X-chromosomal heterosis inDrosophila melanogaster

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Abstract
SUMMARY: Population cages were set up containing anX-chromosome balancer, and either a single wild-type chromosome(homozygous cages) or a mixture of wild-type chromosomes(heterozygous cages). The balancer chromosome was eliminated more rapidly from the heterozygous cages, indicating that chromosome heterozygotes are at an advantage over chromosome homozygotes. The disadvantage ofX-chromosome homozygosity in the female is estimated to be about 40%. From earlier studies it is known that the average disadvantage of homozygosity for either of the two major autosomes ofD. melanogasteris approximately 80%. Since these autosomes are both about twice as long as theXchromosome, the disadvantage per unit length is similar for both chromosomal types.BothX-chromosomal and autosomal heterosis can be explained by either dominance or overdominance at individual loci. However, a dominance model can only explain the similarity if many of theX-linked loci (about 50%) are limited in expression to the female.