Double-stranded (ds) RNA can induce sequence-specific inhibition of gene function in several organisms. However, both the mechanism and the physiological role of the interference process remain mysterious. In order to study the interference process, we have selected C. elegans mutants resistant to dsRNA-mediated interference (RNAi). Two loci, rde-1 and rde-4, are defined by mutants strongly resistant to RNAi but with no obvious defects in growth or development. We show that rde-1 is a member of the piwi/sting/argonaute/zwille/eIF2C gene family conserved from plants to vertebrates. Interestingly, several, but not all, RNAi-deficient strains exhibit mobilization of the endogenous transposons. We discuss implications for the mechanism of RNAi and the possibility that one natural function of RNAi is transposon silencing.

Recently it has been proposed that di-methylation of histone H3 on lysine 4 (H3K4me2) acts as an epigenetic memory to maintain transcriptional patterns in developing tissues. This model suggests that there may be a requirement to reprogram this modification in the germline to prevent transcriptional memory from being inappropriately transmitted to the next generation. We asked if SPR-5, the C. elegans ortholog of the H3K4me2 demethylase LSD1/KDM1, plays a role in epigenetically reprogramming H3K4me2. We show that spr-5 mutants exhibit progressive sterility over many generations due to defects in oogenesis and spermatogenesis. These defects correlate with a progressive failure to erase H3K4me2 in the primordial germ cells, resulting in the misregulation of spermatogenesis-expressed genes due to the transgenerational accumulation of H3K4me2 at these loci. These results suggest that H3K4me2 can serve as an epigenetic memory and that LSD1/KDM1 demethylases play a critical role in the reprogramming of this memory in the germline, preventing inappropriate epigenetic information from being propagated from one generation to the next.

In many multicellular organisms, males have one X chromosome and females have two. Dosage compensation refers to a regulatory mechanism that insures the equalization of X-linked gene products in males and females. The mechanism has been studied at the molecular level in model organisms belonging to three distantly related taxa; in these organisms, equalization is achieved by shutting down one of the two X chromosomes in the somatic cells of females, by decreasing the level of transcription of the two doses of X-linked genes in females relative to males, or by increasing the level of transcription of the single dose of X-linked genes in males. The study of dosage compensation in these different forms has revealed the existence of an amazing number of interacting chromatin remodeling mechanisms that affect the function of entire chromosomes.

The genetic imprinting of individual loci or whole chromosomes, as in imprinted X-chromosome inactivation in mammals 1,2 , is established and reset during gametogenesis; defects in this process in the parent can result in disease in the offspring 3 . We describe a sperm-specific chromatin-based imprinting of the X chromosome in the nematode Caenorhabditis elegans that is restricted to histone H3 modifications. The epigenetic imprint is established during spermatogenesis and its stability in the offspring is affected by the presence of a pairing partner during meiosis in the parental germ line. We observed that DNA lacking a pairing partner during meiosis, the normal situation for the X chromosome in males, is targeted for methylation of histone H3 at Lys9 (H3-Lys9) and can be silenced. Targeting unpaired DNA for silencing during meiosis, a potential hallmark of genome defense, could therefore have a conserved role in imprinted X-chromosome inactivation and, ultimately, in sex chromosome evolution.The mechanisms by which imprinting is established during gametogenesis are not understood, but all such epigenetic phenomena clearly involve DNA methylation or histone modifications 4-7 . Methylation of H3-Lys9 is a highly conserved modification that is enriched in facultative and constitutive heterochromatin 8 . Methylation of histone H3 Lys4 (H3-Lys4), an opposing mark to methylation of H3-Lys9, is even more highly conserved and is enriched in transcriptionally competent or active DNA in all eukaryotes that have been examined 8 . Cytosine methylation, which is not found in all organisms, requires histone methylation in some species 9 , suggesting that histone modification may be the underlying mechanism guiding epigenetic processes. Indeed, methylation of histone H3 may provide an epigenetic mark during gametogenic imprinting of individual loci 10,11 . Histone modifications established during gametogenesis might therefore create a heritable epigenetic code that controls imprinted transcriptional competency in the offspring.The X chromosome in early adult germ cells of C. elegans lacks histone modifications associated with transcriptional competence 12 . The X chromosome is silenced in immature gametes of both sexes but shows sex-specific regulation during later gametic stages. In XX hermaphrodites, histones on the X chromosome become highly modified during early oogenesis, coincident with a burst of X-linked oocyte-specific gene expression 12 . In contrast, the unpaired X chromosome in (XO) males never accumulates 'activating' modifications during spermatogenesis but becomes enriched in methylated H3-Lys9 and condenses prematurely in pachytene nuclei 13 . This is markedly similar to what is observed for the XY body in mammalian spermatogenesis 13 . The X chromosome is also depleted of genes encoding sperm-enriched transcripts in C. elegans 12,14 . XX crossprogeny (produced from fertilization of a hermaphrodite by a male) therefore inherit sex chromosomes from each parent that differ substantially both in the spectrum ...

