Somatic chromosome karyotype of tomato based on in situ hybridization of the TGRI satellite repeat

Abstract

A karyotype of tomato mitotic chromosomes was constructed based on in situ hybridization to a 162-bp telomeric DNA repeat, TGRI. Variation in the spatial and quantitative distribution of this repeat creates distinct patterns for most of the chromosomes, which along with other morphological characteristics (i.e., length and arm length ratio), allow the identification of each of the 12 mitotic chromosomes of tomato. The structure and physical size of the TGRI clusters were further investigated by means of pulsed-field gel electrophoresis. Approximately 30 hybridizing fragments were observed in the range of 25 to 1000 kb when high molecular weight DNA was digested with BglII and probed with TGRI. The total molecular weight of these fragments is approximately 14 million bp, which is close to the estimated total length of TGRI in the genome (12.5 million bp) based on genomic reconstruction experiments. The results suggest that most of the TGRI clusters consist of single, uninterrupted blocks of satellite DNA. Assignment of somatic chromosomes, identified by TGRI hybridization to the previously established tomato linkage groups, was accomplished via in situ hybridization to mitotic spreads of primary trisomic lines. Using this information, we estimate the somatic length and DNA content of each of the tomato chromosomes and chromosome arms. Key words: Lycopersicon esculentum, somatic karyotype, in situ hybridization, satellite DNA, pulsed-field gel electrophoresis.