The type of variegation of eyes of white-mottled mutants of D. hydei, either small-spotted or large-spotted, depends on the specific chromosome rearrangement involved. This distinction between mutants, though handsome, is not absolute because very seldomly small-spotted types do show a larger pigment aggregate and some "large-spotted" flies have no large spots at all, but only minor spots. Because of a pleiotropic action of the white gene, we could study the variegation of Malpighian tubules. The quasi-linear array of Malpighian cells enables a thorough statistical analysis. The problem was in how far the variegation of the tubes that is correlated with the variegation of eyes (in as far as numbers of pigmented and unpigmented cells are concerned) is also connected with a cell-lineage type of determination. Statistics now show that in spite of temperature-induced great variation in the percentage of pigmented cells, all types studied show a nearly random distribution of pigmented and unpigmented cells in the Malpighian tubules. This implies that the cell-lineage type of determination is not only largely mutant-specific but also organ-specific, i.e., limited to eyes. A basic gradient, however, as characteristic for eyes, was also found in Malpighian tubes.
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