Serum lipocalin-like protein in Sebastes rockfish: A novel biomarker for reproductive stage males

Abstract

Viviparous rockfishes (Sebastes spp.) are ecologically and economically important fisheries and aquaculture species. Because the males lack obvious morphological characteristics related to reproductive status, it is challenging to select semen donors for artificial insemination. We previously identified lipocalin-type prostaglandin D2 synthase homolog protein (LPGDShp) in the urine of male black rockfish (Sebastes schlegelii). Subsequently, LPGDShp-like protein (hereafter, lipocalin-like protein) was immunologically detected in the serum of two Sebastes species, black rockfish and white-edged rockfish (S. taczanowskii). In the present study, an enzyme-linked immunosorbent assay (ELISA) was developed to explore the potential of serum lipocalin-like protein as a biomarker for selecting mature males. The ELISA measured lipocalin-like protein in the sera of black rockfish and white-edged rockfish over an assay range between 0.391 and 12.5 ng/mL. In male black rockfish, serum levels of lipocalin-like protein were higher during the copulation season, and they were higher than levels in females during both the copulation and non-copulation seasons. In male white-edged rockfish sampled nearly monthly for a year, serum lipocalin-like protein levels showed synchronous dynamics with gonadosomatic index (GSI). Histological examination of testis revealed that the serum levels were elevated in the late/functional maturation stages, as compared to early/mid-maturation and resting stages. In both rockfish species, while serum levels of lipocalin-like protein were positively correlated with GSI in males, no such trend was observed in females. These results indicate, for the first time, that serum lipocalin-like protein can be employed as a hematological biomarker to detect the reproductive status of male rockfishes.