Ribosomal RNA (rRNA) production represents the most active transcription in the cell. Synthesis of the large rRNA precursors (35&#8211;47S) can be achieved by up to 150 RNA polymerase I (Pol I) enzymes simultaneously transcribing each rRNA gene. In this paper, we present recent advances made in understanding the regulatory mechanisms that control elongation. Built-in Pol I elongation factors, such as Rpa34/Rpa49 in budding yeast and PAF53/CAST in humans, are instrumental to the extremely high rate of rRNA production per gene. rRNA elongation mechanisms are intrinsically linked to chromatin structure and to the higher-order organization of the rRNA genes (rDNA). Factors such as Hmo1 in yeast and UBF1 in humans are key players in rDNA chromatin structure in vivo. Finally, elongation factors known to regulate messengers RNA production by RNA polymerase II are also involved in rRNA production and work cooperatively with Rpa49 in vivo.

Benjamin Albert

Jorge Perez-Fernandez

Isabelle L&#233;ger-Silvestre 

Olivier Gadal

Regulation of Ribosomal RNA Production by RNA Polymerase I: Does Elongation Come First?

Chromosome pairing is usually discussed in the context of meiosis. Association of homologues in germ cells enables chromosome segregation and is necessary for fertility. A few organisms, such as flies, also pair their entire genomes in somatic cells. Most others, including mammals, display little homologue pairing outside of the germline. Experimental evidence from both flies and mammals suggests that communication between homologues contributes to normal genome regulation. This paper will contrast the role of pairing in transmitting information between homologues in flies and mammals. In mammals, somatic homologue pairing is tightly regulated, occurring at specific loci and in a developmentally regulated fashion. Inappropriate pairing, or loss of normal pairing, is associated with gene misregulation in some disease states. While homologue pairing in flies is capable of influencing gene expression, the significance of this for normal expression remains unknown. The sex chromosomes pose a particularly interesting situation, as females are able to pair X chromosomes, but males cannot. The contribution of homologue pairing to the biology of the X chromosome will also be discussed.

Manasi S. Apte

Victoria H. Meller

Homologue Pairing in Flies and Mammals: Gene Regulation When Two Are Involved

The Stellate-made crystals formation in spermatocytes is the phenotypic manifestation of a disrupted crystal-Stellate interaction in testes of Drosophila melanogaster. Stellate silencing is achieved by the piRNA pathway, but many features still remain unknown. Here we outline the important role of the crystal-Stellate modifiers. These have shed light on the piRNA pathways that defend genome integrity against transposons and other repetitive elements in the gonads. In particular, we illustrate the finding that HSP90 participates in the molecular pathways of piRNA production. This observation has relevance for the mechanisms underlying the evolutionary canalization process.

Maria Pia Bozzetti

Laura Fanti

Silvia Di Tommaso

Lucia Piacentini

Maria Berloco

Patrizia Tritto

Valeria Specchia

The &#8220;Special&#8221; crystal-Stellate System in Drosophila melanogaster Reveals Mechanisms Underlying piRNA Pathway-Mediated Canalization

This project aims at investigating the link between individual epigenetic variability (not related to genetic variability) and the variation of natural environmental conditions. We studied DNA methylation polymorphisms of individuals belonging to a single genetic lineage of the clonal diploid fish Chrosomus eos-neogaeus sampled in seven geographically distant lakes. In spite of a low number of informative fragments obtained from an MSAP analysis, individuals of a given lake are epigenetically similar, and methylation profiles allow the clustering of individuals in two distinct groups of populations among lakes. More importantly, we observed a significant pH variation that is consistent with the two epigenetic groups. It thus seems that the genotype studied has the potential to respond differentially via epigenetic modifications under variable environmental conditions, making epigenetic processes a relevant molecular mechanism contributing to phenotypic plasticity over variable environments in accordance with the GPG model.

Rachel Massicotte

Bernard Angers

General-Purpose Genotype or How Epigenetics Extend the Flexibility of a Genotype

Historically, evolutionary 
            biologists have taken the view that an understanding 
            of development is irrelevant to theories of evolution. 
            However, the integration of several disciplines in 
            recent years suggests that this position is wrong. The 
            capacity of the organism to adapt to challenges from 
            the environment can set up conditions that affect the 
            subsequent evolution of its descendants. Moreover, 
            molecular events arising from epigenetic processes can 
            be transmitted from one generation to the next and 
            influence genetic mutation. This in turn can 
            facilitate evolution in the conditions in which 
            epigenetic change was first initiated.

Patrick Bateson

The Impact of the Organism on Its Descendants

At its broadest sense, to say that a phenotype is epigenetic suggests that it occurs without changes in DNA sequence, yet is heritable through cell division and occasionally from one organismal generation to the next. Since gene regulatory changes are oftentimes in response to environmental stimuli and may be retained in descendent cells, there is a growing expectation that one&#39;s experiences may have consequence for subsequent generations and thus impact evolution by decoupling a selectable phenotype from its underlying heritable genotype. But the risk of this overbroad use of &#8220;epigenetic&#8221; is a conflation of genuine cases of heritable non-sequence genetic information with trivial modes of gene regulation. A look at the term &#8220;epigenetic&#8221; and some problems with its increasing prevalence argues for a more reserved and precise set of defining characteristics. Additionally, questions arising about how we define the &#8220;sequence independence&#8221; aspect of epigenetic inheritance suggest a form of genome evolution resulting from induced polymorphisms at repeated loci (e.g., the rDNA or heterochromatin).

Keith A. Maggert

Genetics: Polymorphisms, Epigenetics, and Something In Between

Dosage compensation balances gene expression levels between the sex chromosomes and autosomes and sex-chromosome-linked gene expression levels between the sexes. Different dosage compensation strategies evolved in different lineages, but all involve changes in chromatin. This paper discusses our current understanding of how modifications of the histone H4 tail, particularly changes in levels of H4 lysine 16 acetylation and H4 lysine 20 methylation, can be used in different contexts to either modulate gene expression levels twofold or to completely inhibit transcription.

Michael B. Wells

Gy&#246;rgyi  Csankovszki

Laura M. Custer

Finding a Balance: How Diverse Dosage Compensation Strategies Modify Histone H4 to Regulate Transcription

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Genetics Research International publishes original research articles and review articles in all areas of genetics and genomics. The journal focuses on articles bearing on heredity, biochemistry, and molecular biology, as well as clinical findings.

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Genetics Research International ceased publication in 2019 and is no longer accepting submissions.

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Genetics Research International publishes original research articles and review articles in all areas of genetics and genomics. The journal focuses on articles bearing on heredity, biochemistry, and molecular biology, as well as clinical findings.

Genetics Research International