that exhibit pigmented sectors on an otherwise albino floral limb (Epperson and Clegg, 1987b, 1992). The pigmented sectors display the color associated with the P/p genotype. Finally, the pigmentation in the floral tube appears to be controlled separately from the outer floral limb, but the genetics of floral tube variation have not been analyzed. Fig. 1 displays the flower color phenotypes determined by these genetic loci.

To put the phenotypic variation into a biochemical context, it is useful to sketch the main outlines of the flavonoid biosynthetic pathway (Fig. 2), which culminates in the production of anthocyanins, the main pigments responsible for flower color. The presence or absence of these pigments affects the coloration of the floral display, which attracts pollinators. The

FIGURE 1. Flower color variation in I. purpurea. Loci are described in the text. The locus that determines the phenotype shown is highlighted in bold. Dashes indicate that the phenotype is dominant and only the dominant allele is therefore indicated. In the aa genotype, for example, the A/a locus is epistatic to the P/p and I/i loci; therefore, the albino phenotype determined by the recessive aa is the same regardless of the state of the other loci.