resistant loci. Since we have markers linked to each Ol‐genes, we could apply MAS. We did a disease test since (i) it is a relatively easy disease assay which can be carried out at seedling stage; and (ii) the resistance phenotype is clear to be scored. In case that the disease assay is not easy to perform, for example due to (i) quarantine pathogens or (ii) a disease test that has be done at the late developmental stage, MAS would be a convenient way to select resistant plants (Figure B5.4).
Figure B5.3 Cross‐pollinating scheme of generation of near‐isogenic lines (NIL) harboring dominant resistance genes to tomato powdery mildew. Via backcrosses, new traits are introduced from wild tomato relatives. During the backcrosses, selection of resistant plants can be performed via (1) disease test and/or (2) marker‐assisted selection (MAS, Figure B5.4). In addition, select plants should have similar morphology to the recurrent parent. * and **: Usually it takes many generations to remove the deleterious genes that go along with the introduced genes due to linkage drag. Therefore, it is useful to start with advance breeding lines having introgression from the wild species in order to shorten the backcrossing procedure. In our practice, we used advance breeding lines derived from S. habrochaites G1.1560 (donor of the Ol‐1 gene) to produce the F1. We checked BC3S1 plants for the uniformity of their genetic background by genotyping these plants with 12 AFLP primer combinations that produce genome‐wide markers. Of the 30 AFLP marker alleles specific for S. habrochaites G1.1560, they were only present in the BC3S1 plants with two fully cosegregating with the Ol‐1 gene, suggesting that the genetic background of these BC3S1 plants are genetically similar to MM. For the Ol‐4 genes derived from S. peruvianum LA2172, we started with the wild accession. With 12 AFLP primer combinations, 48 AFLP marker alleles were identified from S. peruvianum LA2172 and 11 of these alleles still segregated in the tested BC3S1 plants. Thus, when the backcross is started from wild accessions, more backcross generations are needed to make NILs.
Figure B5.4 Illustration of marker‐assisted selection (MAS). On the left, a genetic linkage map of tomato chromosome 6 showing that the Ol‐1 and Ol‐3 genes, conferring resistance to tomato powdery mildew, are located at the same locus and are flanked by Markers 3 and 4. On the right, electrophoretic patterns of PCR markers showing marker genotypes of 6 plants; the upper panel for Marker 3 and the lower panel for Marker 4. Plant 1–4 are either BC3 plants (for Marker 3) and BC3S1 plants (for Marker 4). Plant 5 and 6 are parental plants that are susceptible and resistant to tomato powdery mildew, respectively. M indicates DNA size marker of 1 kb ladder. For MAS, marker flanking the target gene is often used. For Marker 3, BC3 plants no. 1 and 3 are selected and expected to be resistant to powdery mildew since they have the marker allele of the resistant parent (plant no. 6). For Marker 4, plant 1 to 3 are selected and expected to be resistant since they have the resistant marker allele as the resistant parental plant no. 6 (homozygous plant no. 1 and heterozygous plant no. 2 and 3).
After several backcrossing generations, homozygous BCnS1 resistant plants of these crosses were selected (Figure B5.3). Since we have facilities for genome‐wide analysis, we genotyped all selected plants with AFLP markers to compare their genetic background with the recurrent parent MM. BCnS1 resistant plants that were genetically most similar to MM were maintained as NILs.
Releasing NILs to companies for production of resistant cultivars
These NILs harboring dominant Ol genes are valuable advanced breeding lines and have been used by seed companies for breeding tomato cultivars with resistance to tomato powdery mildew, which are now available on the market. The NILs for the Ol‐qtls are still in development via MAS.
References
1 Egashira, H., Ishihara, H., Takshina, T., and Imanishi, S. (2000). Genetic diversity of the ‘peruvianum‐complex’ (Lycopersicon peruvianum (L.) Mill. and L. chilense Dun.) revealed by RAPD analysis. Euphytica. 116: 23–31.
2 Huang, Y., Komoto, J., Konishi, K. et al. (2000). Mechanisms for auto‐inhibition and forced product release in glycine N‐methyltransferase: crystal structures of wild‐type, mutant R175K and S‐adenosylhomocysteine‐bound R175K enzymes. J Mol Biol 298 (1): 149–162.
3 Kiss, L., Cook, R.T.A., Saenz, G.S. et al. (2001). Identification of two powdery mildew fungi, Oidium neolycopersici sp. nov. and O. lycopersici, infecting tomato in different parts of the world. Mycological Research 105 (2001): 684–697.
4 Kiss, L. and Takamatsu, S. (2005). Cunnington Molecular identifications of Oidium neolycopersici as the causal agent of the recent tomato powdery mildew epidemics in the North America. Plant Disease (89): 491–496.
5 Paternotte, S.J. (1988). Occurrence and chemical control of powdery mildew (Oidium sp.) in tomatoes. Mededelingenvan de Faculteit Landbouwwetenschappen RijksuniversiteitGent (53/2b): 657–661.
6 Picken, A.J.F., Hurd, R.G., and Vince‐Prue, D. (1985). Lycopersicon esculentum. In: Handbook of flowering III (ed. A.H. Halevy), 330–346. Boca Raton: CRC Press.
7 Rick, C.M. (1986). Germplasm resources in the wild tomato species. Sci. Hort 200: 45–55.
8 Rick, C.M. (1988). Tomato‐like nightshades: affinities, auto‐ecology, and breeders opportunities. Economic Botany. 42: 145–154.
9 Taylor, I.B. (1986). Biosystematics of the tomato. In: The Tomato Crop ‐ A scientific Basis for Improvement (eds. J.G. Atherton and J. Rudich), 1–34. London: Chapman and Hall.
Repeated selfing has no genetic consequence in self‐pollinated species (no inbreeding depression or loss of vigor following selfing). Similarly, self‐incompatibility does not occur. Because a self‐pollinated cultivar is generally one single genotype reproducing itself, breeding self‐pollinated species usually entails identifying one superior genotype (or a few) and multiplying it. Specific breeding methods commonly used for self‐pollinated species are pure line selection, and also pedigree breeding, bulk populations, and backcross breeding.
5.6 Genotype conversion programs
To facilitate breeding of certain major crops, projects have been undertaken by certain breeders to create breeding stock of male sterile lines that plant breeders can readily obtain. In barley, over 100 spring and winter wheat cultivars have been converted to male sterile lines by USDA researchers.
