Ancient and Recent Horizontal Invasions of Drosophilids by P Elements

P elements of two different subfamilies designated as M- and O-type are thought to have invaded host species in the Drosophila obscura group via horizontal transmission from external sources. Sequence comparisons with P elements isolated from other species suggested that the horizontal invasion by the O-type must have been a rather recent event, whereas the M-type invasion should have occurred in the more distant past. To trace the phylogenetic history of O-type elements, additional taxa were screened for the presence of O- and M-type elements using type-specific PCR primers. The phylogeny deduced from the sequence data of a 927-bp section (14 taxa) indicate that O-type elements have undergone longer periods of regular vertical transmission in the lineages of the saltans and willistoni groups of Drosophila. However, starting from a species of the D. willistoni group they were transmitted horizontally into other lineages. First the lineage of the D. affinis subgroup was infected, and finally, in a more recent wave of horizontal spread, species of three different genera were invaded by O-type elements from the D. affinis lineage: Scaptomyza, Lordiphosa, and the sibling species D. bifasciata/D. imaii of the Drosophila obscura subgroup. The O-type elements isolated from these taxa are almost identical (sequence divergence <1%). In contrast, no such striking similarities are observed among M-type elements. Nevertheless, the sequence phylogeny of M-type elements is also not in accordance with the phylogeny of their host species, suggesting earlier horizontal transfer events. The results imply that P elements cross species barriers more frequently than previously thought but require a particular genomic environment and thus seem to be confined to a rather narrow spectrum of host species. Consequently, different P element types acquired by successive horizontal transmission events often coexist within the same genome. Received: 15 May 2000 / Accepted: 19 July 2000     

Zinc toxicity in various lung cell lines is mediated by glutathione and GSSG reductase activity

In a previous work, it was shown that in cells after a decrease of cellular glutathione content, toxic zinc effects, such as protein synthesis inhibition or GSSG (glutathione, oxidized form) increases, were enhanced. In this study, zinc toxicity was determined by detection of methionine incorporation as a parameter of protein synthesis and GSSG increase in various lung cell lines (A549, L2, 11Lu, 16Lu), dependent on enhanced GSSG reductase activities and changed glutathione contents. After pretreatment of cells with dl-buthionine-[R,S]-sulfoximine (BSO) for 72 h, cellular glutathione contents were decreased to 15–40% and GSSG reductase activity was increased to 120–135% in a concentration-dependent manner. In BSO pretreated cells, the IC₅₀ values of zinc for methionine incorporation inhibition were unchanged as compared to cells not pretreated. The GSSG increase in BSO pretreated cells by zinc was enhanced in L2, 11Lu, and 16Lu cells, whereas in A549 cells, the GSSG increase by zinc was enhanced only after pretreatment with the highest BSO concentration. Inhibition of GSSG reductase in alveolar epithelial cells was observed at lower zinc concentrations than needed for methionine incorporation inhibition, whereas in fibroblastlike cells, inhibition of GSSG reductase occurred at markedly higher zinc concentrations as compared to methionine incorporation inhibition. These results demonstrate that GSSG reductase is an important factor in cellular zinc susceptibility. We conclude that reduction of GSSG is reduced in zinc-exposed cells. Therefore, protection of GSH oxidation by various antioxidants as well as enhancement of GSH content are expected to be mechanisms of diminishing toxic cellular effects after exposure to zinc.     

rRNA based identification and detection systems for rhizobia and other bacteria

Ribosomal ribonucleic acids are excellent marker molecules for the elucidation of bacterial phylogeny; they also provide useful target sites for identification and detection with nucleic acid probes. Based on the currently available 16S rRNA sequence data, bacteria of the rhizobial phenotype (plant nodulation, nitrogen fixation) are members of three moderately related phylogenetic sub-groups of the -subclass of the Proteobacteria: i.e. the rhizobia group, the bradyrhizobia group, and the azorhizobia group. All rhizobia, azo-, brady-, meso- and sinorhizobia are closely related to and in some cases phylogenetically intermixed with, non-symbiotic and/or non-nitrogen-fixing bacteria. Especially in the case of Bradyrhizobium japonicum strains, the 16S rRNA sequence data indicate substantial heterogeneity. Specific probe design and evaluation are discussed. A multiprobe concept for resolving specificity problems with group specific probes is presented. In situ identification with group specific probes of rhizobia in cultures as well as rhizobia and cyanobacteria within plant material is shown.     

Strontium-Induced Repetitive Calcium Spikes in a Unicellular Green Alga

The divalent cation Sr²⁺ induced repetitive transient spikes of the cytosolic Ca²⁺ activity [Ca²⁺]_cy and parallel repetitive transient hyperpolarizations of the plasma membrane in the unicellular green alga Eremosphaera viridis. [Ca²⁺]_cy measurements, membrane potential measurements, and cation analysis of the cells were used to elucidate the mechanism of Sr²⁺-induced [Ca²⁺]_cy oscillations. Sr²⁺ was effectively and rapidly compartmentalized within the cell, probably into the vacuole. The [Ca²⁺]_cy oscillations cause membrane potential oscillations, and not the reverse. The endoplasmic reticulum (ER) Ca²⁺-ATPase blockers 2,5-di-tert-butylhydroquinone and cyclopiazonic acid inhibited Sr²⁺-induced repetitive [Ca²⁺]_cy spikes, whereas the compartmentalization of Sr²⁺ was not influenced. A repetitive Ca²⁺ release and Ca²⁺ re-uptake by the ER probably generated repetitive [Ca²⁺]_cy spikes in E. viridis in the presence of Sr²⁺. The inhibitory effect of ruthenium red and ryanodine indicated that the Sr²⁺-induced Ca²⁺ release from the ER was mediated by a ryanodine/cyclic ADP-ribose type of Ca²⁺ channel. The blockage of Sr²⁺-induced repetitive [Ca²⁺]_cy spikes by La³⁺ or Gd³⁺ indicated the necessity of a certain influx of divalent cations for sustained [Ca²⁺]_cy oscillations. Based on these data we present a mathematical model that describes the baseline spiking [Ca²⁺]_cy oscillations in E. viridis.