KORPELAINEN, HELENA. Department of Biosciences, Division of Genetics, P.O. Box 56, 00014 University of Helsinki, Finland. - The evolutionary processes of chloroplast and mitochondrial genomes differ from those of nuclear genomes.
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
Key words: chloroplasts, endosymbiosis, evolution, gene transfers, inheritance, mitochondria