分类： 生物学 >> 生物进化论 提交时间： 2018-04-20
摘要： 古花Archaeanthus是1984年由Dilcher和Crane根据美国堪萨斯州白垩纪Albian-Cenomanian期的化石材料建立起来的被子植物化石属。当时其胚珠/种子被解释成是沿着果实的腹缝线着生的。这种解释在当时听起来很合理，尤其是考虑到当时盛行的被子植物演化理论。但是Dilcher和Crane关于胚珠/种子着生位置的说法连他们自己的证据都不支持。考虑到古花较早的时代、在古植物学和植物系统学中的重要性和影响力，有必要重新确认古花中胚珠的着生位置及其对心皮同源性及被子植物起源的启示。对于古花模式标本的重新观察表明，除了前人所说的腹面着生外，古花中至少有些胚珠是着生在果实的背面的。这个信息动摇了原作者认为的古花与木兰类之间的亲缘关系，也否定了其对传统的被子植物心皮同源性理论的支持。仔细分析前人关于早期被子植物化石（包括古果、古花和其他化石）的错误解读，可以发现当年Arber和Parkin想当然提出的所谓理论是一百多年以来长期折磨植物学和植物学家的元凶。
分类： 地球科学 >> 空间物理学 提交时间： 2016-05-12
摘要： MHD discontinuities are ubiquitous in the solar wind and are often found at the origin of turbulence intermittency. They may also play a key role in the turbulence dissipation and heating of the solar wind. The tangential discontinuities (TDs) and rotational discontinuities (RDs) are the two most important types of discontinuities. Recently, the connection between turbulence intermittency and proton thermodynamics has been observationally investigated. Here, we present numerical results from a three-dimensional MHD simulation with pressure anisotropy and we define new methods for identifying and distinguishing TDs and RDs. Three statistical results obtained for the relative occurrence rates and heating effects are highlighted: (1) RDs tend to take up the majority of the discontinuities along with time;(2) the thermal states embedding TDs tend to be associated with extreme plasma parameters or instabilities while RDs do not;(3) TDs have a higher average T as well as perpendicular temperature T.. The simulation shows that TDs and RDs evolve and contribute to solar wind heating differently. These results will improve our understanding of the mechanisms that generate discontinuities and cause plasma heating.
分类： 地球科学 >> 空间物理学 提交时间： 2016-05-12
Yang, Liping
Zhang, Lei
He, Jiansen
Tu, Chuanyi
Wang, Linghua
Marsch, Eckart
Wang, Xin
Zhang, Shaohua
Feng, Xueshang
摘要： Measurements of solar wind turbulence reveal the ubiquity of discontinuities. In this study we investigate how the discontinuities, especially rotational discontinuities (RDs), are formed in MHD turbulence. In a simulation of the decaying compressive three-dimensional (3D) MHD turbulence with an imposed uniform background magnetic field, we detect RDs with sharp field rotations and little variations of magnetic field intensity, as well as mass density. At the same time, in the de Hoffman-Teller frame, the plasma velocity is nearly in agreement with the Alfven speed, and is field-aligned on both sides of the discontinuity. We take one of the identified RDs to analyze its 3D structure and temporal evolution in detail. By checking the magnetic field and plasma parameters, we find that the identified RD evolves from the steepening of the Alfven wave with moderate amplitude, and that steepening is caused by the nonuniformity of the Alfven speed in the ambient turbulence.
分类： 生物学 >> 生物物理学 提交时间： 2016-05-05
Guo, Yu
Ma, Chao
Wang, Xu
Wang, Xin
Liu, Pi
Lin, Jianping
Guo, Yu
Ma, Chao
Wang, Xu
Wang, Xin
Liu, Pi
Lin, Jianping
Lou, Zhiyong
Lou, Zhiyong
Sun, Yuna
Lou, Zhiyong
Shen, Shu
Deng, Fei
Wang, Hualin
Wang, Wenming
摘要： Hantaviruses, which belong to the genus Hantavirus in the family Bunyaviridae, infect mammals, including humans, causing either hemorrhagic fever with renal syndrome (HFRS) or hantavirus cardiopulmonary syndrome (HCPS) in humans with high mortality. Hantavirus encodes a nucleocapsid protein (NP) to encapsidate the genome and form a ribonucleoprotein complex (RNP) together with viral polymerase. Here, we report the crystal structure of the core domains of NP (NPcore) encoded by Sin Nombre virus (SNV) and Andes virus (ANDV), which are two representative members that cause HCPS in the New World. The constructs of SNV and ANDV NPcore exclude the N- and C-terminal portions of full polypeptide to obtain stable proteins for crystallographic study. The structure features an N lobe and a C lobe to clamp RNA-binding crevice and exhibits two protruding extensions in both lobes. The positively charged residues located in the RNA-binding crevice play a key role in RNA binding and virus replication. We further demonstrated that the C-terminal helix and the linker region connecting the N-terminal coiled-coil domain and NPcore are essential for hantavirus NP oligomerization through contacts made with two adjacent protomers. Moreover, electron microscopy (EM) visualization of native RNPs extracted from the virions revealed that a monomer-sized NP-RNA complex is the building block of viral RNP. This work provides insight into the formation of hantavirus RNP and provides an understanding of the evolutionary connections that exist among bunyaviruses. IMPORTANCE Hantaviruses are distributed across a wide and increasing range of host reservoirs throughout the world. In particular, hantaviruses can be transmitted via aerosols of rodent excreta to humans or from human to human and cause HFRS and HCPS, with mortalities of 15% and 50%, respectively. Hantavirus is therefore listed as a category C pathogen. Hantavirus encodes an NP that plays essential roles both in RNP formation and in multiple biological functions. NP is also the exclusiv
分类： 生物学 >> 生物物理学 提交时间： 2016-05-05
Hao, Huijing
Liang, Junrong
Duan, Ran
Chen, Yuhuang
Liu, Chang
Xiao, Yuchun
Li, Xu
Jing, Huaiqi
Wang, Xin
Liu, Chang
Su, Mingming
摘要： API 20E strip test, the standard for Enterobacteriaceae identification, is not sufficient to discriminate some Yersinia species for some unstable biochemical reactions and the same biochemical profile presented in some species, e.g. Yersinia ferderiksenii and Yersinia intermedia, which need a variety of molecular biology methods as auxiliaries for identification. The 16S rRNA gene is considered a valuable tool for assigning bacterial strains to species. However, the resolution of the 16S rRNA gene may be insufficient for discrimination because of the high similarity of sequences between some species and heterogeneity within copies at the intra-genomic level. In this study, for each strain we randomly selected five 16S rRNA gene clones from 768 Yersinia strains, and collected 3,840 sequences of the 16S rRNA gene from 10 species, which were divided into 439 patterns. The similarity among the five clones of 16S rRNA gene is over 99% for most strains. Identical sequences were found in strains of different species. A phylogenetic tree was constructed using the five 16S rRNA gene sequences for each strain where the phylogenetic classifications are consistent with biochemical tests; and species that are difficult to identify by biochemical phenotype can be differentiated. Most Yersinia strains form distinct groups within each species. However Yersinia kristensenii, a heterogeneous species, clusters with some Yersinia enterocolitica and Yersinia ferderiksenii/intermedia strains, while not affecting the overall efficiency of this species classification. In conclusion, through analysis derived from integrated information from multiple 16S rRNA gene sequences, the discrimination ability of Yersinia species is improved using our method.
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