The L. pennellii Introgression Lines (ILs)

A) Population structure
We have developed, using whole genome marker analysis, a permanent mapping population designed for QTL analysis. This resource is composed of a tomato variety (Lycopersicon esculentum CV. M82) which includes single introgressed genomic regions from the wild green-fruited species L. pennellii. This congenic resource is composed of 76 introgression lines (ILs) where amongst them there is complete coverage of the wild species genome. The ILs have been produced through successive introgression backcrossing and marker-assisted selection to generate a set of recurrent parent lines with single introgressed segments (Eshed and Zamir 1994).

Figure 1

The IL map is connected to the high-resolution F2 map composed of 1500 markers (IL chromosome maps). Seed of the ILs are distributed by The C.M. Rick Tomato Genetics Resource Center, University of California Davis and the ILs were assigned accession numbers LA4028 - LA4103.

B) The genetic basis of QTL variation
Gene action of quantitative trait loci (QTL) can be determined by comparing an IL homozygous for a particular wild species introgression to its nearly isogenic hybrid with M82 which is heterozygous for the same chromosome segment. Three nearly isogenic lines (M82, IL1-1 and ILH1-1), their genome composition and the genetic components of the QTL variation are cartooned in Figure 2. Based on the mean phenotypic values of the lines it is possible to estimate the mode of action of the QTL (Eshed and Zamir 1995).

Figure 2

C) IL bin-mapping of 'trait QTL'
The bin mapping method is exemplified in Figure 3. The exotic accession of L. pennellii is represented by a pair of homologous chromosomes of the donor parent (green chromosomes), the recurrent parent M82 (red chromosome) and six ILs from chromosome 1 that are presented in Figure 3. The ILs, all of which are homozygous for the red chromosomes in the rest of the genome, constitute a set of nearly isogenic lines for chromosome 1. The six ILs covering chromosome 1 create 10 mapping bins (1A - 1J), each with a unique donor parent composition. A 'trait QTL' is mapped by phenotyping all lines in randomised replicated trials and presenting the results as percent difference from the recurrent parent M82 (0 value; black bars indicate a significant difference from the common control; empty bars indicate non-significant differences). Combined analysis of the data for all lines defines a QTL that increases the phenotypic value to bin 1C and a reducing QTL to bin 1H. The mapping scheme presented here was adopted from actual tomato data (Fridman et al. 2002).

Figure 3

Selected IL publications :