Effect of Metamorphic Hormones on Gene Expression

Effect of metamorphic hormones on gene expression in moulting and metamorphosing insects, the first report of a particular hormone being capable to influence specific gene loci, control their transcription and lastly influence cell differentiation was reported in the 1960s by Clever and Karlson on the basis of their studies on Dipteran larva Chironomus. These findings were based upon experiments in which Clever injected minute quantities of ecdysone into the larva and take place within fifteen minutes, hormone induced puffings (swellings) at specific regions of a specific chromosome. Clever also studied patterns of puffing in polytene chromosomes at various larval developmental stages. He extra established that puffing of specific chromosomal bands takes place at specific times in usual course of development and follows a fixed sequence whereby certain bands puff at one stage to synthesize RNA and regress at a later stage. For instance he injected ecdysone into an intermoult larva in which puffing of a band 1-19A was known to be particularly active, and observed that in 10- 15 minutes, puff 1-19,A regressed, band 1-18C started to puff, followed by puffing of band IV-2B. This puffing sequence was exactly identical to the sequence that occurs just before moulting, a time at which the insect spontaneously releases ecdysone into the hemolymph Figure diaplys similar findings in Drosophila as observed by Becker, 1959.

Figure: Puffs appearing and disappearing during the third larval instar and the prepupal stage at the base of chromosome arm 111 L of Drosophila melanogaster salivary glands. Numbers indicate hours before or after pupanum formation. After H. Becker, Chromosome. 10-654 (1959).