Role of ecdysteroids in the molting and reproductive cycles of the white shrimp, Penaeus vannamei

Abstract

The molt cycle was characterized in the white shrimp Penaeus vannamei based upon changes in the morphology of pleopod setae. These characters were used subsequently to determine changes in ecdysteroids and related biosynthetic events during the stages of the molt cycle and in relation to reproduction. Ecdysteroid titers were measured during the molt cycle. They increased during the premolt, decreased at molting and remained minimal through the intermolt. 20-hydroxyecdysone (20-OHE) was the major ecdysteroid present during the premolt. Injection of [3H]ecdysone resulted in label accumulation in the epidermis and hepatopancreas in the form of 20-OHE which was subsequently hydroxylated further to a polar metabolite. [3H]ecdysone was not metabolized by the Y-organ, hemolymph, muscle or intestine. Eyestalk ablation elevated hemolymph ecdysteroid titers and reduced the duration of all molt stages. However, 20-OHE injection into intact animals caused a disproportionate reduction in the durations of only the intermolt and early premolt stages. Total hemolymph protein levels were high during the premolt and low during the postmolt and intermolt and showed the same general pattern as the ecdysteroid titers. No changes were noted for specific proteins. Likewise, protein synthesis by both the epidermis and the hepatopancreas tissues was high during the premolt and low during the post- and intermolts. Although no changes were found for the pattern of epidermal polypeptides between the stages of the molt cycle, treatment with 20-OHE in vitro enhanced protein synthesis by intermolt epidermis and changed the pattern of polypeptide synthesis to that of premolt tissue. In the hepatopancreas, the pattern of polypeptide synthesis remained the same for both the intermolt and premolt stages. Treatment in vitro with 20-OHE resulted in the synthesis of new polypeptides and the suppression of others in intermolt tissue only. No significant differences were found for the pattern of hemolymph ecdysteroid titers in adult females and juveniles; however, the intermolt period was prolonged in adults and had the highest frequency of animals with ovarian maturation. The ovaries contained ecdysteroids that increased in quantity during oocyte maturation...