A segmental gene duplication generated differentially expressed myb-homologous genes in maize

The myb-homologous pi gene regulates the synthesis of flavonoid pigments in maize kernel pericarp and cob; these floral organs are greatly modified in size and shape compared with their counterparts in teosinte, the progenito r of maize. To elucidate the molecular evolution of the p1 gene in relation to its expression and possible functions in maize and teosinte, we have is olated a second maize gene (p2) that is highly homologous with the p1 gene, and a related gene (p2-t) from Zea mays subsp parviglumis. We present evid ence that the maize p1 and p2 genes were generated by duplication of an anc estral p gene (p(pre)) and its downstream sequences; the duplicated 3' flan king sequences were inserted upstream of the p(pre) gene, thereby changing its transcription pattern. This model accounts for the structural organizat ion and the observed differential expression of the p1 and p2 genes: pi tra nscripts accumulate in kernel pericarp, cob, tassel glumes, and silk, where as p2 transcripts are found in developing anther and silk. The duplication is estimated to have occurred 2.75 million years ago; subsequently, multipl e retroelements have been inserted between the pi and p2 genes. Our results demonstrate the evolution of a single gene into a compound locus containin g two component genes with different tissue specificities. Expression of th e p1 gene in the kernel pericarp may have provided a selective advantage du ring the evolution of maize kernel morphology.