Sexual hybridization of two divergent lines of , which had been selected for increased penicillin titre through successive cycles of mutagenesis, released considerable variation for this character. The recovery of segregants with titres equivalent to that of the unselected ancestor suggested that mutations in different genes had been selected in the two lines. However, complementary segregants with substantially improved titres were not found, indicating interactions, probably of a duplicate nature, among the induced mutations. All the genetic variation released by hybridization was fixed following two generations of selection for high titre, but only a small gain over the initial selection lines was achieved. Hybridization of divergent strains has been widely advocated as a means of strain development. The failure to achieve the anticipated gains in this programme is attributed primarily to the unfavourable interactions amongst the induced mutations. Whether similar interactions occur generally in crosses between strains selected by mutagenesis remains to be established and will be an important factor in determining the contribution of recombinational approaches to yield improvement.


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