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Modification of heat resistance in Drosophila by selection


WHAT functional component of an animal is most sensitive to heat? We discuss here two reasonable possibilities which have quite different implications. If heat injury is mediated primarily by enzyme inactivation, then natural selection may well have brought a large number of enzymes to the point where a given duration at a high temperature should cause a comparably critical loss in the activity of each. If this is the case, then selection for increased heat resistance would require simultaneous responses at many enzyme loci, as one sensitive enzyme would make the increased stability of the others worthless. In genetic terms, the sensitive enzyme would be epistatic to the rest. On the other hand, if cell or tissue damage is the first effect of excessive heat, then heat resistance is likely to be a quantitative characteristic, with a considerable additive genetic component. Selection for increased thermostability should be possible. In either case, selection for reduced heat resistance should be relatively rapid, as temperature-sensitive mutants would, as a class, probably be frequent enough to dominate the situation.

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