Molecular cloning and daily mRNA levels of prolactin and somatolactin in aquacultured Pacific bluefin tuna (Thunnus orientalis)

We cloned the prolactin (PRL) and somatolactin (SL) genes of Pacific bluefin tuna (PBT: Thunnus orientalis) and investigated their daily mRNA expression patterns under aquacultured conditions. The PRL and SL cDNAs of PBT encode 211 and 231 aa prehormones which give rise to 187 and 207 aa mature peptides, respectively, and which belong to the same clades as those of other Perciformes. The highest gene expression levels of PRL and SL in PBT were found at 6 am under natural light conditions, which is about 3 times higher than the constitutive levels in both cases. Interestingly, scatter diagrams show a strictly correlated expression between PRL and SL (r = 0.82; p < 0.01) while no correlation was found between PRL or SL and the growth hormone (GH). These results suggest that PRL and SL play synchronized role(s) in daily physiological phases of PBT under aquacultured conditions.