Endogenously imprinted genes in Drosophila melanogaster.

Genomic imprinting is an epigenetic state that results from differential processing of chromosomes during gametogenesis and which can cause differential expression of genes depending on the sex of the parent transmitting that gene. In Drosophila, many examples of imprinted marker genes have been documented and imprinting of these genes involves highly conserved epigenetic regulators. However, no endogenously imprinted genes have yet been identified. Here we present a phenotypic and transcriptional analysis of parthenogenetic (gynogenetic) and genotypically identical but sexually produced adult female Drosophila. We find that while parthenogenetic females have a superficially normal phenotype and are viable and fertile, their lifespan is extended relative to their sexually-produced counterparts. Microarray/transcriptional analysis of parthenogenetic versus sexually-produced females reveals 76 genes with consistently altered patterns of expression, 36 upregulated and 40 downregulated, some with known effects on aging. Analysis of individuals with uniparental inheritance of only portions of their genome suggest that many of these genes may be indirectly imprinted, responding to either other imprinted genes or redistribution of chromatin components that are differentially allocated to sex and autosomal heterochromatin in a sex-dependent manner during gametogenesis. As gene expression dependent on the parental origin of the genome meets the definition of genomic imprinting, our study provides evidence that endogenous genes are imprinted in Drosophila.

The Drosophila homolog of the mammalian imprint regulator, CTCF, maintains the maternal genomic imprint in Drosophila melanogaster.

BACKGROUND: CTCF is a versatile zinc finger DNA-binding protein that functions as a highly conserved epigenetic transcriptional regulator. CTCF is known to act as a chromosomal insulator, bind promoter regions, and facilitate long-range chromatin interactions. In mammals, CTCF is active in the regulatory regions of some genes that exhibit genomic imprinting, acting as insulator on only one parental allele to facilitate parent-specific expression. In Drosophila, CTCF acts as a chromatin insulator and is thought to be actively involved in the global organization of the genome. RESULTS: To determine whether CTCF regulates imprinting in Drosophila, we generated CTCF mutant alleles and assayed gene expression from the imprinted Dp(1;f)LJ9 mini-X chromosome in the presence of reduced CTCF expression. We observed disruption of the maternal imprint when CTCF levels were reduced, but no effect was observed on the paternal imprint. The effect was restricted to maintenance of the imprint and was specific for the Dp(1;f)LJ9 mini-X chromosome. CONCLUSIONS: CTCF in Drosophila functions in maintaining parent-specific expression from an imprinted domain as it does in mammals. We propose that Drosophila CTCF maintains an insulator boundary on the maternal X chromosome, shielding genes from the imprint-induced silencing that occurs on the paternally inherited X chromosome. See commentary: http://www.biomedcentral.com/1741-7007/8/104.

Daphnia as an emerging epigenetic model organism.

Daphnia offer a variety of benefits for the study of epigenetics. Daphnia's parthenogenetic life cycle allows the study of epigenetic effects in the absence of confounding genetic differences. Sex determination and sexual reproduction are epigenetically determined as are several other well-studied alternate phenotypes that arise in response to environmental stressors. Additionally, there is a large body of ecological literature available, recently complemented by the genome sequence of one species and transgenic technology. DNA methylation has been shown to be altered in response to toxicants and heavy metals, although investigation of other epigenetic mechanisms is only beginning. More thorough studies on DNA methylation as well as investigation of histone modifications and RNAi in sex determination and predator-induced defenses using this ecologically and evolutionarily important organism will contribute to our understanding of epigenetics.