INTEGRATION OF MULTIPLE REPEATS OF GEMINIVIRAL DNA INTO THE NUCLEAR GENOME OF TOBACCO DURING EVOLUTION

Integration of viral DNA into the host nuclear genome, although not un usual in bacterial and animal systems, has surprisingly not been repor ted for plants, We have discovered geminivirus related DNA (GRD) seque nces, in the form of distinct sets of multiple direct repeats comprisi ng three related repeat classes, situated in a unique locus in the Nic otiana tabacum (tobacco) nuclear genome, The organization of these seq uences is similar or identical in eight different tobacco cultivars we have examined, DNA sequence analysis reveals that each repeat has seq uences most resembling those of the New World geminiviral DNA replicat ion origin plus the adjacent AL1 gene, encoding the viral replication protein, We believe these GRD sequences originated quite recently in N icotiana evolution through integration of geminiviral DNA by some comb ination of the processes of illegitimate recombination, amplification, deletions, and rearrangements, These events must have occurred in pla nt tissue that was subsequently able to contribute to meristematic tis sue yielding gametes, GRD may have been retained in tobacco by selecti on or by random fixation in a small evolving population, Although we c annot detect transcription of these sequences, this does not exclude t he possibility that they may originally have been expressed.