Abstract

Maize is a domesticated form of a wild Mexican grass called teosinte. The domestication of maize from teosinte occurred about 9,000 years ago. As a result of human (artificial) selection during the domestication process, dramatic changes in morphology arose such that maize no longer closely resembles its teosinte ancestor in ear and plant architecture. We have identified and analyzed three of the genes involved in these morphological changes. First, teosinte branched (tb1) is largely responsible for the difference between the long branches of teosinte versus the short branches of maize. tb1 encodes a transcriptional regulator that functions as a repressor of branch elongation. Gene expression analysis indicates that the product of the teosinte allele of tb1 accumulates at about half the level of the maize allele. Fine-mapping experiments show that the differences in phenotype and gene expression are controlled in part by an upstream transposon insertion that acts as an enhancer of gene expression. Second, teosinte glume architecture (tga1) is largely responsible for the formation of a casing that surrounds teosinte seeds but is lacking in maize. tga1 also encodes a transcriptional regulator, however in this case a single amino acid change represents the functional difference between maize and teosinte. This single amino acid change appears to convert the maize allele into a transcriptional repressor of target genes. Third, grassy tillers (gt1) contributes to differences in plant architecture and encodes an HD-ZIP transcription factor. Causative changes at gt1 appear to be complex, involving multiple changes. tb1, tga1 and gt1 are members of the same developmental network which regulates shade avoidance. This pathway was a target of human selection during the domestication process.