Genomic imprinting and position-effect variegation in Drosophila melanogaster

Genomic imprinting is a phenomenon in which the expression of a gene or chr omosomal region depends on the sex of the individual transmitting it. The t erm imprinting was first coined to describe parent-specific chromosome beha vior in the dipteran insect Sciara and has since been described in many org anisms, including other insects, plants, fish, and mammals. In this article we describe a mini-X chromosome in Drosophila melanogaster that shows geno mic imprinting of at least three closely linked genes. The imprinting of th ese genes is observed as mosaic silencing when the genes are transmitted by the male parent, in contrast to essentially wild-type expression when the same genes are maternally transmitted. We show that the imprint is due to t he sex of the parent rather than to a conventional maternal effect, differe ntial mitotic instability of die mini-X chromosome, or an allele-specific e ffect. Finally, we have examined the effects of classical modifiers of posi tion-effect variegation on the maintenance and the establishment of the imp rint. Factors that modify position-effect variegation alter the somatic exp ression but not the establishment of the imprint. This suggests that chroma tin structure is important in maintenance of the imprint, but a separate me chanism may be responsible for its initiation.