高山松的群体历史及其适应性进化研究

物种形成一直以来是进化生物学的一个中心议题。因为杂交能够快速创造出新的遗传变异和提高基因组重组率，加速物种形成和适应性进化，所以杂交物种形成是其重要的组成部分。但是直到最近几年，人们才逐渐意识到同倍体杂种形成的重要性和普遍性。此外，人工杂交种群结果证实了杂种的适应性起源于亲本基因的超亲分离现象，这些基因之间通过加和效应或上位效应相互连锁，并在杂种后代中逐渐固定，因此，探讨维持这种适应机制的杂种的连锁不平衡模式就显得尤为重要。到目前为止，关于自然杂种的核苷酸多态性和连锁不平衡结构的报道非常稀见，对杂种的进化历史知之更少。高山松能生长于亲本种不能正常繁殖生长的青藏高原上，这种极端环境为杂种的适应性进化和与亲本的生态隔离造就了绝佳的条件，其生态和生殖的稳定性极大地方便了我们研究植物物种形成和适应性进化的遗传机理，对高山松开展的生态学、遗传学的系列分析已使高山松成为同倍体杂种形成的经典范例。 该论文通过随机挑选7个核基因位点对5个高山松群体、3个油松群体和3个云南松群体的164个大配子体单倍体基因组进行了核苷酸多态性分析。研究发现：所有基因座位的单倍型组成和基因谱系的拓扑结构都支持高山松的多次杂交起源。高山松的平均核苷酸多态性与油松相当，θW达到0.0107;比云南松高出一倍;高于已报道的裸子植物类群遗传变异水平。高山松如此高的遗传多态性与其杂交特性和有效群体大有关。我们运用分子钟和共祖模拟分析，推测出高山松的有效群体为7.32 x 105。等位基因共祖时间模拟分析发现，杂交过程早于青藏高原的隆升，也即是说，在杂种稳定成种以前，两个亲本种之间存在广泛的渐渗杂交。此外，高山松杂种的群体间分化严重，并且在不同的群体中，我们找到了多个偏离中性的基因座位。由此说明，高山松复杂的进化历史和适应的策略区域化。 在显著偏离中性的基因座位上，选取群体历史清晰的高山松群体，对其全基因序列多态性和连锁不平衡结构进行了深入调查，以期找到选择作用位点或区域。Ara-like和Dhn1两个基因在高山松与亲本之间都没有发现固定变异。在高山松中，Ara-like基因和Dhn1基因核苷酸多态性式样和连锁不平衡结构存在明显差异，这说明在两个基因上选择作用的程度和方向不尽相同。 我们对Dhn1基因的多态性分布、LD结构和中性偏离水平进行了分析，结果表明，该基因可能在PdNX和PdLZ群体中受到平衡选择的作用。从Dhn1基因谱系结构可以看出有两种来源的等位基因造成这种平衡多态性，一种属于祖先类型，另一种是从云南松继承衍生而来。这两种等位基因之间的分化很大，极有可能造成编码蛋白的亚功能化。事实上，Dhn1编码的脱水蛋白在植物对环境的抗逆过程中发挥着重要作用。由此我们推测高山松在高海拔极端环境中的适应性进化与Dhn1基因的平衡作用有关。 对Ara-like和Dhn1基因进行HKA检测，结果表明，与Dhn1基因相比，在Ara-like基因上，双维管束亚属与单维管束亚属之间存在显著分化，沉默突变位点的分化Ksil达到了0.1392，远远高于平均水平0.0508;在双维管束亚属共发现了43个固定突变位点，其中有6个能导致氨基酸突变，它们有可能导致了Ara-like基因功能的分化。结合裸子植物近缘物种间共享多态性的普遍性，我们推测Ara-like基因可能在单维管束亚属中的进化速率加快，暗示其在单维管束亚属中的适应性进化。 最后，基因内连锁不平衡分析结果显示，随机筛选的基因座位之间不存在连锁不平衡。高山松平均的基因内LD程度非常低，仅有18%的信息位点之间显著连锁。平均基因内LD在油松和高山松中的衰减速率很快，尤其在杂种中下降最快，在不到200bp以内就降到0.1以下。 LD的结果印证了高山松的有效群体大和多次起源特性。另外，我们也怀疑杂交物种形成过程中染色体组的重组和重排频繁发生，也是造成自然杂种现有群体的LD水平低的一个主要原因。 通过LD衰减曲线估计高山松的平均单位重组率,比亲本油松高17倍，比云南松高45倍。这个结果表明，杂种共适应的基因间要维持超亲分离，需要强烈的自然选择压力，才能保持它们之间的连锁不平衡。

Influence of convections on dopant segregation in floating-zone crystal-growth system

The steady and axisymmetric crystal growth process of floating zone model was studied numerically to concern with the influence of convection and phase change on effective segregation. An iteration method of numerical simulation considering both thermocapillary and buoyancy effects for GaAs crystal growth gave the effective segregation coefficient, which was compared with the space experiment of GaAs on board the Chinese recoverable satellite. The calculated segregation coefficient of a two-dimensional model was found to be smaller than the one suggested by space experiment with the simplified assumption of an one-dimensional model.

