ABSTRACT
As an experimental organism, tomato presents a number of genetic, biological, and economic advantages that have led to its development as a model for cytogenetic and evolutionary studies. The cultivated tomato, Lycopersicon esculentum (= Solanum lycopersicum), and related wild species traditionally classified as genus Lycopersicon, more recently as Solanum sect. Lycopersicon (Spooner et al. 2005) are diploids, with a chromosome number of 2n=2x=24. Eleven of the 12 chromosomes in the haploid tomato nucleus are metacentric or submetacentric (Lapitan et al. 1989). The exception, chromosome 2, is acrocentric with a heterochromatic short arm consisting primarily of the nucleolus organizing region (NOR). During late prophase of meiosis (diakinesis), only chromosome 2 can be distinguished from the others, by virtue of its association with the nucleolus. However, in early prophase (pachytene), each of the 12 chromosomes can be identified by the position of the centromere, the length of chromatic and achromatic segments, and the pattern of heterochromatic knobs (chromomeres) (Khush 1963). These features are illustrated in corresponding cytological maps for each chromosome in the set (Rick and Butler 1956, Khush and Rick 1968).
