An estimate of heterosis inDrosophila melanogaster

SUMMARY Twenty-five population cages of D. melanogaster were set up, each containing a different wild-type second chromosome and the marker chromosome Cy . In all but one case where contamination apparently occurred, the Cy chromosome persisted in the population at high frequency, showing a selective advantage of Cy / + heterozygotes over wild-type homozygotes. Overall, the results indicate that homozygosity of the entire second chromosome causes a depression in fitness of the order of 85%.