|Authors: ||C.M. Rick, R.T. Chetelat|
|Keywords: ||Lycopersicon, Solanum, germplasm resources, plant breeding, wide crosses|
Typical of self-pollinated crops, traditional cultivated tomatoes severely lack genetic diversity.
Consequently, since ca. 1940, breeders have relied increasingly on exotic sources -- particularly related wild spp. -- for desired traits.
Since then, accelerated introgression of useful exotic traits conttibuted to spectacular improvement, manifest in a 4 to 5-fold yield increase.
Nearly unknown in tomato cultivars prior to 1940, resistance to at least 42 major diseases has been discovered in exotics and 20 of them bred into horticultural tomatoes -- numbers that are continually increasing.
Current progress is exemplified by detection gemini virus resistances in Lycopersicon chilense and transfer of TYLCV tolerance from this source.
Future gains from this area as well as resistance to arthropod pests are anticipated.
Fruit quality traits (soluble solids, etc.) have been improved thereby.
Tolerances to such environmental stresses as temperature extremes, drought, excess moisture, salinity and other edaphic problems have been ascertained and transfers therefore are in progress.
Time and other constraints on introgression delay progress in such exploitation of exotic sources.
Molecular techniques and prebreeding (transfer of wild traits to largely L. esculentum background) can allay these problems.
Regarding the latter, a battery of prebreds that cover the complete genome of L. pennellii has been synthesized and others are in progress for Solanum lycopersicoides and other spp.
Stocks of these libraries are being increased and inventoried for distribution in the near future to facilitate utilization.
Introgressed chromosomal segments are mapped via molecular markers, which can also facilitate monitoring the transfer of desired traits.
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