Principles of Plant Genetics and Breeding. George Acquaah

Self‐incompatible 7 S. pennellii L. pennellii Green 8 S. chilense L. chilense Green Old “peruvianum” group, crossable between these two species, but difficult to cross them with cultivated tomato and embryo rescue is often needed 9 S. peruvianum L. peruvianum Green 10 S. lycopersicoides L. lycopersicoides Green‐black Most closely related to old Lycopersicon species and crossable to S. lycopersicum, S. cheesmaniae, S. pimpinellifolium, and S. pennellii 11 S. sitiens L. sitiens Green Also known as S. rickii, Crossable with S. lycopersicoides 12 S. ochranthum L. ochranthum Green Unknown crossability with other Solanum species Section Juglandifolium 13 S. juglandifolium L. juglandifolium Green Unknown crossability with other Solanum species

       Introgression breeding

      Wild tomatoes have large genetic diversity, especially within the self‐incompatible species like S. chilense and S. peruvianum (Rick 1986). Tremendous variation has been revealed by molecular markers and it is striking that more genetic variation was observed within a single accession of the self‐incompatible species than in all accessions of any of the self‐compatible species (Egashira et al. 2000). Compared to the rich reservoir in wild species, the cultivated tomato is genetically poor due to the inbreeding during tomato domestication. It is estimated that the genomes of tomato cultivars contain less than 5% of the genetic variation of their wild relatives. The lack of diversity in the cultivated tomato can be visualized using DNA technologies. 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.

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