ALBERT

Notes: Summary 1. 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. 2. 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. 3. 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. 4. 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. 5. 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). 6. 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. 7. 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.

Notes: Summary 1. In salivary gland chromosomes of Drosophila melanogaster approximately 70 puffs have been registered. Those of the tip of the X-chromosome and of the chromosome arm IIIL have been investigated in detail. 2. Puffs have been found during all investigated ages, i. e. the last 24 hours of the 2-day third larval instar and the following 12-hour prepupal stage. Puffs are, however, most numerous during two distinct puffing periods, one at the end of each stage. Both periods are alike in that (a) each one extends over not more than six hours, mainly before, but also during and possibly slightly after entering the next developmental stage, (b) puffs are formed in specific chromosome sections, and (c) formation and disapparance of the puffs follows a characteristic order. Both periods differ in that (a) some puffs appear in only one of the two periods, and (b) the other puffs appear in both periods, but in a different order in each one. 3. There is some evidence for a puffing period also at the end of the second larval instar. This, together with the observations of paragraph two suggests that puffing in salivary gland chromosomes is closely connected with the molting from one developmental stage to the next. 4. The view given in paragraph three is supported by puff formation in larvae of the mutant giant (gt), whose third larval instar may be three times as long as normal. In spite of this prolongation their puffs are also formed at the end of this instar, identical in place and sequence with puffs of normal third instar larvae. 5. Salivary gland chromosomes of another mutant, lethal-giant-larvae (lgl), on the other hand, do not develop typical puffs before or at the time of their puparium formation that occurs up to 14 days later than in their normal sisters. The independance of puffing and puparium formation in this case does not, however, necessarily contradict a possible interdependance of them in normal development, since the salivary glands of this mutant are abnormal in their growth. 6. Both salivary glands were removed from normal larvae. One of each pair was fixed immediately, while the remaining partners were kept for different intervals in Ringer's solution. Depending on the ages of the larvae, three different types of puffing behavior in the second gland have been observed. Glands of larvae younger than a certain stage of the third larval instar showed a surprising reverse development of their puffing pattern; glands of larvae older than this stage went on with the development of their puffing pattern almost normally; and glands of larvae at precisely the pivotal stage showed in different cells different kinds of development: some developed in the reverse, others in the forward direction and still others showed abnormal puff combinations. These latter ones showed in addition a puff in subsection 67C that has not been found otherwise. 7. A difference between the anterior and the posterior part of the gland was found in the puffing behavior of X-chromosome subsection 15BC, which forms a typical puff only in the anterior part. 8. The results are discussed in connection with current interpretations of the puffing phenomenon.

Notes: Summary In a Y-unit maze wild-type flies of Drosophila melanogaster were tested for their chemotactic behavior reactions to insect repellents. Selection over 12 generations in two parallel experiments yielded two insensitive lines. Crosses indicated that the genes that were responsible for insensitivity were at least in part dominant. Lines selected for insensitivity to one repellent were also insensitive to a second repellent.

Notes: Summary Twin mosaic spots of dark-apricot and light-apricot ommatidia were found in the eyes of w a/wa females, of w a males, of females homozygous for In(1)sc 4, wa and of attached-X females homozygous for w a. The flies were raised from larvae which had been treated with 1630 R of X-rays at the age of 48–52 hours. An additional group of w a/wa males came from larvae that had been fed with triethylene melamine (TEM) at the age of 22–24 hours. The twin spots apparently were the result of induced unequal mitotic recombination, i.e. from unequal sister-strand recombination in the males and from unequal sister-strand recombination as well as, possibly, unequal recombination between homologous strands in the females. That is, a duplication resulted in w aDp wa/wa dark-apricot ommatidia and the corresponding deficiency in an adjacent area of w a/Df wa light-apricot ommatidia. In an additional experiment sister-strand, mitotic recombination in the ring-X chromosome of ring-X/rod-X females heterozygous for w and w co is believed to be the cause for X-ray induced single mosaic spots that show the phenotype of the rod-X marker.

Notes: Summary The origin and phenotypes of a number of zeste mutant stocks with mutable white loci are described. Each newly arising form was lighter in eye color than the mutant it originated from. In each case the lighter pigmentation is believed to be due to an increase in genetic material in the proximal region of the white locus, the increase supposedly being the result of unequal crossing over. Some of the mutations which arose in the mutable stocks are reversions. They occurred in males as well as in homo- and heterozygous females. The reversions are believed to be due to a decrease in genetic material in the proximal region of the white locus. The decrease is assumed to be the result of intrachromosomal recombination. At least some of these events took place premeiotically. New mutants which originate frequently from mutable stocks are stable. In addition to the structure of the mutable white locus there is probably at least one still unknown factor which affects its mutability since the frequency of mutations arising in the mutable stocks decreases over the years.