormone levels in plasma, eggs, and yolk-sac larvae, and its effect on larval growth and survival

Abstract The thyroid hormone levels in female rabbitfish, Siganus guttatus , plasma, eggs, and yolk-sac larvae were measured before and after thyroxine (T 4 ) injection in female spawners at doses of 1, 10, and 100 μ g T 4 /g body weight (BW) fish. T 4 and triiodothyronine (T 3 ) levels in maternal plasma, eggs, and yolk-sac larvae were elevated following T 4 administration. Apparently, there is conversion of T 4 into T 3 in the broodfish which suggests the presence of the enzyme, 5′-monodeiodinase, in rabbitfish. T 4 and T 3 in maternal circulation were easily transferred into the oocytes and subsequently into the larvae. Larvae from spawners treated with 10 and 100 μ g T 4 /g BW tended to be longer and showed sornewhat better survival compared to the control and those injected with 1 μ g T 4 /g BW 7 days after hatching. These findings suggest that thyroid hormones may play an important role during early larval development of rabbitfish.

The effect of stress on spawning of brood fish and survival of larvae of the rabbitfish, Siganus guttatus (Bloch)

The effects of stress due to handling, and repeated sham and human chorionic gonadotropin (HCG) injections on spawning and survival were studied. Results showed that stress significantly enhanced spawning in captive females (P<0.05), but apparently has no significant effect on the survival of larvae. The results indicate that factors other than stress are responsible for the high variability in larval survival in the hatchery. In addition, the results clearly demonstrate the necessity of exogenous gonadotropin to ensure 100% monthly spawning of captive S. guttatus females.

Milkfish (Chanos chanos) growth hormone cDNA cloning and mRNA expression in embryos and early larval stages

In an attempt to understand growth regulation in milkfish, the milkfish growth hormone (GH) and its cDNA were characterized and the expression of GH mRNA in embryos and larvae was examined by RT-PCR. The milkfish GH was purified from an alkaline extract of the pituitary by reverse-phase high-performance liquid chromatography and detected as an immuno-positive protein with anti-salmon GH serum. The complete sequence of milkfish pre-GH was determined by cDNA cloning and nucleotide sequencing. On the basis of the N-terminal amino acid analysis of the native protein, the pre-GH was found to consist of a signal peptide of 22 amino acids and a mature protein of 188 amino acids. Milkfish GH shows higher amino acid sequence identity with GHs of carps (91–94%) and salmonids (70%) than with GHs of more advanced teleosts (<60%) in good accordance with its taxonomic position in teleosts. It has five half Cys residues, four of which are at positions homologous with those of other known GHs and the extra Cys with those of carp GHs. The molecular weight of milkfish GH was estimated to be 22 kDa, which is comparable to the theoretical value. This suggests that milkfish GH is a simple protein, although it has two potential N-glycosylation sites. Semiquantitative RT-PCR showed that GH mRNA expression was relatively weak in embryos and newly hatched larvae but was already strong in 2-day old and older larvae.

Induced spawning of rabbitfish, Siganus guttatus (Bloch) using human chorionic gonadotropin (HCG)

Abstract A positive spawning response of rabbitfish females to two injections of HCG at 2 I.U./g BW given 24 h apart was observed. The latency period after hormone injection was inversely related to the initial oocyte size. Minimum initial oocyte size required for spawning without hormone injection was 0.46 mm. HCG induction of spawning was necessary for females with initial oocyte size of ≤0.45 mm. Number of eggs spawned (424000), fertilization rate (96%), and hatching rate (59%) did not differ from those of naturally spawned fish.

Gonadal response of juvenile protogynous grouper (Epinephelus fuscoguttatus) to long term recombinant follicle-stimulating hormone administration

Abstract The role of follicle‐stimulating hormone (FSH) in the gonadal development of protogynous hermaphroditic grouper (Epinephelus fuscoguttatus) was investigated. Recombinant giant grouper (E. lanceolatus) FSH (rggFSH) was produced in yeast. Its receptor‐binding capacity and steroidogenic potency were confirmed in vitro. Weekly injections of rggFSH to juvenile tiger grouper for 8 weeks (100 &mgr;g/kg body weight, BW) resulted in significantly larger and more advanced oocytes (cortical alveolar stage vs primary growth stage in control). Sustained treatment with rggFSH (20 to 38 weeks at 200 &mgr;g/kg BW) resulted in significant reduction in gonad size, degeneration of oocytes, and proliferation of spermatogonial cells, indicative of female to male sex change. Gene expression analysis showed that, while initiating female to male sex change, the rggFSH significantly suppressed the steroidogenic genes cyp11b, cyp19a1a, and foxl2 which restrained the endogenous production of sex steroid hormones and thus prevented the differentiation of spermatogonial cells. Expression profile of sex markers dmrt1, amh, figla, and bmp15 suggests that the observed sex change was restricted at the initiation stage. Based on these results, we propose that the process of female to male sex change in the protogynous grouper is initiated by FSH, rather than sex steroids, and likely involves steroid‐independent pathway. The cortical alveolar stage in oocyte development is the critical point after which FSH‐induced sex change is possible in grouper.