Introduction Marker assisted breeding (MAB)

In this assignment we will show how to design a commercial tomato variety which is resistant to whiteflies (Bemisia tabaci). Whiteflies damage the tomato plant and its fruits and are the vectors of many viruses. At the moment viruses are the main thread for commercial tomato growers. Since screening for whitefly resistance is a tedious and laborious procedure, we want to replace the screening during the breeding process with a screening based on molecular markers. However, screenings with living whiteflies are still necessary in the research part when you are looking for sources of resistance in wild tomato plants and when you search for linkage to molecular markers. Also, at the final stage in breeding you need a screening to make sure that you have truly bred a new variety with whitefly prior to commercializing. Resistance against whiteflies in tomato plants is mainly important in field grown varieties because in protected greenhouses natural enemies of whiteflies can be used to control the population build-up of whiteflies (biological control).

Figure: The concept of Marker Assisted Breeding for Research and Application

In the figure above you can see a picture describing the concept of Marker Assisted Breeding (MAB) for research and application. For research you need a lot of markers (like SNPs), a large segregating population (F1, F2, BC1 or RIL) and a good analysis of the desired trait/phenotype. The data from the markers (genotyping) and phenotyping of the trait of the segregating population are all combined in a Linkage analysis study where you hope to find markers that are associated/linked to the desired trait. Once these markers have been found they can be applied in the breeding and selection program. Now the trait can be followed in the selection process with the linked marker this saves time and money.

Assignment: Breeding for resistance to whitefly in tomato:

In this assignment we will go through the different steps of this concept of MAB. We will look at different marker technologies (SNPs, KASP, next generation sequencing for genotyping), discover the influence of the different mapping populations on the linkage analysis, make genetic linkage maps and eventually find makers linked to whitefly resistance.

But first we need to introduce the trait/phenotype that we are looking for: whitefly resistance