Reducing the genome size of organelles favours gene transfer to the nucleus

Endosymbiotic organelles exhibit strong genetic erosion du ri ng their evol ution as a result of the loss of unnecessary genes and of gene transfer to the nucleus. The reasons for this erosion are much debated. Unidirectionali ty of DNA exchange between cell compartments could favour biased gene trans fer, but selection mig ht also act to favour nuclear localization of genes, for example, because organelles accumulate more mutations than do nuclei, Selection for rapid replication might be a general cause of organelle genom e reduction. This selection also accounts for the compactness of organelle genomes.