Time and Origin of Cichlid Colonization of the Lower Congo Rapids

Most freshwater diversity is arguably located in networks of rivers and streams, but, in contrast to lacustrine systems riverine radiations, are largely understudied. The extensive rapids of the lower Congo River is one of the few river stretches inhabited by a locally endemic cichlid species flock as well as several species pairs, for which we provide evidence that they have radiated in situ. We use more that 2,000 AFLP markers as well as multilocus sequence datasets to reconstruct their origin, phylogenetic history, as well as the timing of colonization and speciation of two Lower Congo cichlid genera, Steatocranus and Nanochromis. Based on a representative taxon sampling and well resolved phylogenetic hypotheses we demonstrate that a high level of riverine diversity originated in the lower Congo within about 5 mya, which is concordant with age estimates for the hydrological origin of the modern lower Congo River. A spatial genetic structure is present in all widely distributed lineages corresponding to a trisection of the lower Congo River into major biogeographic areas, each with locally endemic species assemblages. With the present study, we provide a phylogenetic framework for a complex system that may serve as a link between African riverine cichlid diversity and the megadiverse cichlid radiations of the East African lakes. Beyond this we give for the first time a biologically estimated age for the origin of the lower Congo River rapids, one of the most extreme freshwater habitats on earth

High pressure induced phase transitions and amorphisation in Gd2(MoO4)3

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Étude Raman de l'effet de taille nanométrique des cristallites sur le diagramme de phase P-T de la zircone

The phase transformations of 7, 13, 18nm nanocrystalline zirconia prepared by pyrosol technique and 10-15nm zirconia prepared by oxidation of zirconium alloys have been monitored by Raman spectrometry. It is shown that for these zirconia, the tetragonal form can be obtained either under its metastable state at ambient conditions or from the monoclinic form by application of pressure below 4 GPa and with temperature increment smaller than 300 K. The nanometric monoclinic-tetragonal equilibrium line is strongly down-shifted in temperature compared to literature data measured on usual polycrystalline zirconia. In addition, the monoclinic-orthorhombic equilibrium line is slightly shifted to higher pressure (i.e. 7 GPa). These results confirm that stresses can play a primordial role in the formation of tetragonal form in the oxidation process of zirconium alloys

Étude Raman de l'effet de taille nanométrique des cristallites sur le diagramme de phase P-T de la zircone

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Zone center phonon frequencies in tetragonal zirconia: lattice dynamical study and new assignment proposition

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Étude Raman et cristallographique de la transition de phase displacive quadratique-cubique dans une zircone nanométrique

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Neutron scattering study of the reorientational motions for NH3 groups in (NH3(CH2)3NH3)MnCl 4

Incoherent quasi-elastic neutron scattering experiments have been performed on oriented crystals and on polycrystalline samples of (NH3(CH 2)3NH3)MnCl4 and (NH3(CD 2)3NH3)MnCl4 compounds in the 4-300 K temperature range. Backscattering and time of flight spectrometers were used; spectra were obtained using different resolutions (1-170 μeV) and a large range of momentum transfers (0.17-2.57 Å-1). It is concluded that NH 3 groups reorientate below 300 K; the experimental spectra have been satisfactorily fitted with a model of 2 π/3 jumps of the NH3 groups, associated with a correlation time of 1 x 10-11 s at 300 K and an activation energy roughly equal to 27 kJ/mol. Finally a comparison with literature data, mainly from NMR and Raman results, is presented.Des expériences de diffusion incohérente quasi-élastique des neutrons ont été réalisées à partir de cristaux orientés et d'échantillons polycristallins des composés (NH3(CH2)3NH3)MnCl 4 et (NH3(CD2)3NH3)MnCl 4 sur le domaine de température 4-300 K. Des spectromètres à rétro-diffusion et à temps de vol ont été utilisés ; les spectres ont été enregistrés à l'aide de différentes résolutions (entre 1 et 170 μeV) et sur un large domaine de transfert de moment (entre 0,17 et 2,57 Å-1). On conclut que les groupes NH3 effectuent des mouvements de réorientation en dessous de 300 K. Les spectres expérimentaux ont été reproduits, de manière satisfaisante, par un modèle de sauts de 2 π/3 des groupes NH3, avec un temps de corrélation de 10-11 s à 300 K et une énergie d'activation d'environ 27 kJ/mole. Enfin, une comparaison de ces résultats avec ceux de la littérature, provenant essentiellement d'études antérieures par RMN et Raman, est présentée