CHROMOSOMAL LOCATION OF ENDOGENOUS GEMINIVIRUS-RELATED DNA-SEQUENCES IN NICOTIANA-TABACUM-L

The N. tabacum (tobacco) nuclear genome carries approximately 25 multi ple direct repeats of a geminivirus-related DNA (GRD) sequence that pr obably arose by illegitimate recombination, following geminivirus infe ction, during Nicotiana evolution. Each GRD repeat carries sequences s imilar to the geminiviral AL1 gene of the tomato golden mosaic virus ( TGMV), encoding a protein required for viral DNA replication, plus the cis-essential replication origin. Using a cloned 14-kb GRD repeat seq uence as a probe for fluorescence in situ hybridization (FISH), we ide ntified a unique tobacco chromosome carrying GRD. Translocations betwe en chromosomes of the tobacco S and T genomes were used as physical ma rkers by sequentially hybridizing chromosomes with labelled GRD and to tal genomic DNA from N. sylvestris (equivalent to the S genome). The 2 5S, 18S and 5.8S ribosomal gene clusters were detected in double-label ling experiments for use as additional markers to identify the chromos omal location of GRD. GRD occupies one site on a homologous pair of sm all submetacentrics from the T genome characterized by a lack of eithe r translocated segments from the S genome or ribosomal genes. GRD prov ides an additional marker for the small chromosomes of the T genome an d a useful phylogenetic tool.