Genome-Wide Analyses Reveal Genes Subject to Positive Selection in Pasteurella multocida

Pasteurella multocida, a Gram-negative opportunistic pathogen, has led to a broad range of diseases in mammals and birds, including fowl cholera in poultry, pneumonia and atrophic rhinitis in swine and rabbit, hemorrhagic septicemia in cattle, and bite infections in humans. In order to better interpret the genetic diversity and adaptation evolution of this pathogen, seven genomes of P. multocida strains isolated from fowls, rabbit and pigs were determined by using high-throughput sequencing approach. Together with publicly available P. multocida genomes, evolutionary features were systematically analyzed in this study. Clustering of 70,565 protein-coding genes showed that the pangenome of 33 P. multocida strains was composed of 1,602 core genes, 1,364 dispensable genes, and 1,070 strain-specific genes. Of these, we identified a full spectrum of genes related to virulence factors and revealed genetic diversity of these potential virulence markers across P. multocida strains, e.g., bcbAB, fcbC, lipA, bexDCA, ctrCD, lgtA, lgtC, lic2A involved in biogenesis of surface polysaccharides, hsf encoding autotransporter adhesin, and fhaB encoding filamentous haemagglutinin. Furthermore, based on genome-wide positive selection scanning, a total of 35 genes were subject to strong selection pressure. Extensive analyses of protein subcellular location indicated that membrane-associated genes were highly abundant among all positively selected genes. The detected amino acid sites undergoing adaptive selection were preferably located in extracellular space, perhaps associated with bacterial evasion of host immune responses. Our findings shed more light on conservation and distribution of virulence-associated genes across P. multocida strains. Meanwhile, this study provides a genetic context for future researches on the mechanism of adaptive evolution in P. multocida

Mid-Neoproterozoic diabase dykes from Xide in the western Yangtze Block, South China: New evidence for continental rifting related to the breakup of Rodinia supercontinent

The petrogenesis of widespread Mid-Neoproterozoic mafic dykes is crucial for the paleographic position of the South China Block (SCB) in Rodinia supercontinent and the mechanism of Rodinia breakup. Here, new detailed geochronological and geochemical data on the diabase dykes from Xide in the western Yangtze Block are presented. Zircon SHRIMP/LA-ICP-MS U-Pb dating shows that four diabase samples yield uniform crystallization age varying from 796 ± 6 Ma to 809 ± 15 Ma, while one sample gives a slight older age of 824 ± 11 Ma that is overlapped with ca. 810 Ma within uncertainties. This suggests that the Xide diabase dykes emplaced at ca. 800–810 Ma and were coeval with regional bimodal magmatism (e.g., the Suxiong bimodal volcanics). The Xide diabase dykes are characterized by low SiO2 contents, high Mg# values and Cr, Ni contents, relative enrichment of light rare-earth elements, and slight depletion of high field strength elements (e.g., Nb, Ta, Zr, and Hf) and nearly constant Zr/Hf, Nb/Ta and Nb/La ratios. Our analyses indicate that the diabase was mainly produced by interaction between lithospheric and asthenospheric mantle. Moreover, the diabase samples display geochemical characteristics affinity with typical intra-plate basalts. Together with the widespread coeval bimodal magmatic suite and sedimentary records in the Kangdian Rift, we proposed that the western Yangtze Block once experienced continental rifting during the Mid-Neoproterozoic, which also occurred in other Rodinia blocks, such as Tarim, Australia and North America. In addition, the Grenville-aged magmatism records throughout SCB with the widespread Mid-Neoproterozoic rift-related magmatism and sedimentation records imply that SCB probably played a key role in the assembly and breakup of Rodinia supercontinent