The Early Stage of Bacterial Genome-Reductive Evolution in the Host

The equine-associated obligate pathogen Burkholderia mallei was developed by reductive evolution involving a substantial portion of the genome from Burkholderia pseudomallei, a free-living opportunistic pathogen. With its short history of divergence (∼3.5 myr), B. mallei provides an excellent resource to study the early steps in bacterial genome reductive evolution in the host. By examining 20 genomes of B. mallei and B. pseudomallei, we found that stepwise massive expansion of IS (insertion sequence) elements ISBma1, ISBma2, and IS407A occurred during the evolution of B. mallei. Each element proliferated through the sites where its target selection preference was met. Then, ISBma1 and ISBma2 contributed to the further spread of IS407A by providing secondary insertion sites. This spread increased genomic deletions and rearrangements, which were predominantly mediated by IS407A. There were also nucleotide-level disruptions in a large number of genes. However, no significant signs of erosion were yet noted in these genes. Intriguingly, all these genomic modifications did not seriously alter the gene expression patterns inherited from B. pseudomallei. This efficient and elaborate genomic transition was enabled largely through the formation of the highly flexible IS-blended genome and the guidance by selective forces in the host. The detailed IS intervention, unveiled for the first time in this study, may represent the key component of a general mechanism for early bacterial evolution in the host

Influences of agricultural landuse and seasonal changes in abiotic conditions on invertebrate colonisation of riparian leaf detritus in intermittent streams

The structure and function of agricultural stream reaches with sparse riparian and floodplain vegetation differ from those of forested reaches, but may be &lsquo;reset&rsquo; as these streams flow through reaches with forested riparian zones. We investigated whether invertebrate colonisation of River Red Gum (Eucalyptus camaldulensis) leaf packs in lowland intermittent streams was influenced by the adjacent reach-scale landuse (cleared farmland or forested reserve) within an agricultural catchment in Victoria, Australia. Further, we examined the influence of seasonal changes in hydrology and associated changes in abiotic conditions on the colonisation of leaves by repeating experiments over two summers and one spring. Across these experiments, there were no consistent differences in the structure of communities that colonised leaves in farmland and reserve reaches. In both seasons, most leaf colonists were collectors and few were shredders in both farmland and reserve reaches. Relative abundances of gastropod grazers were much higher in summer than in spring. The structure of invertebrate communities colonising leaves in the different reaches converged over time when streams flowed in spring, but diverged over time as the streams dried and abiotic conditions within disconnected pools became increasingly harsh in summer. Thus, patterns of leaf pack colonisation were influenced by the regional climate causing large seasonal changes in hydrology, but not by reach-scale landuse. The large-scale disturbances of agricultural landuse across the catchment and a supra-seasonal drought probably contributed to low diversities of invertebrate communities in the streams.<br /

Food webs in Mediterranean rivers

River food webs are subject to two regimes of longitudinally varying ecological control: productivity and disturbance. Light-limited productivity increases as channels widen downstream. Time windows for growth, however, shrink as discharge increases, substrate particle size decreases, and the frequency of flood-driven bed mobilization increases downstream. Mediterranean rivers are periodically reset by hydrologic events with somewhat predictable timing. Typically, a rainy winter with high river discharge is followed by summer drought with little or no rainfall and slowly declining river flow. The magnitude and timing of winter floods and severity of subsequent summer drought can vary considerably from year to year, however. Episodic scouring floods or prolonged periods of drought are experienced as disturbances, stressors, or opportunities by river biota. The timing, duration, and intensity of these hydrologic controls affect performances of individuals, distribution and abundances of populations, and outcomes and consequences of species interactions. These interactions in turn determine how river food webs will assemble, develop, and reconfigure after disturbance. We discuss how spatial variation in solar radiation and spatial and temporal variations in disturbance affects river food webs under Mediterranean climate seasonality, focusing primarily on long-term observations in the Eel River of northwestern California, USA