Tomato DNA markers
The cultivated tomato (Solanum lycopersicum L.) is the second most commonly consumed vegetable crop (after potato) and unquestionably the most popular garden crop in the world. In the United States, it is the most economically important vegetable crop, and is fifth in crop value after maize, soybean, wheat, and cotton.
With the advent of molecular markers and genetic maps, there has been an increased interest in using markers technology to facilitate tomato crop improvement. Tomato was among the first crop species for which genetic markers and maps were developed and utilized for breeding purposes (Tanksley, 1983; Tanksley et al., 1992).
Tomato was one of the first crops for which molecular markers were suggested as indirect selection criteria for breeding purposes (Rick and Fobes, 1974; Tanksley and Rick, 1980; Tanksley, 1983).
More recently, with the development of new molecular markers and maps in tomato, Marker Assisted Selection- MAS has become a routine practice in many tomato breeding programs, in particular in the private sector.
Recently, several years ago, also other morphological tomato traits were mapped and now are becoming further used in MAS for genotyping and enhancing commercial breeding programs. Traits like ripening inhibitor tomato lycopene and beta carotene mutations. Plant growth structures along fruit peel and color and sugar content (BRX) traits are now routinely genotypes together with the resistance markers. Below are elaborations for several of these tomato morphological traits:
nor non-ripening Similar to the rin mutant, nor fruits fail to ripen and have a green pericarp . The nor mutation comprises a 2 bp deletion in the third exon of NAC-NOR, resulting in a truncated protein . The mutation reduces both ethylene production and lycopene biosynthesis
U Uniform ripening
Modern tomato (Solanum lycopersicum) varieties are bred for uniform ripening (u) light green fruit phenotypes to facilitate harvests of evenly ripened fruit. U encodes a Golden 2-like (GLK) transcription factor, SlGLK2, which determines chlorophyll accumulation and distribution in developing fruit.
SGR stay green, green-flesh (gf)
Fruit of the green-flesh (gf) mutant of tomato (Solanum lycopersicum) ripen to a muddy brown color due to the accumulation of lycopene coupled with a lack of chlorophyll degradation (Kerr, 1956). The lack of chlorophyll degradation in gf is not restricted to fruits because dark- and nutrient-induced chlorophyll loss in leaves is also compromised in the mutant (Akhtar et al., 1999).
Dark Green dg
Several light-hypersensitive mutants have been described in tomato (Lycopersicon esculentum). Among these, mutants carrying the monogenic recessive high pigment (hp-1 and hp-2) and dark green (dg) mutations are characterized by their exaggerated light responsiveness.
The hp-1 mutant was originally discovered as a spontaneous mutant in 1917 at the Campbell Soup Company farms (Riverton, N.J.) (Reynard 1956), the hp-2 mutant was reported in the Italian San Marzano variety in 1975 (Soressi 1975) and the dg mutant appeared in trellised planting of the Manapal variety (Konsler 1973). Despite some initial confusion, it is now clear that hp-1 and hp-2 mutations map to the tomato chromosomes 2 and 1, respectively, and are therefore non-allelic
The dg mutant is phenotypically similar to other hp mutants, but has a much darker mature-green fruit, resulting from a higher total chlorophyll content (Konsler 1973; Wann et al. 1985).
Phytoene synthase (PSY) catalyzes the dimerization of two diterpenes from geranylgeranyl pyrophosphate to phytoene, which is the rate-limiting step in carotenoid biosynthesis. Mutations in the tomato PSY1, yellow flesh, are widely used for breeding yellow tomatoes.
Fruit of tangerine are orange and accumulate prolycopene (7Z,9Z,7′Z,9′Z-tetra-cis-lycopene) instead of the all-trans-lycopene, which normally is synthesized in the wild type.
The color of tomato fruit is mainly determined by carotenoids and flavonoids. Phenotypic analysis of an introgression line (IL) population derived from a cross between Solanum lycopersicum ‘Moneyberg’ and the wild species Solanum chmielewskii revealed three ILs with a pink fruit color. These lines had a homozygous S. chmielewskii introgression on the short arm of chromosome 1, consistent with the position of the y (yellow) mutation known to result in colorless epidermis, and hence pink-colored fruit, when combined with a red flesh. Metabolic analysis showed that pink fruit lack the ripening-dependent accumulation of the yellow-colored flavonoid naringenin chalcone in the fruit peel, while carotenoid levels are not affected.
Vegetative and reproductive phases alternate regularly during sympodial growth in tomatoes. In wild-type ‘indeterminate’ plants, inflorescences are separated by three vegetative nodes. In ‘determinate’ plants homozygous for the recessive allele of the SELF-PRUNING (SP) gene, sympodial segments develop progressively fewer nodes until the shoot is terminated by two consecutive inflorescences.