During European eel assisted reproduction, timely administration of hormones that induce oocyte maturation and ovulation is a major factor influencing subsequent egg quality. This treatment commonly comprises one injection of fish pituitary extract (PE) as a primer followed by a 17α, 20β-dihydroxy-4-pregnen-3-one (DHP) injection. In this context, the present study aimed at optimizing timing of the dual hormone administration by applying a lipid droplet-based oocyte maturation scale, previously developed for Japanese eel to determine the maturational status of each female. Using wild-caught female eels, the potential effect of female size, egg fatty acid composition and dry weight on egg quality was also analyzed. Larval survival at 3 days post hatch was used to differentiate High-and Low-quality egg batches. Results showed that lipid droplet diameter was significantly smaller in High-quality eggs than in Low-quality egg batches, indicating that females producing High-quality eggs received the PE primer and DHP generally at an earlier developmental stage than those producing Low-quality batches. These results confirm that oocyte lipid droplet diameter is a useful indicator of female maturational status for optimization of induction of oocyte maturation and ovulation in European eel. Additional parameters, including female size, egg fatty acid composition and dry weight, were similar between high and low quality egg batches. This insight regarding the fatty acid composition of eggs obtained from wild-caught female eels may help advancing the development of tailored diets for increased reproductive success of farmed broodstock.
In captivity, oogenesis and ovarian follicle maturation in European eel can be induced experimentally using hormonal therapy. The follicle's ability to respond effectively to the induction of maturation and ovulation, resulting in viable eggs, depends on the oocyte stage at the time of induction. We hypothesized that variation in the expression of key hormone receptors in the ovary and size of oocyte lipid droplets are associated with changes in oocyte stage. Thus, we induced ovarian follicle maturation using a priming dose of fish pituitary extract followed by the administration of a 17α, 20β-dihydroxy-4-pregnen-3-one (DHP) injection. Females were then strip-spawned, the eggs were fertilized in vitro, incubated and larval survival was recorded at 3 days post hatch (dph). The expression of gonadotropin receptors (fshr, lhcgr1 and lhcgr2) and estrogen receptors (esr1, esr2a, esr2b, gpera and gperb) was quantified and the size of oocyte lipid droplets measured. Larval survival at 3 dph was used to differentiate high- and low-quality egg batches. Results showed significantly higher abundance of lhcgr1 and esr2a at priming for high-quality egg batches whereas fshr and gperb transcripts were significantly higher at DHP injection for low-quality egg batches. Therefore, high levels of lhcgr1 and esr2a may be important for attaining follicular maturational competence, while high fshr and gperb mRNA levels may indicate inadequate maturational competence. Furthermore, lipid droplet size at DHP and in ovulated eggs was significantly smaller in high-quality egg batches than in low-quality, which indicates that droplet size may be a useful marker of follicular maturational stage.
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