Structure of a fig wasp community: Temporal segregation of oviposition and larval diets

Observations were made on six fig wasp species on Ficus racemosa growing in the Xishuangbanna Tropical Botanic Garden, Yunnan Province, China. The oviposition sequence was determined for Apocryptophagus testacea, Apocrypta sp2, Apocryptophagus mayri, Cera

Interlayer segregation in magnetic multilayers and its influence on exchange coupling

Experimental results show that the exchange coupling field (H-ex) of NiFe/FeMn for Ta/NiFe/FeMn/Ta multilayers is higher than that for spin-valve multilayers Ta/NiFe/Cu/NiFe/FeMn/Ta. X-ray photoelectron spectroscopy shows that Cu atoms segregate to the NiFe/FeMn interface for Ta/NiFe/Cu/NiFe/FeMn/Ta multilayers. While studying Ta/X(X=Bi,Pb,Ag,In)/NiFe/FeMn multilayers, we also find that X atoms segregate to the NiFe/FeMn interface, which results in a decrease of the H-ex. However, a small amount of Bi, Pb, etc. deposited between Cu and pinned NiFe layer for Ta/NiFe/Cu/NiFe/FeMn/Ta multilayers can increase H-ex. (C) 2003 American Institute of Physics.

Magnetic property and interlayer segregation in spin valve metal multilayers

The experimental results show that the exchange coupling field of NiFe/FeMn for Ta/ NiFe/FeMn/Ta multilayers is higher than that for the spin valve multilayers Ta/NiFe/Cu/NiFe/FeMn/ Ta. In order to find out the reason, the composition and chemical states at the surfaces of Ta(12nm)/ NiFe(7nm), Ta(12nm)/NiFe(7nm)/Cu(4nm) and Ta(12nm)/NiFe(7nm)/Cu(3nm)/NiFe(5nm) were studied using the X-ray photoelectron spectroscopy (XPS). The results show that no elements from lower layers float out or segregate to the surface for the first and second samples. However, Cu atoms segregate to the surface of Ta(12nm)/NiFe(7nm)/Cu(3nm)/NiFe(5nm) multilayers, i.e. Cu atoms segregate to the NiFe/FeMn interface for Ta/NiFe/Cu/NiFe/FeMn/Ta multilayers. We believe that the presence of Cu atoms at the interface of NiFe/FeMn is one of the important factors causing the exchange coupling field of Ta/NiFe/FeMn/Ta multilayers to be higher than that of Ta/NiFe/Cu/NiFe/ FeMn/Ta multilayers.

Detection of indium segregation effects in InGaAs/GaAs quantum wells using reflectance-difference spectrometry

The influence of the Indium segregation on the interface asymmetry in InGaAs/GaAs quantum wells have been studied by reflectance-difference spectroscopy (RDS). It is found that the anisotropy of the 2H1E (2HH --> 1E) transition is very sensitive to the degree of the interface asymmetry. Calculations taking into account indium segregation yield good agreement with the observed anisotropy structures. It demonstrates that the anisotropy intensity ratio of the 1L1E (1LH --> 1E) and 2H1E transitions measured by RDS can be used to characterize the interface asymmetry. (C) 2002 Elsevier Science B.V. All rights reserved.

Interlayer segregation of Cu atoms in Ta/NiFe/Cu/NiFe/FeMn/Ta spin-valve multilayers and its influence on magnetic properties

