Chapter Two - Gene Content and Gene Transfer from Mitochondria to the Nucleus During Evolution

Organelles such as mitochondria and chloroplasts are derived from endosymbionts. Gene transfer events from organelles to the nucleus, in which organelle genes are translocated and activated in the nucleus, have been occurring over evolutionary time. Complete gene transfer requires several steps because of the differences in transcriptional and translational machinery between the organelles and the nucleus (cytoplasm). In addition, protein signals appropriate for sorting need to be acquired. In angiosperms, more genes (in particular, ribosomal protein genes) are encoded in the mitochondrial genome than in vertebrates and fungi. Furthermore, the number of genes in the mitochondrial genome varies among plant species. These clues suggest that mitochondrial gene transfer to the nucleus is still ongoing in angiosperms. Thus, the mitochondrial genome in angiosperms is a good tool for the study of gene transfer events from the mitochondria to the nucleus and provides a way of understanding the steps of symbiosis in angiosperms. In this review, we discuss insights into gene transfer and substitution revealed by comparative genomic analysis in angiosperms.