Genetics and evolution of the chloroplast : (cytoplasmic inheritance, nucleus-cytoplasm interaction, plastome recombination, plastome mutator, oenothera)

As a photosynthetic organelle and as the site of several important biosynthetic pathways, the chloroplast is essential to the survival of the plant cell. In evolutionary terms, the chloroplast genetic material can be considered conservative: from the green algae to higher plants, chloroplast DNAs have similar sizes, gene contents and arrangements, although some modifications of the chloroplast DNA have occurred as it has coevolved with the nucleus. Mutation and recombination are traditionally considered responsible for evolutionary change, but these may be quite limited in the chloroplast genetic system. Gene conversion may result in the elimination of new mutations, while the widespread occurrence of uniparental inheritance of chloroplasts reduces the likelihood of two different plastids meeting and their DNAs recombining. In other systems, transposons contribute to genetic change. One possible explanation for plastid mutator genes is that they activate transposable elements in the chloroplast DNA. Among diverse genera in primitive and higher plants, both uniparental and biparental transmission of chloroplasts occur in sexual crosses. When maternal inheritance of plastids occurs, the paternal plastids may be excluded or eliminated from the male gamete in a variety of ways. Thus maternal inheritance has probably appeared in the algae, mosses, ferns, gymnosperms, and angiosperms through parallel evolution.BARBARA B. SEARS, Department of Botany and Plant Pathology, Michigan State University, East Lansing, Michigan