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

Comparison of the fitness of three types of populations of Drosophila melanogaster has been made. These are (1) inbred monomorphic controls, (2) heterotic polymorphic populations, and (3) monomorphic populations reselected from the polymorphic ones. These reselected populations have the genetic background of the polymorphic populations but the phenotypes of the monomorphic controls. Fitness was compared by measuring the weights and numerical sizes of the populations when maintained under controlled environmental conditions where natural selection operates. The reselected populations have been found to closely parallel the original heterotic polymorphic populations in fitness. Their performance is only slightly below these heterotic populations but very significantly above the inbred monomorphic controls. These data indicate that the heterosis found originally by Carson in the polymorphic populations is largely the result of heterozygosity for unknown genetic factors, perhaps polygene blocks, and only due in small degree to heterozygosity at the loci of sepia, spineless, and rough, the mutant marker genes in the monomorphic populations. Single gene heterosis apparently contributes little.