cDNAs for three subunits of gonadotropin, namely follicle-stimulating hormone (FSH) β, luteinizing hormone (LH) β and glycoprotein hormone (GP) α, of the New Zealand longfinned eel Anguilla dieffenbachii were cloned. Based on their nucleotide sequences, which were more than 98% identical to those of the Japanese eel Anguilla japonica, a real-time reverse transcription-polymerase chain reaction method for the quantitative assay of gonadotropin subunit mRNAs of the two eel species was developed using the same primer sets. Accordingly, changes in pituitary gonadotropin mRNA levels were analyzed in female Japanese eels induced to develop artificially by salmon pituitary homogenate (SPH) treatment and in naturally maturing female New Zealand longfinned eels collected from the wild. In Japanese eels, the FSH β mRNA level was very high at the previtellogenic stage (PV) and decreased remarkably after SPH injection, whereas the reverse was seen in New Zealand longfinned eels. The mRNA levels of LH β and GP α increased during gonadal development in both species, albeit to a much greater extent in Japanese eels. As the Japanese and New Zealand longfinned eels are closely related species of the same genus, the different gonadotropin expression patterns are likely to reflect differences in maturational conditions rather than species. Our findings are indicative of aberrant gonadal development and may explain the poor quality eggs obtained from artificially matured Japanese eels.
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