The susceptibility to recombination of a plasmid inserted into a chromosome
varies with its genomic position. This recombination position effect is known to
correlate with the average G+C content of the flanking sequences. Here we
propose that this effect could be mediated by changes in the susceptibility to
superhelical duplex destabilization that would occur. We use standard
nonparametric statistical tests, regression analysis and principal component
analysis to identify statistically significant differences in the
destabilization profiles calculated for the plasmid in different contexts, and
correlate the results with their measured recombination rates. We show that the
flanking sequences significantly affect the free energy of denaturation at
specific sites interior to the plasmid. These changes correlate well with
experimentally measured variations of the recombination rates within the
plasmid. This correlation of recombination rate with superhelical
destabilization properties of the inserted plasmid DNA is stronger than that
with average G+C content of the flanking sequences. This model suggests a
possible mechanism by which flanking sequence base composition, which is not
itself a context-dependent attribute, can affect recombination rates at
positions within the plasmid.
Mining below groundwater tables is increasing globally, yet little is known of how associated large-scale modification of water tables impact functioning of surrounding ecosystems. We used measurements of foliage density (ρ F ) and sapwood-related sap flow (Q S ) to assess effects of depth to groundwater on Eucalyptus victrix, a tree species that is common in riparian zones in central and northern parts of Australia.Foliage density (ρ F ) varied with season and among sites. Of itself, ρ F provided a partial indicator of how trees responded to falling (more than 10 m) and rising (more than 9 m) water tables. Assessment of Q S was highly informative. Across all sites, Q S was least (90-130 l m À2 sapwood h
À1) where groundwater was naturally deep (30 m) or had fallen substantially over the past 4 years (from 8 to 19 m). Fastest rates of Q S (>245 l m À2 sapwood h À1 ) were recorded where groundwater had risen to a depth similar to a site where depth to groundwater remained stable at 6-7 m. Our analyses of daytime and night-time Q S emphasize that water use by E. victrix is highly plastic and opportunistic. We discuss how empirical analysis of Q S , coupled with a sound understanding of local hydrogeology, can help assess responses in ecosystem function to large-scale modification of groundwater levels -an important issue globally, as well as in Australia.
Summary In recent times managers have become increasingly aware of the South
African bulbous species Yellow Soldier (Lachenalia reflexa) becoming a serious
weed of bushland on the Swan Coastal Plain. In 1998, trials were implemented to investigate
control options for Yellow Soldier invading the understorey of a Banksia (Banksia
attenuata) Woodland west of Perth. Our trials showed that hand removal over two seasons left all natives intact but was very labour intensive, only reducing cover of Yellow Soldier by 44%. It also triggered germination by ephemeral weeds. Wiping the leaves of individual plants with a 10% glyphosate solution was not effective and was also highly labour intensive. Spot spraying with metsulfuron methyl at 0.2 g/15 L (5 g/ha) reduced the cover of Yellow Soldier by 65%, was easier to implement and appeared to have had insignificant effects on natives. We hope that this trial will encourage other workers in the field to undertake controlled trials to refine treatments at restoration sites.
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