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.
SUMMARYWe studied position-effect variegation in left eyes of w"4 males with none, one or two Y chromosomes, reared at 25°C or 15°C. The results showed that an additional Y chromosome reduces both the number of variegated flies and the mean number of affected ommatidia, whereas loss of the Y chromosome markedly enhances both. Low culture temperature also enhances the variegation but is less effective than loss of the Y chromosome. Comparisons of variegation patterns between the different experimental groups were hampered by considerable variation between individuals within a group. "Average" patterns were therefore constructed for each group, which are presented in the form of computer-smoothed contour maps and three-dimensional perspectives. They revealed that the distribution of pigment differs in various respects from that observed in other white-variegated strains. Notably, the highest concentration of unpigmented ommatidia occurs in the middle of the eye, decreasing more or less radially though not all peripheral regions are equally affected. Pattern differences associated with differences in Y-chromosome constitution are ascribed to a combination of the factors which determine regions of sensitivity and those which enhance variegation.
The pigmented cells in Malpighian tubules of Drosophila melanogaster contain yellow globules which are blackened by osmium tetroxide. Permanent preparations of the tubules showing pigmentation can thus be made. Following prestaining acid hydrolysis, osmium-fixed tubules can be stained with orcein. For observations on pigment distribution and Feulgen-DNA content in the same preparation, tubules are fixed in mercuric chloride. Color remains visible for several hours in unfixed tubules mounted in modified buffered Ringer solution.
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