Some effects of the molt-inhibiting hormone and 20-hydroxyecdysone upon molting in the grass shrimp, Palaemonetes pugio

Abstract

Several aspects of the chemistry and physiological effects of molt-inhibiting hormone (MIH) and 20-hydroxyecdysone and their interaction in the control of molting in Palaemonetes pugio were investigated. More MIH activity was extracted from eyestalks using phosphate buffer (pH 11) than with distilled water, saline, ethanol, or methanol-chloroform-water. A crude eyestalk extract was filtered through a Sephadex G-25 gel column. The retention constant ( R = void vol elution vol ) for MIH was 0.67. A direct relationship between dosage and response for MIH was found for crude eyestalk extracts over a range of 0.25 to 2.5 eyestalk equivalents per animal. A direct dosage-response relationship for 20-hydroxyecdysone from concentrations of 0.005 to 5.0 μg/animal was also shown. Higher doses caused abnormal setogenesis in the uropods. Precocious proecdysis was also induced in ovigerous females when injected with 20-hydroxyecdysone. When MIH was injected into the last abdominal segment and followed by an injection of 20-hydroxyecdysone into the thoraco-abdominal junction of the same shrimp, a delay in setogenesis in the uropods, as compared to the accelerated setogenesis in the antennal scale, was observed. The results of this latter experiment are consistent with the hypotheses that concentration gradients of the two hormones were established in the shrimp and that MIH acted antagonistically to 20-hydroxyecdysone at the level of the target cells of the epidermis.