Experimental results show that the exchange coupling field (H-ex) of NiFe/FeMn for Ta/NiFe/FeMn/Ta multilayers is higher than that for spin-valve multilayers Ta/NiFe/Cu/NiFe/FeMn/Ta. In order to find out the reason, the composition and chemical states at the surface of Ta(12 nm)/NiFe(7 nm), Ta(12 nm)/NiFe(7 nm)/Cu(4 nm), and Ta(12 nm)/NiFe(7 nm)/Cu(3 nm)/NiFe(5 nm) were studied using x-ray photoelectron spectroscopy. The results show that no elements from lower layers float out or segregate to the surface in the first and second samples. However, Cu atoms segregate to the surface of Ta(12 nm)/NiFe(7 nm)/Cu(3 nm)/NiFe(5 nm) multilayers, i.e., Cu atoms segregate to the NiFe/FeMn interface for Ta/NiFe/Cu/NiFe/FeMn/Ta multilayers. We believe that the presence of Cu atoms at the interface of NiFe/FeMn is one of the important factors which causes the exchange coupling field (H-ex) of Ta/NiFe/Cu/NiFe/FeMn/Ta to be weaker than that of Ta/NiFe/FeMn/Ta. (C) 2002 American Institute of Physics.

Interlayer segregation of Cu atoms in metal multilayers

The experimental results show that the exchange coupling field H.. of NiFe/FeMn for TalNiFe/FeMn/Ta multilayers is higher than that for the spin valve multilayers Ta/NiFe/Cu/NiFe/FeMn/Ta. The composition and chemical states at the surface of Ta(12nm)/NiFe(7nm), Th(12nm)/NiFe(7nm)/Cu(4nm) and Ta(12nm)/NiFe(7nm)/Cu(3 nm)/NiFe(5 mn) were studied by using x-ray photoelectron spectroscopy. The results show that no element from the underlayers Boats out or segregates to the surface for Th(12 nm)/NiFe(7nm), Ta(12 nm)/NiFe(7nm)/Cu(4 mn). However, Cu atoms segregate to the surface of Ta(12 nm)/NiFe(7nm)/Cu(3nm)/NiFe(5nm) multilayers, i.e. to the NiFe/FeMn interface for Ta/NiFe/Cu/NiFe/FeMn/Ta multilayers. We believe that the presence of Cu atoms at the interface of NiFe/FeMn is one of the important factors which will cause the exchange coupling field H.. of Ta/NiFe/FeMn/Ta multilayers to be higher than that of Ta/NiFe/Cu/NiFe/FeMn/Ta multilayers.

Detection of indium segregation effects in InGaAs/GaAs quantum wells using reflectance-difference spectrometry

The influence of the Indium segregation on the interface asymmetry in InGaAs/GaAs quantum wells have been studied by reflectance-difference spectroscopy (RDS). It is found that the anisotropy of the 2H1E (2HH --> 1E) transition is very sensitive to the degree of the interface asymmetry. Calculations taking into account indium segregation yield good agreement with the observed anisotropy structures. It demonstrates that the anisotropy intensity ratio of the 1L1E (1LH --> 1E) and 2H1E transitions measured by RDS can be used to characterize the interface asymmetry. (C) 2002 Elsevier Science B.V. All rights reserved.

Is there a time-transgressive Holocene optimum in the East Asian monsoon area?

IEECAS SKLLQG

Inheritance of 15 microsatellites in the Pacific oyster Crassostrea gigas: segregation and null allele identification for linkage analysis

Microsatellites were screened in a backcross family of the Pacific oyster, Crassostrea gigas. Fifteen microsatellite loci were distinguishable and polymorphic with 6 types of allele-combinations. Null alleles were detected in 46.7% of loci, accounting for 11.7% of the total alleles. Four loci did not segregate in Mendelian Ratios. Three linkage groups were identified among 7 of the 15 segregating loci. Fluorescence-based automated capillary electrophoresis (ABI 310 Genetic Analyzer) that used to detect the microsatellite loci, has been proved a fast, precise, and reliable method in microsatellite genotyping.

Chromosome segregation in fertilized eggs from triploid Pacific oysters, Crassostrea gigas (Thunberg), following inhibition of polar body 1

Chromosome segregation in fertilized eggs from triploid Pacific oysters, following inhibition of the first polar body (PB1), was studied with acetic orcein staining techniques. To block the release of PB1, fertilized eggs were treated with 0.5 mg/l of cytochalasin B (CB). Four types of segregation were observed, namely, ''tripolar segregation'' (54.5%), ''united bipolar segregation'' (12%), ''separated bipolar segregation'' (2.5%), and ''incomplete united bipolar segregation'' (4%). The remaining 23% could not be classified because of chromosome disorganization, but appeared to be variants of the above. It seemed clear that the predominant pattern that gave rise to tetraploids was united bipolar segregation, although certain separated bipolar segregations might also lead to the formation of tetraploids. The sequential events of meioses observed in CB-treated eggs are described. The asynchrony of meiotic events and possible mechanisms for the various types of chromosome segregation are discussed.