The centromere histone is conserved and associated with tandem repeats sharing a conserved 19 bp box in the holocentromere of Meloidogyne nematodes

Although centromeres have conserved function, centromere-specific histone H3 (CenH3) and centromeric DNA evolve rapidly. The centromere drive model explains this phenomenon as a consequence of the conflict between fast evolving DNA and CenH3, suggesting asymmetry in female meiosis as a crucial factor. We characterized evolution of the CenH3 protein in three closely related, polyploid mitotic parthenogenetic species of the Meloidogyne incognita group (MIG), and in the distantly related meiotic parthenogen Meloidogyne hapla. We identified duplication of the CenH3 gene in a putative sexual ancestral Meloidogyne. We found that one CenH3 (αCenH3) copy remained conserved in all extant species, including in distant Meloidogyne hapla, while the other copy evolved rapidly and under positive selection into four different CenH3 variants. This pattern of CenH3 evolution in Meloidogyne species suggests the sub-specialization of CenH3s in ancestral sexual species. Immunofluorescence performed on mitotic Meloidogyne incognita revealed a dominant role of αCenH3 on its centromere, while the other CenH3s have lost their function in mitosis. The observed αCenH3 chromosome distribution disclosed cluster-like centromeric organization. The ChIP-Seq analysis revealed that in M. incognita αCenH3-associated DNA dominantly comprises tandem repeats, composed of divergent monomers which share a completely conserved 19 bp-long box. Conserved αCenH3-associated DNA are also confirmed in the related mitotic MIG species suggesting preservation of both centromere protein and DNA constituents. We hypothesize that the absence of centromere drive in mitosis might allow for CenH3 and its associated DNA to achieve an equilibrium in which they can persist for long periods of time.

DNA Nucleobase Under Ionizing Radiation: Unexpected Proton Transfer by Thymine Cation in Water Nanodroplets

<p>The effects of ionizing radiation on DNA constituents is a widely studied fundamental process using experimental and computational techniques. In particular radiation effects on nucleobases are usually tackled by mass spectrometry in which the nucleobase is embedded in a water nanodroplet. Here we present a multiscale theoretical study revealing the effects and the dynamics of water droplets towards neutral and ionized thymine. In particular, by using both hybrid quantum mechanics/ molecular mechanics and fully ab initio molecular dynamics we reveal an unexpected proton transfer from thymine cation to a nearby water molecule. This leads to the formation of a neutral radical thymine and a Zundel structure, while the hydrated proton localizes at the interface between the deprotonated thymine and the water droplet. This observation opens entirely novel perspective concerning the reactivity and further fragmentation of ionized nucleobases.</p>

DNA Nucleobase Under Ionizing Radiation: Unexpected Proton Transfer by Thymine Cation in Water Nanodroplets

<p>The effects of ionizing radiation on DNA constituents is a widely studied fundamental process using experimental and computational techniques. In particular radiation effects on nucleobases are usually tackled by mass spectrometry in which the nucleobase is embedded in a water nanodroplet. Here we present a multiscale theoretical study revealing the effects and the dynamics of water droplets towards neutral and ionized thymine. In particular, by using both hybrid quantum mechanics/ molecular mechanics and fully ab initio molecular dynamics we reveal an unexpected proton transfer from thymine cation to a nearby water molecule. This leads to the formation of a neutral radical thymine and a Zundel structure, while the hydrated proton localizes at the interface between the deprotonated thymine and the water droplet. This observation opens entirely novel perspective concerning the reactivity and further fragmentation of ionized nucleobases.</p>