Major recent and independent changes in levels and patterns of expression have occurred at the b gene, a regulatory locus in maize

The b locus encodes a transcription factor that regulates the expression of genes that produce purple anthocyanin pigment. Different b alleles are exp ressed in distinct tissues, causing tissue-specific anthocyanin production, Understanding how phenotypic diversity is produced and maintained at the b locus should provide models for how other regulatory genes, including thos e that influence morphological traits and development, evolve. We have inve stigated how different levels and patterns of pigmentation have evolved by determining the phenotypic and evolutionary relationships between 18 allele s that represent the diversity of b alleles in Zea mays, Although most of t hese alleles have few phenotypic differences, five alleles have very distin ct tissue-specific patterns of pigmentation, Superimposing the phenotypes o n the molecular phylogeny reveals that the alleles with strong and distinct ive patterns of expression are closely related to alleles with weak express ion, implying that the distinctive patterns have arisen recently. We have i dentified apparent insertions in three of the five phenotypically distinct alleles, and the fourth has unique upstream restriction fragment length pol ymorphisms relative to closely related alleles. The insertion in B-Peru has been shown to be responsible for its unique expression and, in the other t wo alleles, the presence of the insertion correlates with the phenotype, Th ese results suggest that major changes in gene expression are probably the result of large-scale changes in DNA sequence and/or structure most likely mediated by transposable elements.