Ribosomal DNA Deletions Modulate Genome-Wide Gene Expression: “rDNA–Sensitive” Genes and Natural Variation

The ribosomal rDNA gene array is an epigenetically-regulated repeated gene locus. While rDNA copy number varies widely between and within species, the functional consequences of subtle copy number polymorphisms have been largely unknown. Deletions in the Drosophila Y-linked rDNA modifies heterochromatin-induced position effect variegation (PEV), but it has been unknown if the euchromatic component of the genome is affected by rDNA copy number. Polymorphisms of naturally occurring Y chromosomes affect both euchromatin and heterochromatin, although the elements responsible for these effects are unknown. Here we show that copy number of the Y-linked rDNA array is a source of genome-wide variation in gene expression. Induced deletions in the rDNA affect the expression of hundreds to thousands of euchromatic genes throughout the genome of males and females. Although the affected genes are not physically clustered, we observed functional enrichments for genes whose protein products are located in the mitochondria and are involved in electron transport. The affected genes significantly overlap with genes affected by natural polymorphisms on Y chromosomes, suggesting that polymorphic rDNA copy number is an important determinant of gene expression diversity in natural populations. Altogether, our results indicate that subtle changes to rDNA copy number between individuals may contribute to biologically relevant phenotypic variation

Examination of the failure of sandwich beams with core junctions subjected to transverse shear loading

The article concerns failure and fatigue phenomena associated with local effects occurring in the vicinity of junctions between different core materials in sandwich beams subjected to transverse shear loading. It is known from analytical and numerical modeling that these effects lead to stress concentrations at such junctions. However, their influence on the failure behavior is not fully understood, and there are indications that the available models overestimate the importance of the local effects. In the present article, typical sandwich beam configurations with glass fiber-reinforced plastic face sheets and core junctions between polymer foams of different densities and rigid aluminium were tested under quasi-static and fatigue loading conditions. The failure behavior was compared with results from finite element analyses using various failure criteria. It was found that for the transverse shear load case the inherent core shear stresses overrule the local effects in terms of being most critical with respect to causing failure for the most common sandwich configurations using low density semi-brittle polymer foam cores. A simple maximum shear stress criterion is appropriate for failure prediction (core shear failure) in those cases. However, if very strong core materials and thin face sheets are used, the stress concentrations induced by local effects can cause failure in the faces. Accordingly, the local face stresses can be of importance and should be taken into account for failure prediction