Die Puffs der Speicheldrüsenchromosomen von Drosophila melanogaster
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In salivary gland chromosomes of Drosophila melanogaster a markedly increased number of puffs appears at both the end of the last larval instar and the end of the prepupal stage (Fig. 1 and 2). Each of these puffing periods has its characteristic pattern of puff development with regard to the location of the puffs, their number and their sequence (Fig. 14). Puffs are considered to be active sites of their respective chromosomal loci.
Experiments were designed to show whether puff formation depends upon the presence of metamorphosis hormones. The first step was the determination of the time of hormone production by the ring gland. To that end late third instar larvae were ligated in or near the fourth larval segment (Fig. 3). The ring gland lay in the portion anterior to the ligature, and the passage of hormones into the posterior portion was effectively prevented when ligating was performed prior to hormone production. If the anterior portion formed a puparium in less than 31/2 hours after ligating, the posterior portion also formed a puparium in all cases. If, however, puparium formation in the anterior portion set in more than 5 hours after ligating, the posterior portion remained larval in all cases (Table 3, Fig. 4 and 11). One can conclude, therefore, that 31/2–5 hours before puparium formation the ring gland produces the hormones that lead to this developmental step.
A ligature in the above mentioned region of the larva has the additional effect of separating each salivary gland into two parts, one lying in the anterior and the other in the posterior portion. 58 such larvae were dissected at the beginning of puparium formation, and the chromosomes of the anterior and posterior parts of the glands were examined separately with regard to their puffing pattern. Whenever the anterior portion of a larva formed a puparium less than 4 hours after ligating then both the anterior and posterior portions of the salivary glands showed the puffing pattern which is characteristic for the end of the third larval instar (Table 6, Fig. 7, 8 and 11). If, on the other hand, puparium formation in the anterior part of a larva set in more than 5 hours after ligating, then only the gland portion contained in this part showed the typical puffing pattern, whereas the gland portion behind the ligature showed the larval pre-puffing-period chromosome structure (Table 7, Fig. 9–11.) Therefore the time between 4 and 5 hours before puparium formation appears to be a critical period for initiating of puff formation. From this close correlation between puparium formation and puffing it is concluded, that the puffing period in larval salivary gland chromosomes is triggered by the hormones of the ring gland.
A comparison of the time span between hormone production and puparium formation with the assumed duration of the puffing period (about 4 hours) shows that the puffing reaction of chromosomes follows immediately on the production of hormones. This, in turn, supports the idea of a hormone-controlled gene activation without many intermediate steps. Finally, ring gland hormones seem to activate the complete gene pool provided for temporary salivary gland function, since the whole set of investigated puffs was affected by the ligating experiment.
The cause for the difference between the puffing patterns in the third instar larva and in the prepupa was the objective of salivary gland transplantations. If a gland from an early third instar larva is transplanted into the abdomen of a late third instar larva (Fig. 16), both implant and host start with puff formation simultaneously, i. e. the implant reacts prematurely under the influence of the host milieu (Fig. 17 and 18).
If a salivary gland of an early prepupa is transplanted back into the abdomen of a third instar larva, the puffing pattern of the implant shows characteristics of the host; i. e. section 78D of the implant chromosomes forms a puff under the influence of the host milieu (Fig. 19, 23 and 24). This puff is characteristic for the larva and normally does not appear in prepupal chromosomes (Fig. 14 and 20). The specificity of the puffing pattern, therefore, depends at least in part on the milieu which surrounds the gland. The milieu difference between the two puffing periods may well be due to a change of the relative amounts of the molting and the juvenile hormones.
The results of both ligating and transplantation experiments are discussed in connection with previously available information. There appear to be two different attributes of the function of metamorphosis hormones with regard to gene activation : (a) they can trigger the activation of the gene pool, which a cell holds in preparation according to its state of differentiation, and (b) the milieu can select to a certain degree from the offered gene pool, possibly by means of changed relative amounts of hormones.
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