The Contribution of Heterozygosity at Certain Gene Loci to Fitness of Laboratory Populations of Drosophila melanogaster

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Abstract
The fitness of 3 populations of Drosophila melanogaster was compared by maintaining these populations under controlled laboratory conditions in which natural selection could take place, using total wet weight of and number of flies in the populations at intervals as criteria of fitness. The populations compared were the following: (1) an inbred population in which the autosomal recessive mutants sepia (se), spineless (ss), and rough (ro) were fixed; (2) a population similar to the above but in which a haploid set of wild-type autosomes had been introduced producing increasedfitness previously found by Carson (Carson, 1958, Proc. Nat. Acad. Sci. 44: 1136-1141); and (3) a population derived from population (2) above in which heterozygosity at the se, ss, and ro loci was replaced by homozygosity for these mutant alleles but the rest of the variability in the parent population left intact. The results of the comparison showed that the fitness of this newly derived population was practically as great as that from which it was derived. Therefore it is concluded that heterozygosity at the se, ss, and ro loci contributed little, if anything, to the previously found fitness of the heterozygous population studied by Carson.