ARRANGEMENT OF CELL-LAYERS IN THE SHOOT APICAL MERISTEMS OF PERICLINAL CHIMERAS INFLUENCES CELL FATE

Utilizing a complete set of six periclinal graft chimeras composed of Nicotiana tabacum and Nicotiana glauca (TGG, GTT, TTG, GGT, TGT, and G TG), the fate of the three apical cell layers in both vegetative and r eproductive organs has been traced. An analysis of leaf phenotype indi cated that only rarely did deviations from expected cell lineage occur and in only TTG did such deviations originate in the shoot apical mer istem rather than during leaf development. In most plants that possess a stratified shoot apical meristem, gametes are derived from the seco nd apical layer (L2). A phenotypic and/or DNA analysis of seed progeny following reciprocal crosses between all chimeras and their component species indicated that pollen and eggs were sometimes derived from no n-L2 lineage in all but one periclinal chimera. There was no evidence for non-L2-derived gametes in 95 crosses where GTT was a parent wherea s 40 of 104 crosses with TTG as a parent yielded some offspring that r esulted from non-L2-derived gametes. Of these 40 cases, non-L2-derived pollen grains were responsible 39 times while non-L2-derived eggs wer e responsible just once. Therefore, the occurrence of non-L2-derived g ametes was not random. The disruption of 'normal' lineage patterns was dependent on the specific arrangement of genetically dissimilar tissu e layers in the shoot apices of the chimeras and was different for dif ferent organs.