Evolution at the tip and base of the X chromosome in an African population of Drosophila melanogaster

Hitchhiking effects of advantageous mutations have been invoked to explain reduced polymorphism in regions of low crossing-over in Drosophila. Besides reducing DNA heterozygosity, hitchhiking effects should produce strong linkage disequilibrium and a frequency spectrum skewed toward an excess of rare polymorphisms (compared to the neutral expectation). We measured DNA polymorphism in a Zimbabwe population of D. melanogaster at three loci, yellow, achaete, and suppressor of forked, located in regions of reduced crossing-over. Similar to previously published surveys of these genomic regions in other populations, we observed low levels of nucleotide variability. However, the frequency spectrum was compatible with a neutral model, and there was abundant evidence for recombination in the history of the yellow and ac genes. Thus, some aspects of the data cannot be accounted for by a simple hitchhiking model. An alternative hypothesis, background selection, might be compatible with the observed patterns of linkage disequilibrium and the frequency spectrum. However, this model cannot account for the observed reduction in nucleotide heterozygosity. Thus, there is currently no satisfactory theoretical model for the data from the tip and base of the X chromosome in D. melanogaster.      

Short chain oligogalacturonides induce ethylene production and expression of the gene encoding aminocyclopropane 1-carboxylic acid oxidase in tomato plants

Oligogalacturonic acids (OGAs), derived from plant cell wall pectin, have been implicated in a number of signal transduction pathways involved in growth, development and defense responses of higher plants. This study investigates the size range of OGAs capable of inducing ethylene synthesis in tomato plants, and demonstrates that in contrast with many other effects, only short chain OGAs are active. Oligomers across a range of DP from 2-15 were separated and purified to homogeneity by QAE-Sephadex anion exchange chromatography using a novel elution system. The OGAs were applied to tomato plants and assayed for their ability to induce ethylene gas release and changes in steady state levels of mRNA encoding the ethylene forming enzyme aminocyclopropane-1-carboxylic acid oxidase (ACO). The study demonstrated that only OGAs in the size range of DP4-6 were active both in eliciting ACO expression and in the production of ethylene.      

A change in twist of actin provides the force for the extension of the acrosomal process in limulus sperm: the false-discharge reaction

One of the most spectacular motions is the generation of the acrosomal process in the limulus sperm. On contact with the egg, the sperm generates a 60-mum-long process that literally drills its way through the jelly surrounding the egg. This irresversible reaction takes only a few seconds. We suggested earlier that this motion is driven by a change in twist of the actin filaments comprising the acrosomal process. In this paper we analyze the so-called false discharge, a reversible reaction, in which the acrosomal filament bundle extends laterally from the base of the sperm and not anteriorly from the apex. Unlike the true discharge, which is straight, the false discharge is helical. Before extension, the filament bundle is coiled about the base of the sperm. In the coil, the bundle is not smoothly bent but consists of arms (straight segments) and elbows (corners) so that the coil looks like a 14-sided polygon. The extension of the false discharge works as follows: starting at the base of the bundle, the filaments change their twist which concomitantly changes the orientations of the elbows relative to each other; that is, in the coil, the elbows all like in a common plane, but after the change in twist, the plane of each elbow is rotated to be perpendicular to that of its neighbors. This change transforms the bundle from a compact coil into an extended left- handed helix. Because the basal end of the bundle is unconstrained, the extension is lateral. The true discharge works the same way but starts at the apical end of the bundle. The apical end, however, is constrained by its passage through the nuclear canal, which directs the extention anteriorly. Unlike the false discharge, during the true discharge the elbows are melted out, making the reaction irreversible. This study shows that rapid movement can be regenerated by actin without myosin and gives us insight into the molecular mechanism.     

Accumulation of soil carbon under elevated CO2 unaffected by warming and drought

Elevated atmospheric CO₂ concentration and climate change may substantially alter soil carbon (C) dynamics, which in turn may impact future climate through feedback cycles. However, only very few field experiments worldwide have combined elevated CO₂ (eCO₂) with both warming and changes in precipitation in order to study the potential combined effects of changes in these fundamental drivers of C cycling in ecosystems. We exposed a temperate heath/grassland to eCO₂, warming, and drought, in all combinations for 8 years. At the end of the study, soil C stocks were on average 0.927 kg C/m² higher across all treatment combinations with eCO₂ compared to ambient CO₂ treatments (equal to an increase of 0.120 ± 0.043 kg C m^?2 year^?1), and showed no sign of slowed accumulation over time. However, if observed pretreatment differences in soil C are taken into account, the annual rate of increase caused by eCO₂ may be as high as 0.177 ± 0.070 kg C m^?2 year^?1. Furthermore, the response to eCO₂ was not affected by simultaneous exposure to warming and drought. The robust increase in soil C under eCO₂ observed here, even when combined with other climate change factors, suggests that there is continued and strong potential for enhanced soil carbon sequestration in some ecosystems to mitigate increasing atmospheric CO₂ concentrations under future climate conditions. The feedback between land C and climate remains one of the largest sources of uncertainty in future climate projections, yet experimental data under simulated future climate, and especially including combined changes, are still scarce. Globally coordinated and distributed experiments with long‐term measurements of changes in soil C in response to the three major climate change‐related global changes, eCO₂, warming, and changes in precipitation patterns, are, therefore, urgently needed.     

Application of phage display to high throughput antibody generation and characterization

We have created a high quality phage display library containing over 10¹⁰ human antibodies and describe its use in the generation of antibodies on an unprecedented scale. We have selected, screened and sequenced over 38,000 recombinant antibodies to 292 antigens, yielding over 7,200 unique clones. 4,400 antibodies were characterized by specificity testing and detailed sequence analysis and the data/clones are available online. Sensitive detection was demonstrated in a bead based flow cytometry assay. Furthermore, positive staining by immunohistochemistry on tissue microarrays was found for 37% (143/381) of antibodies. Thus, we have demonstrated the potential of and illuminated the issues associated with genome-wide monoclonal antibody generation.