In C. elegans, mRNA production is initially repressed in the embryonic germline by a protein unique to C. elegans germ cells, PIE-1. PIE-1 is degraded upon the birth of the germ cell precursors, Z2 and Z3. We have identified a chromatin-based mechanism that succeeds PIE-1 repression in these cells. A subset of nucleosomal histone modifications, methylated lysine 4 on histone H3 (H3meK4) and acetylated lysine 8 on histone H4 (H4acetylK8), are globally lost and the DNA appears more condensed. This coincides with PIE-1 degradation and requires that germline identity is not disrupted. Drosophila pole cell chromatin also lacks H3meK4, indicating that a unique chromatin architecture is a conserved feature of embryonic germ cells. Regulation of the germline-specific chromatin architecture requires functional nanos activity in both organisms. These results indicate that genome-wide repression via a nanos-regulated, germ cell-specific chromatin organization is a conserved feature of germline maintenance during embryogenesis.

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Much attention has been given to the idea of transgenerational epigenetic inheritance, but fundamental questions remain regarding how much takes place and the impact that this might have on organisms. We asked five leading researchers in this area — working on a range of model organisms and in human disease — for their views on these topics. Their responses highlight the mixture of excitement and caution that surrounds transgenerational epigenetic inheritance and the wide gulf between species in terms of our knowledge of the mechanisms that may be involved.

The Rodalquilar epithermal Au alunite deposit occurs within the Rodalquilar caldera complex in the Miocene Cabo de Gata volcanic field in southeastern Spain. The Rodalquilar caldera formed by eruption of the rhyolitic Cinto ash-flow tuff at about 11 Ma; continued resurgence of the core of the caldera resulted in structural doming and was followed by emplacement of large ring domes, eruption of the Lfizaras ash-flow tuff, and development of the smaller, nested Lomilla caldera. Hydrothermal circulation associated with the emplacement of shallow hornblende andesitic intrusions late in the evolution of the caldera led to formation of the epithermal deposits along fractures related to multistage caldera collapse and resurgence.Ore deposits within the Rodalquilar caldera complex consist of low-sulfidation Pb-Zn-(Cu-Ag-Au) quartz veins and the economically most important high-sulfidation Au-(Cu-Te-Sn) ores. The latter are enclosed in areas of acid sulfate alteration present on the east margin of the Lomilla caldera. Drilling indicates that hydrothermally altered rocks are present to depths of >900 m, with a gradational change with depth from silicic, to advanced argillic, to intermediate argillic, to sericitic zones; an envelope of propylitic alteration surrounds these zones. The sericitic zone is present at depths >400 m and occurs under the advanced argillic (stage i alunite, diaspore, zunyite, pyrophyllite) and silicic (vuggy silica and massive silicified rock) zones, which are well developed to present depths of •300 and 100 m, respectively. Vuggy silica and massive silicified rock are structurally controlled and spatially related to the Au deposits. K/Ar dating of stage i alunite and hydrothermal illitc indicates an age of mineralization of about 10.4 Ma.The results of paragenetic, fluid inclusion, and stable isotope studies indicate an evolution of the hydrothermal system, consisting of an early period of acidic wall-rock alteration and a late period of Au mineralization. A significant magmatic fluid component was present throughout, contributing acidity in the form of H2SO4 and HC1. Salinities in some samples of deep, hot (>400øC) fluids exceeded 40 wt percent NaC1 equiv, consistent with the presence of a magmatic brine. In addition, the O and H isotope ratios of hypogene alteration minerals (alunite, kaolinitc, sericite, and hydrothermal quartz) indicate that hydrothermal fluids (6•SO = 7 ___ 3%0, 6D = -20 ___ 10%o) during the main period of wall-rock alteration were dominantly magmatic in origin. The 634Szs was •9 per mil and H2S/SO4 of the bulk hydrothermal system during acidic alteration was •5. The 634S values of stage 1 alunite (22.3-31.0%o) and pyrite (0.3-8.0%0) in the advanced argillic zone reflect isotopic equilibrium between sulfate and sulfide at T --220 ø to 330øC, with the lower values corresponding with present-day surface samples. * Present address: Geological Survey of Japan, 1-1-3 Higashi, Tsukuba, 305 Japan. 0361-0128/95/1704/795-2854.00 795 796 ,4RRIB,•S, JR. ET ,•L. Gold mineralization extend...