The replication and partitioning of chloroplast and mitochondrial genomes to daughter cells at cell division occur in a flexible manner, unlike in the case of nuclear genomes. Many of the genes present in the prokaryotic endosymbionts that became chloroplasts and mitochondria have been lost or transferred to the nucleus of the host, where they have joined the stringent genetic system of the nuclear genome, including also sexual recombination and more efficient DNA repair. However, genes retained within the cytoplasmic organelles can be involved in selection processes both within and among individuals. In the case of heteroplasmy, which is attributed to mutations or biparental inheritance, within-individual selection on cytoplasmic DNA may provide a mechanism by which to adapt rapidly. The persistence of genes in cytoplasmic genomes indicates that chloroplast and mitochondrial genomes can be maintained in the process of evolution. The inheritance of cytoplasmic genomes is not universally maternal (e.g. about one-third of the angiosperm genera seem to display biparental chloroplast inheritance to some degree) and, therefore, unlikely to be a mere consequence of the asymmetry in gamete sizes. The presence of inheritance patterns other than the strictly maternal pattern indicates that different strategies have been adopted among different organisms.

Key words: chloroplasts, endosymbiosis, evolution, gene transfers, inheritance, mitochondria