Abstract
SYNOPSIS. During larval life the insect epidermis makes a larval cuticle and certain pigments due to the presence of juvenile hormone (JH) at critical times during the molt cycle. The presence of JH also permits growth of imaginal discs and maintains strictly larval epidermis. At metamorphosis the lepidopteran epidermis responds to a low level of 20- hydroxyecdysone (20HE) in the absence of JH by becoming pupally committed, then later it forms a pupal cuticle when more 20HE appears, even though JH is present. During the change of commitment, DNA synthesis occurs but is not essential, whereas both RN A and protein synthesis are. The major changes in the translatable mRNA population at this time are threefold: a decline in most larval cuticle mRNAs, a transient increase followed by a disappearance of a few larval cuticle mRNAs, and an appearance of at least one ‘pupal commitment’ mRNA and two to three mRNAs for small pupal cuticular proteins. Similar changes are seen in the protein synthetic patterns. Thus, a pupally committed cell is one which can no longer make larval products but which is not yet able to make most pupal products. Juvenile hormone prevents the change to pupal commitment by directing some of both the primary and the secondary actions of 20HE on the genome.
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