Pituitary mRNA expression of FSH and LH in yellowbelly flounder (Rhombosolea leporina)

Abstract

Recent interest has been expressed in cultivating the native, yellowbelly flounder (Rhombosolea leporina). This is largely due to its high market and cultural value. Currently, there is little research on this species, both in an aquaculture setting and from an ecological perspective. This study focused on assessing pituitary mRNA expression of two gonadotropin-beta subunit genes, follicle-stimulating hormone (fshβ) and luteinising hormone (lhβ), in wild-caught yellowbelly flounder, to better understand captive reproduction for aquaculture.

The first study characterised the expression of both pituitary fshβ and lhβ in wild-caught yellowbelly flounder in different stages of ovarian development: 1) previtellogenic (PVO), 2) early vitellogenic (EVit), 3) vitellogenic (VIT), and 4) final oocyte maturation (FOM). Both genes showed significant increases in expression between stages 1 and 4. The expression patterns of both pituitary fshβ and lhβ were similar, with a trend of increasing levels across all ovarian stages to peak at stage 4. However, peak expression levels were much greater in fshβ. This is unexpected compared to typical fish models, where pituitary lhβ expression is often many-fold greater than fshβ during oocyte maturation. The reason for this apparent anomaly in peak expression levels is unknown. However, the general profile of increasing levels of co-expressed pituitary fshβ and lhβ throughout oogenesis likely reflects the multiple-batch group synchronous ovarian development found in yellowbelly flounder.

The second study focused on quantifying fshβ and lhβ expression over a five-day period following the injection of gonadotropin-releasing hormone analogue (GnRHa). Each female yellowbelly flounder was assigned to one of four treatment groups: 25 μg/kg, 50 μg/kg, 100 μg/kg, or a control group (0 μg/kg). Although there were no statistically significant increases in fshβ or lhβ expression from day 1 to day 5, an upward trend in both lhβ and fshβ expression was observed in fish treated with 25 μg/kg of GnRHa. Similarly, an increasing trend in lhβ expression was observed in fish treated with 50 μg/kg of GnRHa. GnRHa-treated fish accounted for the highest number of individuals entering FOM, comprising 87% of all fish that reached this stage. Additionally, 85% of all fish treated with GnRHa progressed to early maturation (EM) or further. Interestingly, the 100 μg/kg GnRHa treatment resulted in a downward trend of both fshβ and lhβ expression on both days 1 and 5 post-treatment. Despite this, the 100 μg/kg GnRHa treatment exhibited the joint-highest proportion of oocytes reaching maturity (33.3%) and had 90% of individuals reaching the early migratory (EM) stage or beyond. However, none of the fish were observed to ovulate over the 5-day experimental period.