. The Biological bulletin. Biology; Zoology; Marine biology. 108 T. C. JEGLA AND J. D. COSTLOW. OOI I 20-HYDR0XYECDYS0NE(ng) Figure 2. Comparison of the Limuhis bioassay and a radioimmuue assay. For bioassay, 20-hydroxyecdysone was dissolved in saline (closed circles and broken lines) or 20% ethanol (open circles and solid lines) and injected into 10-day-old larvae on a 23-day molt cycle. RIA results (closed circles and solid lines) are plotted as the opposite of per cent bound; namely, per cent radioactivity displaced off antibody sites in the presence of measured amount
. The Biological bulletin. Biology; Zoology; Marine biology. 108 T. C. JEGLA AND J. D. COSTLOW. OOI I 20-HYDR0XYECDYS0NE(ng) Figure 2. Comparison of the Limuhis bioassay and a radioimmuue assay. For bioassay, 20-hydroxyecdysone was dissolved in saline (closed circles and broken lines) or 20% ethanol (open circles and solid lines) and injected into 10-day-old larvae on a 23-day molt cycle. RIA results (closed circles and solid lines) are plotted as the opposite of per cent bound; namely, per cent radioactivity displaced off antibody sites in the presence of measured amounts of ecdysteroid. The three points with vertical lines represent means of 17 to 20 experiments ± two standard errors of the mean; the others represent duplicate samples. Data in Table I show a large (600- to 900-fold) spread in sensitivity of larvae of different ages to 20-hydroxyecdysone and ecdysone. Three doses were in- jected into groups of larvae of the youngest two ages, four in groups of the other ages. Animals used in these experiments were collected at the same time and all hatched within four days of each other; all experiments were terminated within three days of each other. These procedures ensured minimal physiological varia- tion. The first significant sensitivity shift (four to sixfold) occurred in 6- to 7-day- old larvae, and coincides with a twofold increase in size of the epidermal cells; also, the cuticle is near its maximal intermolt thickness. A second increase in sensitivity occurred in 10- to 11-day—old larvae (seven to ninefold) and is asso- ciated with premolt events, based on data from histological work. A third sensi- tivity change occurred in 12- to 13-day-old larvae (five to sevenfold). At this time the new cuticle is just detectable in histological material. Larvae used after this time are not reliable for the bioassay, since there are many early molts that are not associated with the injected hormone. Whatever the length of the molt cycle, the
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