Walter Plaut

Dosage compensation in Drosophila melanogaster triploids

In order to determine the presence or absence of dosage compensation in triploid intersexes (2X3A) relative to triploid females (3X3A), the rate of RNA synthesis in a segment of the X chromosomes was indirectly measured by autoradiography. Using an autosomal segment as basis for comparison it was found that, over all, there is a tendency toward equalization of the relative activity of X chromosomes in 2X3A and 3X3A individuals. When one allows for differences in selfabsorption of beta particles, it is apparent that the phenomenon of dosage compensation is incomplete. It is suggested that the present as well as other instances of dosage compensation in Drosophila can be explained if it is assumed that the rate limiting element in the transcriptional process is not the X-chromosome DNA template but a factor(s) originating elsewhere in the genome whose presence is necessary for the expression of a given sex-linked gene. Other hypotheses of dosage compensation are discussed.

The synthesis and distribution of DNA in the macronucleus of Euplotes eurystomus.

In an attempt to recognize any ordering of DNA synthesis which might occur in ciliates, the distribution of labelled DNA in the G1 and S periods of the first, second, and third generations of a synchronized population of Euplotes eurystomus was studied by means of autoradiography. The results presented here show that the replicating DNA which at the time of label incorporation is restricted to a morphologically identifiable narrow region of the nucleus (the replication band), becomes dispersed and is evenly distributed throughout the nucleus. This dispersal of labelled DNA occurs previous to division and is observable throughout the G1 period of the following generation. During the S phase of this second generation, this previously labelled DNA once again becomes restricted to a small portion of the nucleus. Now, however, it is present at the tips of the macronucleus independently of the position of the replication band. Again the labelled DNA is found to be dispersed during G1 of the third generation. In the S period of this third generation however the radioactive DNA again appears localized in the replication bands which are found at the same position in the nucleus where they were when the pulse was given two generations earlier. The observations derived from the autoradiographic analysis suggest a non-permanent organization for at least those DNA molecules which are replicated during the first third of the S period. This DNA can be associated in either of two specific locations, replication band or tip, and these two patterns of organization alternate from generation to generation.

Alteration of ringer pH and its effects on precursor incorporation into Drosophila salivary gland nuclei

Drosophila salivary glands were explanted and incubated with 3H-uridine (or 3H-thymidine) in Ringers solution (Ephrussi-Beadle modified saline) adjusted to pH values in the integral range, 4 to 10. The results of autoradiographic investigations indicate a differential effect of altering Ringer pH on 3H-uridine as opposed to 3H-thymidine incorporation: a) Relatively uniform levels of chromosomal incorporation of 3H-thymidine occurred over the range of test pH. Some decrease of incorporation was noted at pH 5 and some increase at pH 9. b) Chromosomal incorporation of 3H-uridine was severely depressed at pH 4 and 7 relative to the high incorporation levels observed at other pH values. Controlling pH of Ephrussi-Beadle Ringers solution in such experiments seems a necessity. This appears especially important for studies involving 3H-uridine incorporation.