Gene activity in the giant chromosomes of Chironomus tentans and its relation to development. I. Gene activation by ecdysone

Abstract

1. Alterations in the pattern of puffs in the salivary gland chromosomes indicate changes in the pattern of gene activities. The relationships between such alterations of gene activity patterns and the behaviour of single gene activities on the one side and developmental processes in fourth stage larvae of Chironomus tentans on the other side are the subject of this paper. 2. Before beginning of the pupal moult there exists a constant pattern of loci which form puffs in the salivary gland cells. This pattern of potentially active loci is identical in all larvae. However, the pattern of such loci which are actually puffed in a given moment, varies from larva to larva. While some puffs always occur in every larva, others are found only infrequently. The puffing frequency of each locus may be altered experimentally but it is the same in young and old larvae if they live under identical conditions. It is concluded that in loci with low puffing frequencies periods of activity alternate with those of inactivity in each larva. These alterations of gene activity are independant of any developmental processes. Puffing of these genes is not specific for development. 3. In the course of moulting the frequency of most puffs which are not specific for development, decreases. However, in old prepupae only a few puffs were not found at all. Thus, the pattern of potentially active loci is hardly altered by the decreasing of puff frequency. It is suggested that the reduction of a part of gene metabolism is related to the breakdown of the glands at the end of the prepupal phase. 4. In studying behaviour of single loci puff size was used as a further measure of the activity. Homologous puffs often vary in size in different larvae. This indicates that gene activities in each larva may vary to a lesser degree than that which leads to the disappearance of the related puff. 5. Puffs I-17-B and I-19-A may be present in larvae during the whole last stage; sometimes they are small, sometimes large. However, some time after the beginning of moulting processes the puffs always reach their maximal size. At the end of the prepupal phase these puffs regress. The number of genes behaving in the same manner as puffs I-17-B and I-19-A, which become more active in the course of moulting, though their activity is not specific for moulting, is relatively small. 6. Puffs I-18-C and IV-2-B are either inactive or only slightly active in larvae before initiation of the moult. After its experimental induction (Clever 1961 a) they are activated within less than one hour and their size is influenced by the concentration of the hormone injected. In normal development the appearance of puff I-18-C in a larva at the end of the stage is the first known sign of the beginning of moulting. The puff size increases during its course and reaches its maximum towards its end. Puff IV-2-B is also activated after the beginning of normal moulting, but very much later than puff I-18-C, although earlier than all other puffs. Its size increases gradually and reaches its maximum in the middle of the prepupal phase. Towards the end of moulting the puff regresses. The behaviour of both puffs in normal development is interpreted to be controlled by the titer of ecdysone in the haemolymph. As an explanation for the difference in the time of their appearance it is assumed that they have different reaction thresholds and that the titer of hormone gradually increases during the moulting phase. 7. Puffs in sections I-1-A, I-14-B, II-14-A and III-9-B are specific for older prepupae. Their appearance is strongly correlated with certain stages of the moulting process and they may, thus, serve as markers for subdividing the prepupal phase. The sequence of puffing of these loci is the same in normal development as after experimental induction. 8. The behaviour of puffs in normal development studied in the present article confirms the conclusions about their role in the physiology of moulting drawn from their behaviour in ecdysone experiments and serves as a basis for discussing regulatory mechanisms of gene activity. Our former hypothesis that the activity of genes I-18-C (and IV-2-B) is directly controlled by ecdysone, is now supplemented by the suggestion that the hormone acts like the effector substances in the genetic regulatory mechanisms of protein synthesis in bacteria.