Plastome organization and evolution of chloroplast genes in Cardamine species adapted to contrasting habitats

© 2015 Hu et al.; licensee BioMed Central. Background: Plastid genomes, also known as plastomes, are shaped by the selective forces acting on the fundamental cellular functions they code for and thus they are expected to preserve signatures of the adaptive path undertaken by different plant species during evolution. To identify molecular signatures of positive selection associated to adaptation to contrasting ecological niches, we sequenced with Solexa technology the plastomes of two congeneric Brassicaceae species with different habitat preference, Cardamine resedifolia and Cardamine impatiens. Results: Following in-depth characterization of plastome organization, repeat patterns and gene space, the comparison of the newly sequenced plastomes between each other and with 15 fully sequenced Brassicaceae plastomes publically available in GenBank uncovered dynamic variation of the IR boundaries in the Cardamine lineage. We further detected signatures of positive selection in ten of the 75 protein-coding genes of the examined plastomes, identifying a range of chloroplast functions putatively involved in adaptive processes within the family. For instance, the three residues found to be under positive selection in RUBISCO could possibly be involved in the modulation of RUBISCO aggregation/activation and enzymatic specificty in Brassicaceae. In addition, our results points to differential evolutionary rates in Cardamine plastomes. Conclusions: Overall our results support the existence of wider signatures of positive selection in the plastome of C. resedifolia, possibly as a consequence of adaptation to high altitude environments. We further provide a first characterization of the selective patterns shaping the Brassicaceae plastomes, which could help elucidate the driving forces underlying adaptation and evolution in this important plant family

Simulation study of uranium content in uranium yellow cake using the active multiplicity method

BackgroundAccurately quantifying the uranium in uranium yellow cake material is the key to selecting the subsequent processing technology. As an essential nondestructive testing method for uranium-containing materials, the active multiplicity method is proposed to quantify uranium by recording and analyzing 238U fission information induced by neutron sources. However, the quantitative results are biased owing to the neutron self-shielding of the uranium yellow cake material itself and differences in water content between samples.PurposeThis study aims to rapidly measure the uranium content of uranium yellow cake material using active multiplicity method and further improve of measurement accuracy.MethodsFirst of all, following the comparison of the excitation effects of different neutron sources on a sample using the MCNP (Monte Carlo N-Particle Transport) program, a 241Am-Be source was selected to simulate the sample measurement process and optimized using MATLAB programming combined with MCNP. Then, the curve of multiplication factor M versus the uranium mass was obtained by simulation, and an appropriate M was selected according to the net content of the sample. Finally, the quantitative error caused by the difference between neutron absorption and water content in the process was investigated, and the double rate was corrected using the relationship between S0/Si and D0/Di and then calculated.ResultsThe simulation results of a series of samples with different masses and water contents show that a large gap is found between M and the leakage multiplication factor ML caused by neutron self-shielding. The error in uranium quantification is less than 5%; neutron self-shielding due to the change in water content affects the single, double, and triple counting rates (S/D/T). The relative error of uranium quantification can be controlled at around 10%.ConclusionsThis study has significance and important reference value for further research on the application of the active multiplicity method in the production and measurement of uranium yellow cake