Screening for sources of resistance
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Whiteflies and their eggs
For further reading, description of the screening for resistance.
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.
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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. 
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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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Plants are infested with whiteflies six weeks after sowing. Five synchronized whitefly females (one to two days old) (n) were anesthetized with CO2 and put into a clip-on cage (2 cm in diameter and 1 cm in height) and placed immediately on the abaxial of a leaflet of the 3rd or 4th leaf from the apex. Five clip-on cages with each 5 females were attached per plant. Four days after infestation (d) the clip-on cages were removed from the leaves and the dead and living whiteflies (m) were counted. The number of eggs (e) was counted under a stereo microscope (10x magnifications). The clip-on cages were reassembled at their original positions on each leaflet before new adult whiteflies started to emerge from the eggs. From these tests the Adult Survival (AS) and the Oviposition Rate (OR) of the white flies are calculated. These parameters are used to determine the level of resistance of a plant.
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OR: Oviposition Rate
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Difference between male and female whiteflies. Click to enlarge.
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3.6 Whiteflies lay the eggs sometimes in circles (see picture). Any idea what is the reason for that?
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3.7 In the free choice test Amaranthus plants are present. They are very susceptible for whiteflies. What is their role in the experimental set-up. Think of a method to make the whiteflies leave the Amaranthus plants.
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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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Questions donorplant
3.1 Which accession has the highest resistance level against white flies? Explain your answer.
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The role of trichomes in resistance (Click for better quality)
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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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