A molecular and cytogenetic survey of major repeated DNA sequences in tomato (Lycopersicon esculentum)

Abstract

The major families of repeated DNA sequences in the genome of tomato (Lycopersicon esculentum) were isolated from a sheared DNA library. One thousand clones, representing one million base pairs, or 0.15% of the genome, were surveyed for repeated DNA sequences by hybridization to total nuclear DNA. Four major repeat classes were identified and characterized with respect to copy number, chromosomal localization by in situ hybridization, and evolution in the family Solanaceae. The most highly repeated sequence, with approximately 77000 copies, consists of a 162 bp tandemly repeated satellite DNA. This repeat is clustered at or near the telomeres of most chromosomes and also at the centromeres and interstitial sites of a few chromosomes. Another family of tandemly repeated sequences consists of the genes coding for the 45 S ribosomal RNA. The 9.1 kb repeating unit in L. esculentum was estimated to be present in approximately 2300 copies. The single locus, previously mapped using restriction fragment length polymorphisms, was shown by in situ hybridization as a very intense signal at the end of chromosome 2. The third family of repeated sequences was interspersed throughout nearly all chromosomes with an average of 133 kb between elements. The total copy number in the genome is approximately 4200. The fourth class consists of another interspersed repeat showing clustering at or near the centromeres in several chromosomes. This repeat had a copy number of approximately 2100. Sequences homologous to the 45 S ribosomal DNA showed cross-hybridization to DNA from all solanaceous species examined including potato, Datura, Petunia, tobacco and pepper. In contrast, with the exception of one class of interspersed repeats which is present in potato, all other repetitive sequences appear to be limited to the crossing-range of tomato. These results, along with those from a companion paper (Zamir and Tanksley 1988), indicate that tomato possesses few highly repetitive DNA sequences and those that do exist are evolving at a rate higher than most other genomic sequences.