Using Ultralow Dosages of Electron Acceptor to Reveal the Early Stage Donor–Acceptor Electronic Interactions in Bulk Heterojunction Blends

Tuning the donor–acceptor (D–A) weight ratio is an essential step to optimize the performance of a bulk heterojunction (BHJ) solar cell. The unoptimized regime with a low acceptor concentration is generally unexplored despite it may reveal the early stage electronic D–A interactions. In this study, PTB7:PC₇₁BM is used to examine factors that limit the device performance in unoptimized regime. The key limiting factor is the creation of traps and localized states originated from fullerene molecules. Photothermal deflection spectroscopy is used to quantify the trap density. Starting with pristine PTB7, addition of small concentration of fullerene increases the electron trap density and lowers the electron mobility. When the D–A weight ratio reaches 1:0.1, fullerene percolation occurs. There is an abrupt drop in trap density and simultaneously a six orders of magnitude increase in the electron mobility. Furthermore, the fill factors of the corresponding photovoltaic devices are found to anticorrelate with the trap density. This study reveals that electron trapping is the key limiting factor for unoptimized BHJ solar cells in low fullerene regime.     

Badge size, paternity assurance behaviours and paternity losses in male house sparrows

Male quality may influence both the outcome of sperm competition and female faithfulness. In male house sparrows Passer domesticus , the size of the black throat patch (badge) signals dominance and perhaps attractiveness. So far, however, no study has reported any significant relationships between badge size, paternity and paternity assurance behaviours in this species. We found that the time mates spent together at the nest was positively correlated with badge size. Furthermore, although paternity losses were influenced by both the time spent at the nest and within-pair copulation frequency, we found no relationship between copulation rate and badge size. It seems therefore that copulation frequency served as a paternity assurance behaviour, whereas the time mates stayed together at the nest may have reflected male attractiveness. Alternatively, females may have decided to stay with large-badged males because they were better able to protect them from harassment by strange males. We also found that paternity losses were related to male badge size; average-badged males cuckolded were more often than males with smaller or larger badges. We suggest that average-badged males suffered higher paternity losses because they had different time allocation strategies than other males.     

High-fidelity KKH variant of Staphylococcus aureus Cas9 nucleases with improved base mismatch discrimination

The Cas9 nuclease from Staphylococcus aureus (SaCas9) holds great potential for use in gene therapy, and variants with increased fidelity have been engineered. However, we find that existing variants have not reached the greatest accuracy to discriminate base mismatches and exhibited much reduced activity when their mutations were grafted onto the KKH mutant of SaCas9 for editing an expanded set of DNA targets. We performed structure-guided combinatorial mutagenesis to re-engineer KKH-SaCas9 with enhanced accuracy. We uncover that introducing a Y239H mutation on KKH-SaCas9’s REC domain substantially reduces off-target edits while retaining high on-target activity when added to a set of mutations on REC and RuvC domains that lessen its interactions with the target DNA strand. The Y239H mutation is modelled to have removed an interaction from the REC domain with the guide RNA backbone in the guide RNA-DNA heteroduplex structure. We further confirmed the greatly improved genome-wide editing accuracy and single-base mismatch discrimination of our engineered variants, named KKH-SaCas9-SAV1 and SAV2, in human cells. In addition to generating broadly useful KKH-SaCas9 variants with unprecedented accuracy, our findings demonstrate the feasibility for multi-domain combinatorial mutagenesis on SaCas9’s DNA- and guide RNA- interacting residues to optimize its editing fidelity.     

The effect of depletion and predictability of distinct food patches on the timing of aggression in red deer stags

For group-foraging ungulates, forage is generally widely and relatively evenly dispersed. However, for free-ranging red deer Cervus elaphus supplementary winter feeding provides distinct patches of predictable food. These patches differ in size, but also in temporal distribution and depletion rate. Interference competition is known to increase with increasing spatial clumping (decreasing patch size), but the influence of temporal clumping and the predictability of food occurrence has received much less attention. Therefore in this study we investigated the effects of different degrees of spatial and temporal clumping of food on interference competition during feeding. Patch size was the main parameter influencing participation in feeding as well as interference competition during feeding on the respective patch. Temporal dispersion and the predictability of food occurrence were however, important parameters for the timing of aggressive interactions. Generally, aggression occurs during feeding and increases with decreasing patch size. But when depletion rate was high food availability was predictably short and the patch occurred predictably (such as hay), middle ranking stags increased aggression already prior to feeding at the respective patch. We suggest that in this way they confirmed hierarchy outside feeding on the quickly depleted patch and as a result gained actual feeding time when feeding on the respective patch. With the patch occurring predictably but varying in size the number of participating subordinates varied concomitantly with variation in patch size. Subordinates assessed patch profitability and left without having fed when patch size was too small for an efficient participation. When patch size was predictably small enough to be defended exclusively (feed blocks), subordinate stags did not assess profitability each time but did not participate at all in feeding at the respective patch. The relative importance of these various food-related parameters (patch size depletion rate, predictability) influencing feeding competition and the timing of aggression will vary with group size, rank, alternative food sources, physical characteristics of the food as well as different hierarchy systems of the feeding animals.