Gene dosage and stochastic effects determine the severity and direction of uniparental ribosomal RNA gene silencing (nucleolar dominance) in Arabidopsis allopolyploids

Nucleolar dominance is an epigenetic phenomenon in which one parental set of ribosomal RNA (rRNA) genes is silenced in an interspecific hybrid. In natural Arabidopsis suecica, an allotetraploid (amphidiploid) hybrid of Arabidopsis thaliana and Cardaminopsis arenosa, the A. thaliana rRNA genes are repressed. Interestingly, A. thaliana rRNA gene silencing is variable in synthetic Arabidopsis suecica F1 hybrids. Two generations are needed for A. thaliana rRNA genes to be silenced in all lines, revealing a species-biased direction but stochastic onset to nucleolar dominance. Backcrossing synthetic A. suecica to tetraploid A. thaliana yielded progeny with active A. thaliana rRNA genes and, in some cases, silenced C. arenosa rRNA genes, showing that the direction of dominance can be switched. The hypothesis that naturally dominant rRNA genes have a superior binding affinity for a limiting transcription factor is inconsistent with dominance switching. Inactivation of a species-specific transcription factor is argued against by showing that A. thaliana and C. arenosa rRNA genes can be expressed transiently in the other species. Transfected A. thaliana genes are also active in A. suecica protoplasts in which chromosomal A. thaliana genes are repressed. Collectively, these data suggest that nucleolar dominance is a chromosomal phenomenon that results in coordinate or cooperative silencing of rRNA genes.

Studies using double mutants of the conformational transitions in influenza hemagglutinin required for its membrane fusion activity

Amino acid substitutions widely distributed throughout the influenza hemagglutinin (HA) influence the pH of its membrane fusion activity. We have combined a number of these substitutions in double mutants and determined the effects on the pH of fusion and on the pH at which the refolding of HA required for fusion occurs. By analyzing combinations of mutations in three regions of the metastable neutral-pH HA that are rearranged at fusion pH we obtain evidence for both additive and nonadditive effects and for an apparent order of dominance in the effects of amino acid substitutions in particular regions on the pH of fusion. We conclude that there are at least three components in the structural transition required for membrane fusion activity and consider possible pathways for the transition in relation to the known differences between neutral and fusion pH HA structures.

Candida albicans strains heterozygous and homozygous for mutations in mitogen-activated protein kinase signaling components have defects in hyphal development

The Candida albicans genes, CST20 and HST7, were cloned by their ability to suppress the mating defects of Saccharomyces cerevisiae mutants in the ste20 and ste7 genes, which code for elements of the mating mitogen-activated protein (MAP) kinase pathway. These Candida genes are both structural and functional homologs of the cognate Saccharomyces genes. The pattern of suppression in Saccharomyces is related to their presumptive position in the MAP kinase cascade. Null alleles of these genes were constructed in Candida. The Candida homozygous null mutants are defective in hyphal formation on some media, but are still induced to form hyphae by serum, showing that serum induction of hyphae is independent of the MAP kinase cascade. The Candida heterozygotes CST20/cst20 and HST7/hst7 are also defective in hyphal formation. This lack of dominance of the wild-type allele suggests that gene dosage is important in Candida.

A model of ocular dominance column development by competition for trophic factor

Recent experimental evidence has shown that application of certain neurotrophic factors (NTs) to the developing primary visual cortex prevents the development of ocular dominance (OD) columns. One interpretation of this result is that afferents from the lateral geniculate nucleus compete for postsynaptic trophic factor in an activity-dependent manner. Application of excess trophic factor eliminates this competition, thereby preventing OD column formation. We present a model of OD column development, incorporating Hebbian synaptic modification and activity-driven competition for NT, which accounts for both normal OD column development as well as the prevention of that development when competition is removed. In the “control” situation, when available NT is below a critical amount, OD columns form normally. These columns form without weight normalization procedures and in the presence of positive inter-eye correlations. In the “experimental” case, OD column development is prevented in a local neighborhood in which excess NT has been added. Our model proposes a biologically plausible mechanism for competition between neural populations that is motivated by several pieces of experimental data, thereby accounting for both normal and experimentally perturbed conditions.

Dominance of one bacterial phylotype at a Mid-Atlantic Ridge hydrothermal vent site.

Microbial community structure in natural environments has remained largely unexplored yet is generally considered to be complex. It is shown here that in a Mid-Atlantic Ridge hydrothermal vent habitat, where food webs depend on prokaryotic primary production, the surface microbial community consists largely of only one bacterial phylogenetic type (phylotype) as indicated by the dominance of a single 16S rRNA sequence. The main part of its population occurs as an ectosymbiont on the dominant animals, the shrimp Rimicaris exoculata, where it grows as a monoculture within the carapace and on the extremities. However, the same bacteria are also the major microbial component of the free-living substrate community. Phylogenetically, this type forms a distinct branch within the epsilon-Proteobacteria. This is different from all previously studied chemoautotrophic endo- and ectosymbioses from hydrothermal vents and other sulfidic habitats in which all the bacterial members cluster within the gamma-Proteobacteria.