Very few polymorphisms within the cultivated tomato gene‐pool are identified, even using sensitive molecular markers. Tomato domestication experienced severe genetic bottleneck as the crop was carried from the Andes to Central America and from there to Europe. The initial domestication process was, in part, reached by selecting preferred genotypes in the existing germplasm. Selection of a horticultural crop like tomato is usually done on a single plant basis and with small numbers of selected plants. In a predominantly inbreeding species, genetic variation tends to decrease, even without selection. As a consequence, genetic drift is a major process that reduces genetic variation.
Most likely, no exchange of genetic information with the wild germplasm took place until the 1940s. By then, the renowned geneticist and plant breeder Charlie Rick (University of California, Davis) observed that crosses between wild and cultivated species generated a wild array of novel genetic variation in the offspring. Since then, breeding from wild species via interspecific crosses and followed by many times of backcrosses to cultivated tomatoes (so‐called introgression breeding), has led to the transfer of many favorable attributes in the cultivated tomato. Breeding barriers are sometimes expected in interspecific crosses, which include unilateral incompatibility, hybrid inviability, sterility, reduced recombination, and linkage drag.
An example of introgression breeding
One of the common breeding objectives in tomato is breeding for resistance to the most destructive pests and pathogens. Tomato hosts more than 200 species of a wide variety of pests and pathogens that can cause significant economic losses. Tomato powdery mildew caused by Oidium neolycopersici occurred for the first time in 1986 in The Netherlands (Paternotte 1988). Within 10 years it had spread to all European countries and is nowadays a worldwide tomato disease, except for Australia where another species (O. lycopersici) occurs (Kiss et al. 2001). Upon the outbreak of O. neolycopersici, all tomato cultivars were susceptible and this fungus was the only one to be controlled by fungicides in greenhouse tomato production in Northwest Europe (Huang et al. 2000). By 1996, our group was invited by Dutch vegetable seed companies to search for resistance genes against O. neolycopersici. Here I will use our practice on breeding tomatoes with resistance to powdery mildew as an example for introgression breeding.
Search for resistance in wild relatives of tomato
As a consequence of inbreeding during tomato domestication, the genetic diversity in cultivated tomato is now very narrow. However, large variation is present and exploitable in the wild Solanum species. Thus, the first step was to find wild tomato accessions with resistance to tomato powdery mildew.
At the Tomato Genetics Resource Center in Davis, California (TGRC, http://tgrc.ucdavis.edu) and Botanical and Experimental Garden (http://www.bgard.science.ru.nl) in the Netherlands, thousands of accessions of the wild Solanum species have been collected and maintained. From these collections, we have selected and tested some Solanum species with tomato powdery mildew. As expected, many wild accessions showed resistance (Figure B5.1).
Figure B5.1 Tomato plants inoculated with tomato powdery mildew. (a) The left plant is from tomato wild species Solanum pervianum LA2172, showing no powdery mildew infection; the right plant is from S. lycoerpsicum cv. Moneymaker (MM), showing fungal clonies growing on infected leaves. (b) A closer look at the colonization of tomato powdery mildew (Oidium neolycopersici) growing on the upper‐side of MM leaf. Pictures were taken 15 days post inoculation.
Inheritance of the resistance
Monogenic resistance is most exploited in tomato breeding programs. Modern tomato cultivars may harbor resistances to more than 10 pathogens. Thus, the second step is to study the inheritance of the resistance identified in the wild tomato species. For this purpose, resistant plants were selected and crossed to a susceptible cultivar, S. lycopersicum cv. Moneymaker to produce populations (usually F2 populations) for inheritance study. Crosses between S. lycopersicum and wild tomato species can be easy but sometimes require strategies such as embryo rescue, especially for the self‐incompatible species like S. peruvianum.
By using F2 populations, inheritance of resistance identified in several wild species was characterized. Monogenic resistance to O. neolycopersici was found in S. peruvianum LA2172, S. habrochaites G1.1560 and G1.1290, and polygenic resistance in S. neorikii G.16101. Rick, C.M. (1988). Further, by screening these F2 plants with molecular markers, such as RAPD, AFLP, and CAPS, the resistance in these species was mapped onto specific chromosomes. The resistance loci in S. peruvianum LA2172 and S. habrochaites G1.1560 and G1.1290, named Ol‐4, Ol‐1, and Ol‐3, respectively, are all located on tomato chromosome 6. The Ol‐4 locus is on the short arm, while Ol‐1 and Ol‐3 are on the long arm and closely linked if not allelic (Figure B5.2). In addition to these monogenic Ol‐genes, three quantitative trait loci (QTLs) were identified governing the resistance in S. neorickii G1.1601. The Ol‐qtl1 interval overlaps with Ol‐1 and Ol‐3, while the other two linked Ol‐qtls are located on chromosome 12 in the vicinity of the Lv locus that confers resistance to another powdery mildew species, Leveillula taurica. Markers with close linkage to these loci were generated and can be applied in marker‐assisted selection (MAS) in breeding programs.
Figure B5.2 The chromosome locations of tomato loci for resistance to tomato powdery mildew caused by Oidium neolycopersici. On the left, genetic distance in cM is shown. On the right, map positions of markers and resistance loci are shown on tomato chromosome 6 and 12, respectively. The donors for Ol‐1, Ol‐3, Ol‐5 are Solanum habrochaites G1. 1560, G1. 1290, and PI247087, respectively; for Ol‐4 is S. peruvianum LA2172 and for Ol‐qtls is S. neorickii G1.1601. Ol‐6 is identified from an advanced breeding line with unknown source. As to Ol‐qtls, bars indicate the QTL interval for which the inner bar shows a one‐LOD support and the outer one shows a two‐LOD support interval.
Generation of near isogenic lines
Near‐isogenic lines (NILs) that carry small introgressed chromosome fragments from related wild species in a cultivated tomato background are most useful pre‐bred in a breeding program. To develop NILs that only differ in Ol genes for resistance to O. neolycopersici, resistant donor accessions were crossed with susceptible S. lycopersicum cv. Moneymaker (MM). BC crosses were made starting from crossing F1 plants back to MM (Figure B5.3). During the backcrossing, selection of resistant BC plants can be performed in two ways. One is by testing BC
