Somatic Variation of Chloroplast DNA Transferred to the Nucleus

Eukaryotic cells arose more than a billion years ago through endosymbiotic engulfment of free-living bacteria that were gradually converted into mitochondria and chloroplasts respectively. Since these events, there has been a continuous deluge of organellar DNA entering the nuclear genome. DNA transfer is still a major driving force in eukaryote evolution by being a source of genetic diversity and by creating genes or nuclear exons encoding parts of novel proteins. In this talk, we will present the recent results of experiments that determined the frequency of DNA transfer from the chloroplast to the nucleus in somatic and gametophytic tissues, the stability of these plastidic integrants in the nuclear genome and the impact of environmental stresses on transfer frequency. We will also describe, at the molecular level, how a chloroplastic gene (prokaryotic) became activated in the nuclear genome (eukaryotic) at low frequency (1 every 2*108 cells) and how it could replace the plastidic copy. Finally, the evolutionary fate of nonfunctional plastidic sequences that inserted in the nuclear genome several millions of years ago will be described and discussed.