Changes in the levels of mRNA coding for gonadotropin Iβ and IIβ subunits during vitellogenesis in the common Japanese conger Conger myriaster

The common Japanese conger Conger myriaster is known to mature up to the mid-vitellogenic stage without hormonal treatment, in contrast to the Japanese eel Anguilla japonica, which does not mature spontaneously under captive conditions. In order to obtain basic information on the process of gonadal development in anguilliformes under natural conditions, we have characterized cDNA of two distinct gonadotropin beta -subunits (GTH-I beta and -II beta) in the common Japanese conger, and examined changes in their mRNA levels during vitellogenesis in the absence of hormonal treatment. Northern blot analysis revealed that the mRNA levels of GTH-I beta were low at the perinucleolus stage, increased at the oil droplet stage, and peaked at the primary yolk globule stage. However, the levels decreased significantly at the secondary yolk globule stage. On the other hand, GTH-II beta mRNA was first detected at the oil droplet stage, and showed a drastic increase with the progression of ovarian development. These results suggest that the two GTHs have distinct functional roles in oogenesis in anguilliformes where GTH-I is involved in the early stages of oogenesis, and GTH-II in the late stages of oogenesis.