Steroid implants for the induction of vitellogenesis in feminized European silver eels (Anguilla anguilla L.)

Abstract

Assisted propagation of the European eel will lead to a closed production cycle supplying the aquaculture industry with juvenile glass eels. Females require long-term weekly treatment with pituitary extract (PE), which is stressful and causes abnormalities in oogenesis. We tested the effects of 17α-methyltestosterone (17 MT), as potent androgen activating the androgen receptor, and 17β-estradiol (E2), as an inducer of vitellogenesis, to shorten the duration of PE treatment.Four groups of feminized eels were subjected to a simulated migration and subsequent injection with implants containing 17 MT (17 MT-group), E2 (E2-group) or 17 MT plus E2 (17 MT + E2-group) to test for synergistic effects, or without any steroids as controls (C-group). The effects of a 2-months treatment were investigated by determining the eye index (EI), hepatosomatic and gonadosomatic index (HSI and GSI, respectively), plasma steroid concentrations by liquid chromatography mass spectrometry (LCMS), gonadal histology, expression of androgen receptors a and b (ara, arb); estrogen receptor 1 (esr1); FSH receptor (fshr); vitellogenin receptor (vtgr) and aromatase (cyp19), and the required number of weekly PE injections to fully mature. For many parameters, both the 17 MT and E2 groups showed an increase vs. controls, with the 17 MT + E2 group showing a synergistic effect, as seen for EI, GSI (3.4 for 17 MT and for E2, 6.6 for 17 MT + E2), oocyte diameter and ara, arb and esr1 expression. Concentrations of almost all focal steroids decreased with simulated migration and steroid treatment. Only eels of the 17 MT-group showed increased expression of cyp19 and of fshr, while fshr expression increased 44-fold in the 17 MT + E2 group, highlighting that co-implantation is most effective in raising fshr mRNA levels. Specific for eels of the E2 groups were vitellogenesis-associated changes such as an increase of HSI, plasma E2, and presence of yolk in the oocytes. Steroid treatments reduced the duration of PE treatment, again synergistically for co-implantation. In conclusion, E2 is necessary to start vitellogenesis, but 17 MT has specific effects on cyp19 and fshr expression. The combination is necessary for synergistic effects and as such, steroid implants could be applied in assisted reproduction protocols for European eel to improve oocyte quality leading to the production of more vital larvae.

The induction of oocyte maturation and ovulation in the European eel (Anguilla anguilla): in vitro and in vivo comparison of progesterone with 17α,20β-dihydroxy-4-pregnen-3-one

“…After the simulated migration, females were anesthetized with 2-phenoxyethanol (2 mL in 10 L water), PIT tagged (TROVAN, Aalten, Netherlands) and transferred to 373-L tanks connected to a recirculating system with artificial seawater (16°C, 36 ppt). Each female was injected in the peritoneal activity with a steroid implant containing 17-methyltestosterone (5 mg) and E2 (2 mg) for 2 months to induce the start of vitellogenesis ( Palstra et al, 2022 ).…”

“…After arrival, females were anesthetized, PIT tagged (TROVAN, Aalten, Netherlands), acclimatized in tanks (16°C, 36 ppt) for 13 weeks and injected with hCG at a dose of 3000 IU.kg −1 (Chorulon, MSD, Kenilworth, New Jersey, United Stated). One month later, wild females were injected in the peritoneal cavity with a steroid implant containing E2 (2 mg) for 2 months to induce the start of vitellogenesis ( Palstra et al, 2022 ).…”

Human Chorionic Gonadotropin Enhancement of Early Maturation and Consequences for Reproductive Success of Feminized European Eel (Anguilla anguilla)

“…The weekly peaks in hormone levels are associated with oocyte developmental abnormalities [21]. Therefore, alternatives for hypophysation are under investigation, such as steroid implants [22] and human chorionic gonadotropin (hCG).…”

Evaluation of the Reproductive Performance of Females of Anguilla anguilla Characterized by Different Levels of Silvering

“…For female eels, this means weekly repeated injection of carp (CPE) or salmon (SPE) [10] pituitary extract and a final injection of 17,20b-dihydroxy-4pregnen-3-one (DHP) [11][12][13]; the routine for males is a protocol of weekly human chorionic gonadotropin (hCG) injections [14]. New protocols have been developed, and there is a continuous effort to optimize the induction of maturity and artificial reproduction [15][16][17][18][19].…”

“…Other than the morphometric parameters, the reproductive performance (quantity and quality of the eggs/spermatozoa) is an important marker often used to determine the quality of the protocols used in artificial reproduction and aquaculture [16,17,22,23] and to evaluate the quality of broodstock for the restocking efforts of the natural populations [14,24,25]. In the case of female eels, the quantity aspect of the reproductive performance is usually determined by the relative fecundity (total egg per female/body weight, expressed in %), and absolute fecundity (total egg production per female) [16,26] while the quality aspect often requires further physiological tests to determine the differential effect of the specific diet or hormone treatment [15][16][17][18][19]26,27]. A quality indicator for Japanese eel (A. japonica) has been approximated by the buoyancy rate expressed as a percentage, as it has been shown to highlight differences in biochemical content [26].…”