CDNA Cloning of Two Gonadotropin β Subunits (GTH-Iβ and -IIβ) and Their Expression Profiles during Gametogenesis in the Japanese Flounder (Paralichthys olivaceus)

Abstract

To clarify the profiles of two distinct gonadotropin (GTH-I and -II) mRNA levels during gametogenesis in a multiple spawner, the Japanese flounder (Paralichthys olivaceus), the cDNAs encoding GTH-Iβ and -IIβ from the pituitary gland have been cloned and sequenced. The nucleotide sequence of GTH-Iβ was 542 bp long, encoding 120 amino acids, and that of GTH-IIβ was 554 bp long, encoding 145 amino acids. In females, Northern blot analysis has revealed that relative mRNA levels of GTH-Iβ and -IIβ were low in immature fish, showed a gradual increase with ovarian development, and reached the highest level at the maturation stage. Both GTH-Iβ and -IIβ mRNA levels were highly correlated with gonadosomatic index (GSI) values and with circulating estradiol-17β and testosterone (T) levels. In males, the mRNA levels of GTH-Iβ increased with the increase in GSI values and in circulating 11-ketotestosterone and T levels, whereas the mRNA levels of GTH-IIβ did not show any correlation with GSI values and with circulating steroid levels, suggesting a difference in regulatory mechanisms of GTH-I and -II synthesis in males. The similar changes in GTH-Iβ and -IIβ mRNA levels during oogenesis are considered to be characteristic of GTH synthesis in multiple spawners, differing from the differential changes reported in annual spawners such as salmonids.