Endosybiotic evolution in action: Real-time observations of chloroplast to nucleus gene transfer.

The origin of new genes has long been considered a fundamental question in evolutionary biology. In eukaryotes, a major pathway for the 'birth' of new nuclear genes has been transfer of genes from the cytoplasmic organelles (mitochondria and plastids) to the nucleus. While the vast majority of gene transfer occurred shortly after endosymbiosis, the process continues today and is still driving the evolution of nuclear genomes. In tobacco (Nicotiana tabacum) a number of studies have indicated that DNA can transfer from the chloroplast to the nucleus at relatively high frequency. Less has been known, however, about how a newly transferred organelle gene can become activated in this new genetic environment. In a recent report we observed, in real-time, the activation of a plastid reporter gene newly transferred to the nucleus. A key observation from this study was that non-homologous repair is an important generator of novel sequence combinations which, in rare instances, can result in the nuclear activation of plastid genes. In addition, the activation of relocated genes can be aided by the fortuitous presence of plastid sequences able to promote nuclear expression.