Changes in Sex Steroids and Ovarian Steroidogenic Enzyme mRNA Levels in Artificially Maturing Japanese Eel (Anguilla japonica) and Naturally Maturing New Zealand Longfin Eel (Anguilla dieffenbachii) during Vitellogenesis

Abstract

Repeated hormone injections are routinely used to induce advanced stages of oogenesis in freshwater eels, but this approach may result in aberrant germ cell development. To investigate the underlying causes, levels of sex steroids (testosterone, T; estradiol-17β, E2) and ovarian steroidogenic enzyme mRNAs were compared between artificially maturing Japanese eels and wild-caught, spontaneously maturing New Zealand longfin eels. The latter were employed as reference, as wild Japanese eels in advanced stages of oogenesis are near-impossible to catch. Serum T levels in artificially maturing Japanese eel changed with stage in a pattern that was comparable to that in longfin eels. Likewise, ovarian mRNA levels of most steroidogenic enzyme genes were not qualitatively dissimilar between both eel species when taking developmental stage into account. However, aromatase (cyp19a) mRNA levels, together with serum E2 levels, rapidly increased in artificially maturing Japanese eels in mid-late stages of oogenesis (gonadosomatic index, GSI = 13.8%), whereas no such increase was evident in longfin eels (GSI ~ 6.9%). In addition, sex steroid and target gene mRNA levels fluctuated drastically with each hormone injection. We contend that expression of most target genes, possibly even that of cyp19a, during induced oogenesis could be “normal”, with the drastic fluctuations due most likely to hormone delivery through repeated injections. The effects of these fluctuations on gamete quality remain unknown and resolving this issue may prove fruitful in the future to further artificial propagation of anguillid eels.

Testicular development and spermatogenesis in fish: insights into molecular aspects and regulation of gene expression by different exogenous factors

“…FSH treatment was found to increase the transcript level of cyp17a1, along with star and hsd3b genes (Rolland et al 2013) whereas androgens downregulated the mRNA expression level of such transcripts. Androgens and oestrogens downregulated the cyp17 isoforms in Japanese flounder, (Meng et al 2019) whereas the gonadotropin hormone treatment upregulated the mRNA levels of cyp17 in female eels (Matsubara et al 2019). The decrease in the expression level of cyp17 with the increased use of androgens may be due to negative feedback effect on HPG axis, which downregulates the neurohormones important for the initiation of steroidogenesis (Bidlingmaier & Knorr 1981).…”

“…2019) whereas the gonadotropin hormone treatment upregulated the mRNA levels of cyp17 in female eels (Matsubara et al . 2019). The decrease in the expression level of cyp17 with the increased use of androgens may be due to negative feedback effect on HPG axis, which downregulates the neurohormones important for the initiation of steroidogenesis (Bidlingmaier & Knorr 1981).…”

“…Vitellogenic follicles incubated with hCG increased the mRNA level of hsd17b3 (Ings & Van Der Kraak 2006). In female New Zealand Longfin Eel (Anguilla dieffenbachia) and Japanese Eel (Anguilla japonica), salmon pituitary homogenates containing gonadotropins up-regulated the mRNA levels of hsd17b along with cyp11a and cyp19a transcripts (Matsubara et al 2019). The inclusion of female data in this section was because of the scarce availability of reports in male fish and to correlate the regulation of hsd17b in both the sexes.…”