Collections of prebred stocks are maintained by the TGRC and are popular for mapping and breeding purposes. These include introgression lines (ILs), alien substitution lines, monosomic alien addition lines, and backcross (BC) recombinant inbreds. The introgression libraries were derived from S. pennellii (Eshed and Zamir 1994a; Liu and Zamir 1999), S. habrochaites (Monforte and Tanksley 2000a), or S. lycopersicoides (Canady etal. 2005). An entire genome of S. pennellii is covered by 50 overlapping introgressions; 26 additional sublines provide increased mapping resolution in some regions (see Sect. 1.8.3). Each line is homozygous for a single introgression from S. pennellii (LA0716) in the background of S. lycopersicum cv. M82. The S. habrochaites and S. lycopersicoides prebreds have a similar genetic makeup but with less complete genome coverage and in some cases more than one introgressed region per line. Because of sterility factors, some of the S. lycopersicoides lines are maintained via heterozygotes. Introgression line libraries are also being developed for other wild species, such as S. chmielewskii (http://www.keygene.com/ services/plants/services_plants_line.htm).
The second type of prebred stocks maintained at the TGRC is a group of BC recombinant inbred lines (RILs) originating from the cross S. lycoper-sicum x S. pimpinellifolium (Doganlar et al. 2002c). Two generations of backcross followed by at least 6 generations of inbreeding via single-seed descent resulted in a high level of homozygosity in these RILs, whose residual heterozygosity is averaged at ~3%. The population has been genotyped at 127 marker loci, and the corresponding maps and relevant data files are available from the Solanaceae Genome Network (SGN) (http://www.sgn.cornell.edu). A set of 99 RILs has been selected for optimum mapping resolution. This provides a permanent, high-resolution mapping population. Numerous other types of unbalanced populations have also been developed to improve mapping resolution; see review by Ji and Scott (2007) for more details.
A few alien substitution and alien addition lines, each containing an intact alien chromosome from one of the wild relatives in the background of cultivated tomato, are also available. The substitutions represent seven of the 12 S. pennellii (LA0716) chromosomes (Rick 1969, 1972; Weide et al. 1993), and four S. lycopersicoides (LA2951) chromosomes (Chetelat and Meglic 2000; Ji and Chetelat 2003). The monosomic alien addition lines, of which there are ten, each contains one extra chromosome (i.e., 2n + 1) from S. ly-copersicoides LA1964 added to the tomato genome (Chetelat et al. 1998).
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