Contribution of Spontaneous Mutations to Quantitative and Molecular Variation at the Highly Repetitive rDNA Locus in Yeast.

The ribosomal DNA array in Saccharomyces cerevisiae consists of many tandem repeats whose copy number is believed to be functionally important but highly labile. Regulatory mechanisms have evolved to maintain copy number by directed mutation, but how spontaneous variation at this locus is generated and selected has not been well characterized. We applied a mutation accumulation approach to quantify the impacts of mutation and selection on this unique genomic feature across hundreds of mutant strains. We find that mutational variance for this trait is relatively high, and that unselected mutations elsewhere in the genome can disrupt copy number maintenance. In consequence, copy number generally declines gradually, consistent with a previously proposed model of rDNA maintenance where a downward mutational bias is normally compensated by mechanisms that increase copy number when it is low. This pattern holds across ploidy levels and strains in the standard lab environment but differs under some stressful conditions. We identify several alleles, gene categories, and genomic features that likely affect copy number, including aneuploidy for chromosome XII. Copy number change is associated with reduced growth in diploids, consistent with stabilizing selection. Levels of standing variation in copy number are well predicted by a balance between mutation and stabilizing selection, suggesting this trait is not subject to strong diversifying selection in the wild. The rate and spectrum of point mutations within the rDNA locus itself are distinct from the rest of the genome and predictive of polymorphism locations. Our findings help differentiate the roles of mutation and selection and indicate that spontaneous mutation patterns shape several aspects of ribosomal DNA evolution.

Comparison of Diplodia Tip Blight Pathogens in Spanish and North American Pine Ecosystems.

Diplodia tip blight is the most ubiquitous and abundant disease in Spanish Pinus radiata plantations. The economic losses in forest stands can be very severe because of its abundance in cones and seeds together with the low genetic diversity of the host. Pinus resinosa is not genetically diverse in North America either, and Diplodia shoot blight is a common disease. Disease control may require management designs to be adapted for each region. The genetic diversity of the pathogen could be an indicator of its virulence and spreading capacity. Our objective was to understand the diversity of Diplodia spp. in Spanish plantations and to compare it with the structure of American populations to collaborate in future management guidelines. Genotypic diversity was investigated using microsatellite markers. Eight loci (SS9-SS16) were polymorphic for the 322 isolates genotyped. The results indicate that Diplodia sapinea is the most frequent Diplodia species present in plantations of the north of Spain and has high genetic diversity. The higher genetic diversity recorded in Spain in comparison to previous studies could be influenced by the intensity of the sampling and the evidence about the remarkable influence of the sample type.

A Species-Specific PCR Assay for Detection of Diplodia pinea and D. scrobiculata in Dead Red and Jack Pines with Collar Rot Symptoms.

A polymerase chain reaction (PCR)-based assay was developed for the specific detection of the fungal pathogens Diplodia pinea and D. scrobiculata from pine host tissues. Variation among mitochondrial small subunit ribosome gene (mt SSU rDNA) sequences of Botryosphaeria species and related anamorphic fungi was exploited to design primer pairs. Forward primer DpF and forward primer DsF, each when used with the nonspecific reverse primer BotR, amplified DNA of D. pinea or D. scrobiculata, respectively. Specificity was confirmed using multiple isolates of each of these two species and those of closely related fungi including Botryosphaeria obtusa. The detection limits for DNA of each pathogen in red and jack pine bark were 50 to 100 pg µl-1 and 1 pg µl-1 in red and jack pine wood. The assay was tested using naturally occurring red and jack pine seedlings and saplings exhibiting symptoms of Diplodia collar rot. Samples from lower stems/root collars of 10 dead trees of each species from each of three sites at each of two locations were tested. Results were positive for D. pinea or D. scrobiculata for the large majorities of symptomatic bark and wood samples from both locations. For positive samples, however, there were effects of location and host species on detection of D. pinea (more frequent on red pine) and D. scrobiculata (more frequent on jack pine) (P < 0.01 in both cases). These results indicate that these new primers are potentially useful for studies in areas or hosts in which both pathogens may be present.

Mucous membrane plasmacytosis: a case report and review of the literature.

Mucous membrane plasmacytosis is a rare, idiopathic condition consisting of a dense plasma-cell infiltrate of the mucous membranes. Zoon first described a plasma-cell infiltrate occurring on the glans penis and plasmacytosis involving other body orifices have been reported under a variety of different terms. White et al. simplified the terminology by suggesting all plasma-cell infiltrates of the mucous membranes of body orifices be termed plasma-cell orificial mucositis. The differential diagnoses and treatment for mucous membrane plasmacytosis are summarized. A middle-aged female with an unusual plasma-cell proliferation disorder of the upper aerodigestive tract is reported for its rarity.

Differentiation of a Fusicoccum sp. Causing Panicle and Shoot Blight on California Pistachio Trees from Botryosphaeria dothidea.

A panicle and shoot blight disease of pistachio trees in California is caused by a fungus previously identified as the anamorph of Botryosphaeria dothidea. We have compared random amplified polymorphic DNA (RAPD) markers, nuclear rDNA internal transcribed spacer (ITS) region sequences, and conidium morphology of 15 isolates of the pistachio Fusicoccum to those of well-characterized isolates of B. dothidea, B. ribis, and F. luteum. Cluster analysis of RAPD markers separated the pistachio Fusicoccum isolates from B. dothidea, as did parsimony analysis of the ITS region sequences. Conidium size and shape were similar to those of B. ribis (Fusicoccum sp.) and F. luteum, but distinguishable from those of F. aesculi (the anamorph of B. dothidea). We conclude that the fungus causing panicle and shoot blight of pistachio is distinguishable from B. dothidea and is part of a complex containing B. ribis, F. luteum, and other fungi with Fusicoccum anamorphs.