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This supports the idea that in embryos from wild-type mothers, dpp H~- mutations cause the transformation of dorsal into ven-tral epidermis. If tissue determined to become dorsal epidermis dies in the dpp rim-null embryos, then one should not see ventral or lateral tissue formed in the double-mutant combination.
Since PDA is a precursor to the drosopterins (Figure 5.2), a mutation in sepia prevents the production of these red eye pigments and increases the production of the yellow pigment sepiapterin. The gene is named sepia because of the eye color resulting from its mutation. Similarly, the genes vermilion and scarlet code for enzymes involved in the ommochrome pathway. Mutations in these genes prevent the production of the brown pigment xanthommatin, resulting in bright red eyes. Not all mutations that affect eye color affect enzymes in the pigment pathways. For eye pigments to be made, several steps must occur. First, the starting substrate must be transported into the cell. Second, the pigments must be made within the cell using the enzyme pathway.