Incorporation and Utilization of Multiple Forms of Vitellogenin and their Derivative Yolk Proteins during Vitellogenesis and Embryonic Development in the Mosquitofish, Gambusia affinis

Abstract

We previously demonstrated the presence of three forms of vitellogenin (Vg), two 600 kDa Vgs (600Vg; VgA and VgB) and a 400 kDa Vg (400Vg; phosvitinless Vg) in plasma from maturing female viviparous mosquitofish, Gambusia affinis. For further quantitative elucidation of the accumulation and utilization of the multiple Vg-derived yolk proteins, two sandwich enzyme-linked immunosorbent assays (ELISA) were developed using antisera against 600Vgs and a 400 kDa yolk protein (400Yp; derived from 400Vg), respectively. Contents of 560 kDa yolk protein (560Yp; lipovitellins derived from 600Vg) and 400Yp measured by the ELISAs increased in accordance with the growth of vitellogenic oocytes, keeping their proportional ratio (mol/mol) at about 4:1. A similar ratio obtained for plasma Vgs suggests that the proportional accumulation of the multiple Vg-derived yolk proteins is regulated by the hepatic synthesis and secretion of their precursor Vgs. When egg homogenate was analyzed by gel chromatography, three peaks, consisting of 560Yp, 400Yp and 28 kDa native beta'-component, were observed. The elution profile showed no change until embryos reached the early neurula stage, however, the relative height of the 560Yp peak as compared to the 400Yp one decreased after retinal pigmentation. Results from measurements of 560Yp and 400Yp at each embryonic stage supported the occurrence of unequal utilization of the two yolk proteins. The proportional ratios (mol/mol) of 560Yp content versus 400Yp content gradually decreased from 4.1 fold in early neurula embryo to 1.4 fold in larva just before parturition. The present study thus demonstrated unequal utilization of the multiple Vg-derived yolk proteins in developing embryos of mosquitofish.