Screening for sources of resistance
Whiteflies and their eggs
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Resistance to Bemisia tabaci in tomato wild relatives Firdaus et al. 2012 (Euphytica 187:31–45 DOI 10.1007/s10681-012-0704-2,
Introduction
Tomato is threatened by many diseases (fungal, bacterial, viruses and pests) and in many cases no resistance is present in the cultivated tomato genepool in Europe and therefore new sources of resistance have to be found in its wild, crossable relatives. Sources of resistance were successfully looked for in the centre of origin of the tomato in South America and many of these resistances were successfully introduced in modern varieties.
Develop a reliable screening method for whitefly resistance
Two types of assessments can be used to evaluate whitefly resistance in tomato plants: a free-choice test or a no-choice test. In a free-choice test, whiteflies are given the choice between two or more different plants, whereby the whitefly is able to choose the most preferred one.
Clip-on cage test
To test the resistance of our tomato plants in this experiment we will use the clip-on cage test, a no-choice test.
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It is very important to determine a reliable score of the level of resistance of each plant. The better and more precise the phenotypic data, the better the linkage analysis can be performed.
Questions phenotyping
3.1 Antibiosis and antixenosis are two methods to combat whiteflies (look for the definition in Google or other search engines). What is the difference between the two methods?
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Which plant is more resistant to whiteflies?
Search for the best donorplant with wildfly resistance
In the article of Firdaus et al. 2012, several tomato accessions* were tested with the clip-on cage test, see results in table 1. *An accession is a collection at a certain location of a certain species. It is a collection of different seeds. Not all plants of an accession are necessarily genetically identical.
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3.10 Which 3 accessions of different species have the highest resistance level?
The role of trichomes in resistance
Glandular trichomes in section Lycopersicon. Wild accessions have high densities of glandular trichomes that confer resistance to several pests. Panel (A) shows the leaflet surface of Solanum habrochaites acc. LA 1777 with high densities of glandular trichome types IV and VI (B), and type I (C). Surface of Solanum pennellii acc. LA 716 is also covered by type IV trichomes (D, E) producing and secreting acyl sugars. This accession also has type VI trichomes, but in low density (F). Panel (G) shows the surface of Solanum lycopersicum cv. Moneymaker. Cultivated tomato has low density of type VI trichomes (H) and type I trichomes. Sometimes, type IV-like trichomes (I) are observed on stems, veins, and on the leaflet edges. White bars represent 500 μm in panel A, C, D, and G. In panels B, E, F, H, and I, bars represent 50 μm.
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Firdaus et al. 2012 table 2 show the means of whitefly resistance parameters and type-trichome density in clip-on cage tests with several accessions of different tomato species.
Questions trichomes
3.11 What are trichomes?
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3.16 What 3 species/accessions are the most resistant to whiteflies in Table 2? Are they the same as Table 1?
Crossability with tomato
To know the phylogenetic relationship between the resistant accession and cultivated tomato (S. lycopersicum) is very important. The more related these species are, the easier they are to cross for making large mapping populations. In the paper of Marcela Viquez-Zamora (BMC Genomics201314:354) the phylogenetic relations between different wild relatives of tomato have been calculated. See figure below.
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