The isolation of ‘pure lines’ fromF1hybrids of tomato, and the problem of heterosis in inbreeding crop species

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Abstract
1. The problem of inbreeding and the fixation of apparent heterosis in the cultivated tomato and in inbreeding crop species generally is discussed. The case is argued that ‘heterosis’ as normally understood in out-breeders is not likely to exist as a general phenomenon in inbreeding species.2. Selections were made among segregating progenies of two successful, commercialF1hybrids, and the performance of the selected lines, up to theF4generation, shows that the superiority of the hybrids over their parents can be fixed in purebreeding lines.3. Owing to selection effects in inbreeding species of crops the best parents will differ in respect of fewer quantitative loci than is to be expected from general experiments on quantitative inheritance. In the present material, desirable recombinants were isolated at a frequency of 1 in every 1000–1500F2individuals.4. The utility ofF1hybrids in the tomato thus depends on the economy of effort involved in their development as compared with the isolation of pure lines. It is concluded that the production of hybrids is to be favoured only when reliable methods of prediction of their performance are available, and then as an interim measure while pure-line forms become available.