On the potential for evolutionary change in meristematic cell lineages through intraorganismal selection

In the absence of sexual recombination somatic mutations represent the only source of genetic variation in clonally propagating plants. We analyse the probability of such somatic mutations in the shoot apical meristem being f ixed in descendant generations of meristems. A model of meristem cell dynam ics is presented for the unstratified shoot apical meristem. The fate of on e mutant initial is studied for a two- and three-celled shoot apical merist em. The main parameters of the model are the number of apical initials, the time between selection cycles, number of selection cycles and cell viabili ty of the mutant genotype. As the number of mitotic divisions per selection cycle and number of selection cycles increases the chimeric state dissipat es and the probability of mutation fixation approaches an asymptote. The va lue of this fixation asymptote depends primarily on cell viability, while t he time to reach it is mainly influenced by the total number of mitotic div isions as well as the number of initials. In contrast to the presumed operation of Muller's Ratchet in plants the chi meric state may represent an opportunity for deleterious mutations to be el iminated through intraorganismal selection or random drift. We conclude tha t intraorganismal selection not only can be a substantial force for the eli mination of deleterious mutations, but also can have the potential to confe r an evolutionary change through a meristematic cell lineage alone.