From Many, One: Genetic Control of Prolificacy during Maize Domestication

When maize was domesticated from teosinte in southwest Mexico approximately 9,000 years ago, the number and arrangement of the female inflorescences (ears) changed dramatically. In teosinte, the long lateral branches are tipped with tassels and multiple ears are found at the lateral nodes of the branch. In maize, the greatly shorted branches are tipped with a single large ear and no ears are formed at the other nodes. To investigate this change in the number of ears on the lateral branch, we performed a QTL analysis in a BC2S3 maize-teosinte mapping population with maize as the recurrent parent. The population was scored for prolificacy, which is the number of ears on the primary lateral branch. A large QTL was detected on the short arm of chromosome one, in a location that has previously been shown to influence multiple domestication traits.  We fine-mapped prol1.1 to a 2.7 kb interval or “causative region” that is 7.5 kb upstream of the known maize gene grassy tillers1 (gt1), which encodes an HD-ZIP class of transcriptional regulator. Population genetic analyses point to positive selection on the maize allele of gt1, resulting in a partial sweep that fixed the maize allele throughout most of the distribution of domesticated maize. Although there is roughly equal overall expression of the maize and teosinte alleles of gt1, RNA in situ hybridizations of young ear buds show a clear expression difference between these alleles in a stripe across the stem node, the region where secondary ears are initiated.