The clonal origin of the lateral meristem generating the ear shoot of maize

The ear shoot of maize (Zea mays L.) consists of the peduncle and reproductive tissues (ear). Genetic mosaics induced by the unstable allele of the chi locus were used for cell lineage analysis of the ear shoot. The unstable chl-ml allele, caused by the insertion of a transposable element, gives rise to yellow-green seedlings with many small revertant green stripes. Rare plants with large revertant sectors comprising 30-50% of the plant were selected. Nineteen plants showing large sectors on the main stem were subjected to sector boundary analysis. Sectoring was recorded for the main stem, leaf subtending the ear shoot, peduncle, prophyll and ear. The reproductive part of the ear shoot, the ear, was scored after removal of the husks and subsequent exposure to light. In 18 cases the ear was non-sectored yellow-green or green. In an additional four cases, peduncle cell lineages entered the ear, but only in the proximal part, while the tip of the ear was non-sectored. Two additional ears showed longitudinal sectors which reached the tip of the ear. These observations indicate that in the lateral meristem of the ear shoot two types of cellular clone exist. One will generate the peduncle, the other will found the ear. Sector boundary analysis indicates that for the vegetative part of the ear shoot the number of meristem founder cells is high, whereas only a few initials are recruited for the formation of the ear. The presence of ear sectors not starting in the peduncle and reaching the ear tip, and the finding that the ear is frequently non-sectored, suggest that this organ derives from an apical type of growth.