Planarian flatworms have an indefinite capacity to regenerate missing or damaged body parts owing to a population of pluripotent adult stems cells called neoblasts (NBs). Currently, little is known about the importance of the epigenetic status of NBs and how histone modifications regulate homeostasis and cellular differentiation. We have developed an improved and optimized ChIP-seq protocol for NBs in Schmidtea mediterranea and have generated genome-wide profiles for the active marks H3K4me3 and H3K36me3, and suppressive marks H3K4me1 and H3K27me3. The genome-wide profiles of these marks were found to correlate well with NB gene expression profiles. We found that genes with little transcriptional activity in the NB compartment but which switch on in post-mitotic progeny during differentiation are bivalent, being marked by both H3K4me3 and H3K27me3 at promoter regions. In further support of this hypothesis, bivalent genes also have a high level of paused RNA Polymerase II at the promoter-proximal region. Overall, this study confirms that epigenetic control is important for the maintenance of a NB transcriptional program and makes a case for bivalent promoters as a conserved feature of animal stem cells and not a vertebrate-specific innovation. By establishing a robust ChIP-seq protocol and analysis methodology, we further promote planarians as a promising model system to investigate histone modification-mediated regulation of stem cell function and differentiation.
Andreev reflection in graphene is special since it can be of two types- retroor specular. Specular Andreev reflection (SAR) dominates when the position ofthe Fermi energy in graphene is comparable to or smaller than thesuperconducting gap. Bilayer graphene (BLG) is an ideal candidate to observethe crossover from retro to specular since the Fermi energy broadening near theDirac point is much weaker compared to monolayer graphene. Recently, theobservation of signatures of SAR in BLG have been reported experimentally bylooking at the enhancement of conductance at finite bias near the Dirac point.However, the signatures were not very pronounced possibly due to theparticipation of normal quasi-particles at bias energies close to thesuperconducting gap. Here, we propose a scheme to observe the features ofenhanced SAR even at zero bias at a normal metal (NM)-superconductor (SC)junction on BLG. Our scheme involves applying a Zeeman field to the NM side ofthe NM-SC junction on BLG (making the NM ferromagnetic), which energeticallyseparates the Dirac points for up-spin and down-spin. We calculate theconductance as a function of chemical potential and bias within thesuperconducting gap and show that well-defined regions of specular- andretro-type Andreev reflection exist. We compare the results with and withoutsuperconductivity. We also investigate the possibility of the formation of ap-n junction at the interface between the NM and SC due to a work functionmismatch.
Global polarization of $\Lambda$ hyperons has been measured to be of theorder of a few tenths of a percent in Au+Au collisions at $\sqrt{s_{_{NN}}}$ =200 GeV, with no significant difference between $\Lambda$ and $\bar{\Lambda}$.These new results reveal the collision energy dependence of the globalpolarization together with the results previously observed at$\sqrt{s_{_{NN}}}$ = 7.7 -- 62.4 GeV and indicate noticeable vorticity of themedium created in non-central heavy-ion collisions at the highest RHICcollision energy. The signal is in rough quantitative agreement with thetheoretical predictions from a hydrodynamic model and from the AMPT (AMulti-Phase Transport) model. The polarization is larger in more peripheralcollisions, and depends weakly on the hyperon's transverse momentum andpseudorapidity $\eta^H$ within $|\eta^H|<1$. An indication of the polarizationdependence on the event-by-event charge asymmetry is observed at the $2\sigma$level, suggesting a possible contribution to the polarization from the axialcurrent induced by the initial magnetic field.