Résumé
Plant pararetroviruses replicate their genome via a transcription–reverse transcription cycle like retroviruses, but unlike them their genomes do not obligatorily integrate into the host chromatin. Nevertheless, one can find complete or fragmented pararetrovirus genomes, as well as those from geminiviruses and even RNA viruses incorporated into the genomes of nearly all plants analysed. Integration events are thought to be rare and even rarer are those that find their way into the germ line. Normally, these integrated viral sequences are incomplete, rearranged and mutated and cannot easily escape as active viruses. However, in some cases apparently more recently acquired and therefore less initiated integrates can escape by direct transcription from tandem insertions or by recombination. This can lead to severe outbreaks in crop and ornamental plants. In anticipation of such events, methods have been developed for the detection and characterization of integrated virus sequences in plant genomes.