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

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.