The tomato genome is well represented by various types of cytogenetic stocks. The TGRC maintains au-totetraploids, sesquidiploids, translocations, and several types of trisomics. These stocks contain cyto-logically detectable changes in chromosome numbers or chromosome structure. The autotetraploids and sesquidiploids are euploids (i.e., have changes in whole sets of chromosomes), while the trisomics are aneuploids, having a single extra chromosome which maybe rearranged in various ways. The autote-traploids arose from spontaneous or induced chromosome doubling, the sesquidiploids (i.e., interspecific triploids) from artificial hybridization between tomato and related wild species (Rick etal. 1986). The trisomics were isolated as spontaneous unfruitful plants in fields of tomato, or were selected in progeny of autotriploids or following mutagenic treatments. A complete set of primary trisomies, containing one extra, nonrearranged tomato chromosome, is available. In addition, there are partial sets of secondary trisomics, which contain an extra isochromosome (i.e., two identical chromosome arms on either side of a centromere), tertiary trisomics, which contain a translocated chromosome (one arm from each of two different chromosomes), telo-trisomics, with an extra chromosome arm, and compensating trisomics, with the loss of a normal chromosome compensated by the presence of two arms in new, translocated associations (Khush and Rick 1968a). All types of trisomics are maintained via seed and have been utilized for gene mapping and other cytogenetic studies. For example, the assignment of certain linkage groups of morphological markers to their respective chromosomes was based on distortion in segregation ratio in the F2 populations of primary trisomics (Lesley 1932; Rick and Barton 1954). Other trisomic types can be used in finding information regarding arm location, position of centromeres and orientation of linkage markers (Frary et al. 1996). In addition, tri-somics have been used for chromosomal assignment of molecular markers by dosage analysis (Fobes 1980; Young etal. 1987), and for identification of individual chromosomes in synaptonemal complex spreads (Sherman and Stack 1992).
Changes in chromosome structure constitute another major type of chromosomal variation in tomatoes. Various kinds of structural alterations, including duplications, deficiencies, inversions, and translocations, have been identified. Only the reciprocal translocations have been maintained, probably because they are transmissible to the next generation (unlike deficiencies), fertile, and easily applied to mapping studies. They have been used for assignment of gene loci to their respective chromosomal arms and as a source of tertiary trisomics (Khush and Rick 1967a). The TGRC collections maintain 37 such lines, eight of which consist of a tester set involving all 12 tomato chromosomes (Gill 1983).
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