Using a strain of D. melanogaster carrying an X-4 translocation, comparison between groups of individuals cultured at 14°, 19° and 25°C. showed good correlation between heterochromatization, variegation in malpighian tubules and Notch. All these phenomena are enhanced by low temperatures. Correlation was less good within each temperature group, and considerable variability was observed between individuals within the group, and between nuclei from one individual.Experiments involving a temperature change during the embryonic period indicate that (1) heterochromatization is especially temperature-sensitive during the early embryonic period but may be increased by low temperature later, (2) larval malpighian tubules are sensitive to temperature only during the early embryonic stage, and (3) N is influenced by temperature during early embryonic life and also during the larval period.Our observations, in conjunction with those of other workers, could be explained as follows: exposure of individuals to low temperature at a time when a specific system is beginning to differentiate will cause the processes concerned to be blocked in some cells. At least during the early embryonic stages this appears to involve heterochromatization of the relevant locus. Once the processes are established which will lead to the formation of a character, further heterochromatization has no effect on the phenotype. Temperature may affect pupal or adult phenotype in this way or by a direct action on the metabolic processes of cells.Further experiments showed: (1) the greatest temperature-sensitivity of all three phenomena within the first 6 hours of embryonic life; (2) striking fluctuations of the effect of temperature, especially within the early embryonic period; (3) close correspondence between all three phenomena in time of response to temperature. Some alternative interpretations are considered.

Summary Instrumental factors affecting the precision and accuracy of measurements with the Vickers M86 scanning and integrating microinterferometer were investigated. ‘Spot’ measurements of OPD (optical path difference) could be made to a precision better than 0.001 Λ (wavelengths); the coefficient of variation of scanning measurements of IOPD (integrated OPD) depended on the object, typical values being approximately ± 5% for a human erythrocyte in water, and ± 10% for a mouse sperm head in water. Repeated measurements enabled the dry mass of a specimen to be estimated to any desired precision. Instrumental calibration to give results in absolute units is described. Early prototypes required frequent calibration, but with production instruments the calibration varies little from day to day, and very similar results are obtained if a specimen is measured in different parts of the scanned field. A slight, approximately sinusoidal deviation from linearity was sometimes observed when the apparent OPD or IOPD was plotted against expected values. The error, which probably occurs with all instruments employing a Jamin double‐refracting interference system, was maximally about ± 0.02 Λ for a specimen of true OPD 0.25 Λ or 0.75 Λ, but was zero for a specimen of true OPD 0.5 Λ. The error could be minimized or eliminated by the correct choice of an interference fringe for ‘spreading’, careful adjustment of the condenser and background OPD, and especially by reduction of the condenser aperture. Scanning measurements of IOPD were shown experimentally and theoretically to be insensitive to changes in the measuring‐spot diameter or errors in focus. Valid measurements of dry mass can therefore be made even if the three‐dimensional specimen (e.g. a living cell or isolated nucleus) is considerably thicker than the depth of focus of the objective used.

SUMMARYThe effect of the Y chromosome on white variegation associated with In(1)w"4 has been investigated by comparing the occurrence, proportion and distribution of colourless cells in the Malpighian tubules of XO, XY and XYY larvae cultured at 25°C or 15°C. The results confirmed previous observations that the Y chromosome suppresses variegation, and showed that its loss from the XY complement has a much greater effect on the proportion of colourless cells than its addition. The variegation is markedly more affected by Y-chromosome constitution than by culture temperature. Colourless cells are non-randomly distributed within and between tubules; neither chromosome constitution nor temperature appears to modify these distribution patterns.

The DNA content of translocated polytene chromosome regions in Drosophila melanogaster is affected by heterochromatic position effect. Microdensitometric studies on wm258-21 translocation heterozygotes showd (Hartmann-Goldstein and Cowell, 1976; Cowell and Hartmann-Goldstein, 1980) that band region 3D1-E2, adjacent to the breakpoint, contained less DNA than the homologous non-translocated region whereas the neighbouring 3C1-10 region contained more DNA than its non-translocated counterpart. In the nuclei selected for measurement the translocated X chromosome was morphologically euchromatic, but both regions undergo heterochromatisation in other nuclei within the same salivary gland. To explore the relationship between changes in DNA content and heterochromatisation, the effect on DNA content of two known modifiers of heterochromatisation has now been studied. Larvae cultured at 15 degrees C, which exhibit more heterochromatisation than those grown in 25 degrees C, have the same relative DNA contents as at the higher temperature. The addition of a Y chromosome markedly reduced heterochromatisation; in XXY larvae there was no difference between the DNA contents of translocated and non-translocated 3D1-E2 regions, and in region 3C1-10 the percentage excess of DNA in the translocated homolgue was approximately double that found in XX larvae. The relationship between replication behaviour and compaction suggested by these results is discussed.

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.