Cell lineage analysis of the gynoecium of maize using the transposable element Ac

The maize gynoecium develops from a primordium comprising two distinct cell lineages: an epidermal LI and a subepidermal LII lineage. We have analyzed somatic gynoecial sectors marked with red flavonoid pigments by excision of Ac from the P locus. Somatic sector analysis indicates the epidermal lineage starts as a single cell layer at the base of the ovary and thickens into several cell layers, presumably by periclinal divisions, near the silk attachment point. The silk attachment point appears to be mainly an epidermal outgrowth of two of three fused carpels and presumably the silk contains only LI, or mostly LI, with only traces of subepidermal LII cells. The third carpel, covering the germinal face of the kernel, retains a multicellular LII and unicellular LI organization but fails to contribute substantially to stylar outgrowth. Ac transpositions in subepidermal somatic sectors are transmitted to the archesporial cell lineage. Ac transpositions that occur in epidermal sectors are not transmitted to offspring. These results demonstrate that the female megasporocyte is derived from subepidermal (LII) cells.