Herbivory could unlock mutations sequestered in stratified shoot apices ofgenetic mosaics

Many higher plants have shoot apical meristems that possess discrete cell l ayers, only one of which normally gives rise to gametes following the trans ition from vegetative meristem to floral meristem. Consequently, when mutat ions occur in the meristems of sexually reproducing plants, they may or may not have an evolutionary impact, depending on the apical layer in which th ey reside. In order to determine whether developmentally sequestered mutati ons could be released by herbivory (i.e., meristem destruction), a characte rized genetic mosaic was subjected to simulated herbivory. Many plants deve lop two shoot meristems in the leaf axils of some nodes, here referred to a s the primary and secondary axillary meristems. Destruction of the terminal and primary axillary meristems led to the outgrowth of secondary axillary meristems. Seed derived from secondary axillary meristems was not always de scended from the second apical cell layer of the terminal shoot meristem as is expected for terminal and primary shoot meristems. Vegetative and repro ductive analysis indicated that secondary meristems did not maintain the sa me order of cell layers present in the terminal shoot meristem. In secondar y meristems reproductively sequestered cell layers possessing mutant cells can be repositioned into gamete-forming cell layers, thereby adding mutant genes into the gene pool. Herbivores feeding on shoot tips may influence pl ant evolution by causing the outgrowth of secondary axillary meristems.