Effect of temperature on morphology and DNA-content of polytene chromosomes in Drosophila

Abstract

Feulgen-DNA contents and chromosome lengths and projected areas were measured in salivary gland nuclei from Drosophila prepupae which had developed at 25 degrees or 15 degrees C. Nuclei from a given prepupa fell into 3 to 5 DNA classes corresponding to different levels of polyteny. The 15 degree nuclei tended to fall into higher classes than those from 25 degree-reared animals, and their chromosomes were, on average, about 50% wider. Chromosomes within a given DNA class did not differ significantly in mean area, length or width between the temperature groups, and slight apparent differences in mean DNA content were attributable to systematic microdensitometric errors associated with differences in the spreading behaviour of the nuclei. On cytological examination, chromosomes from the two temperature groups differed mainly in width and stain intensity, but some other differences in appearance could not be accounted for by levels of polyteny. The mean length of the chromosome complement was about 400 microns. From one polytenic level to the next the chromosomes increased by about 10% in length, 40% in width and 17% in mean absorbance. The DNA content approximately doubled; small apparent deviations from the 1:2 ratio could have been due to microdensitometric error or to underreplication of heterochromatin.

Three-dimensional organization of Drosophila melanogaster interphase nuclei. I. Tissue-specific aspects of polytene nuclear architecture.

“…2 do not agree with those from any previous investigation except the length measurements of lax salivary gland chromosomes made by Bridges (7). Most or all of the discrepancies are probably a consequence of the squashing and/or fixation steps required in these studies (4,6,18,29). Squashing stretches chromosomes to an unknown extent, and fixation also generally alters chromosome and nuclear dimensions.…”

“…The data in Fig. 2 suggest that the ~/-2 increase of chromosome bandwidth with each DNA doubling determined from squashes (18,28) is an accurate reflection of the situation in vivo. This value indicates that when DNA content doubles, the new chromatids are packed laterally into chromomeres at a density similar to that of the previous polyteny level.…”

“…The bottles were either left at 16°C for the entire period to third instar or wore transferred to 16°C after allowing egg laying for several days at 25°C. Growth at low temperature increases the fraction of nuclei that reach higher levels of polyteny (18). It does not alter chromosome organization inasmuch as the salivary gland nuclei from Hochstrasser et al (22) that were from OR-isoX larvae raised at 16°C did not differ in any measure from those of larvae raised at 25°C, including average chromosome dimensions.…”

“…Fig. 2 (18,28); it increases by a factor of ~V~ with each round of replication. Three of the peaks in the histograms fit the prediction well (peaks centered at 1.5, 2.1, 3.1 tma), but the final peak in the salivary gland nuclei (3.5 txm) is at a lower width than expected.…”

“…Tables II and III provide data on several other nuclear size parameters. Studies comparing (squashed) chromosome bandwidth to DNA content indicate that width is a fairly accurate indicator of degree ofpolyteny (18,28); it increases by a factor of ~V~ with each round of replication. Three of the peaks in the histograms fit the prediction well (peaks centered at 1.5, 2.1, 3.1 tma), but the final peak in the salivary gland nuclei (3.5 txm) is at a lower width than expected.…”