抗原变异可使得多种致病微生物易于逃避宿主免疫应答。表达在感染红细胞表面的恶性疟原虫红细胞表面蛋白1（PfPMP1）与感染红细胞、内皮细胞、树突状细胞以及胎盘的单个或多个受体作用，在黏附及免疫逃避中起关键的作用。每个单倍体基因组var基因家族编码约60种成员，通过启动转录不同的var基因变异体为抗原变异提供了分子基础。

疟原虫var基因转换速率变化导致抗原变异[英]／Paul H, Robert P, Christodoulou Z, et al//Proc Natl Acad Sci U S A

If a stable layer of dense melt formed at the base of the mantle early in Earth's history, it would have undergone slow fractional crystallization and could provide an unsampled geochemical reservoir hosting a variety of incompatible geochemical species (most notably the missing budget of heat producing elements). The distribution of geochemical species in the Earth’s interior is largely controlled by fractional melting and crystallization processes that are intimately linked to the thermal state and evolution of the mantle. The existence of patches of dense partial melt at the base of the Earth’s mantle1, together with estimates of melting temperatures for deep mantle phases2 and the amount of cooling of the underlying core required to maintain a geodynamo throughout much of the Earth’s history3, suggest that more extensive deep melting occurred in the past. Here we show that a stable layer of dense melt formed at the base of the mantle early in the Earth’s history would have undergone slow fractional crystallization, and would be an ideal candidate for an unsampled geochemical reservoir hosting a variety of incompatible species (most notably the missing budget of heat-producing elements) for an initial basal magma ocean thickness of about 1,000 km. Differences in 142Nd/144Nd ratios between chondrites and terrestrial rocks4 can be explained by fractional crystallization with a decay timescale of the order of 1 Gyr. These combined constraints yield thermal evolution models in which radiogenic heat production and latent heat exchange prevent early cooling of the core and possibly delay the onset of the geodynamo to 3.4–4 Gyr ago5.

A crystallizing dense magma ocean at the base of the Earth’s mantle

Plumes of hot upwelling rock rooted in the deep mantle have been proposed as a possible origin of hotspot volcanoes, but this idea is the subject of vigorous debate. On the basis of geodynamic computations, plumes of purely thermal origin should comprise thin tails, only several hundred kilometres wide, and be difficult to detect using standard seismic tomography techniques. Here we describe the use of a whole-mantle seismic imaging technique--combining accurate wavefield computations with information contained in whole seismic waveforms--that reveals the presence of broad (not thin), quasi-vertical conduits beneath many prominent hotspots. These conduits extend from the core-mantle boundary to about 1,000 kilometres below Earth's surface, where some are deflected horizontally, as though entrained into more vigorous upper-mantle circulation. At the base of the mantle, these conduits are rooted in patches of greatly reduced shear velocity that, in the case of Hawaii, Iceland and Samoa, correspond to the locations of known large ultralow-velocity zones. This correspondence clearly establishes a continuous connection between such zones and mantle plumes. We also show that the imaged conduits are robustly broader than classical thermal plume tails, suggesting that they are long-lived, and may have a thermochemical origin. Their vertical orientation suggests very sluggish background circulation below depths of 1,000 kilometres. Our results should provide constraints on studies of viscosity layering of Earth's mantle and guide further research into thermochemical convection.

Broad plumes rooted at the base of the Earth's mantle beneath major hotspots

We propose a theoretically and practically improved density-based, hierarchical clustering method, providing a clustering hierarchy from which a simplified tree of significant clusters can be constructed. For obtaining a “flat” partition consisting of only the most significant clusters (possibly corresponding to different density thresholds), we propose a novel cluster stability measure, formalize the problem of maximizing the overall stability of selected clusters, and formulate an algorithm that computes an optimal solution to this problem. We demonstrate that our approach outperforms the current, state-of-the-art, density-based clustering methods on a wide variety of real world data.

Density-based clustering based on hierarchical density estimates

Emerging evidence for threefold higher heat flow across the core–mantle boundary prompts a re-evaluation of the role of thermal plumes in geodynamics and the thermal history of the Earth's core and lower mantle.

Core–mantle boundary heat flow

Temperature gradients in a low-shear-velocity province in the lowermost mantle (D'' region) beneath the central Pacific Ocean were inferred from the observation of a rapid S-wave velocity increase overlying a rapid decrease. These paired seismic discontinuities are attributed to a phase change from perovskite to post-perovskite and then back to perovskite as the temperature increases with depth. Iron enrichment could explain the occurrence of post-perovskite several hundred kilometers above the core-mantle boundary in this warm, chemically distinct province. The double phase-boundary crossing directly constrains the lowermost mantle temperature gradients. Assuming a standard but unconstrained choice of thermal conductivity, the regional core-mantle boundary heat flux (approximately 85 +/- 25 milliwatts per square meter), comparable to the average at Earth's surface, was estimated, along with a lower bound on global core-mantle boundary heat flow in the range of 13 +/- 4 terawatts. Mapped velocity-contrast variations indicate that the lens of post-perovskite minerals thins and vanishes over 1000 kilometers laterally toward the margin of the chemical distinct region as a result of a approximately 500-kelvin temperature increase.

https://science.sciencemag.org/content/sci/314/5803/1272.full.pdf

A Post-Perovskite Lens and D'' Heat Flux Beneath the Central Pacific

[1] Anomalous boundary layer structure at the core-mantle boundary (CMB) is investigated using a global set of broadband SKS and SPdKS waves from permanent and portable broadband seismometer arrays. SPdKS is an SKS wave that intersects the CMB at the critical angle for ScP, thus initiating a diffracted P wave (Pdiff) along the CMB at the core entry and exit locations. The waveshape and timing of SPdKS data are analyzed relative to SKS, with some SPdKS data showing significant delays and broadening. Broadband data from several hundred deep focus earthquakes were analyzed; retaining data with simple sources and high signal-to-noise ratios resulted in 53 high-quality earthquakes. For each earthquake an empirical source was constructed by stacking pre-SPdKS distance range SKS pulses (∼90°–100°). These were utilized in our synthetic modeling process, whereby reflectivity synthetic seismograms are produced for three classes of models: (1) mantle-side ultralow-velocity zones (UVLZ), (2) underside CMB core rigidity zones, and (3) core-mantle transition zones. For ULVZ structures, ratios of P-to-S velocity reductions of 1:1 and 1:3 are explored, where 1:3 is appropriate for a partial melt origin of ULVZ. Over 330 unique CMB boundary layer models have been constructed and tested, corroborating previous work suggesting strong trade-offs between the three model spaces. We produce maps of inferred boundary layer structure from the global data and find evidence for extremely fine-scale heterogeneity where our wave path sampling is the densest. While uncertainties are present relating to the source versus receiver sides of the SPdKS wave path geometry, our data are consistent with the hypothesis that ULVZ presence (or absence) correlates with reduced (or average) heterogeneity in the overlying mantle.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2004JB003010

Inferences on ultralow-velocity zone structure from a global analysis of SPdKS waves

Geographic correlation between hot spots and deep mantle lateral shear-wave velocity gradients

Mega ultra low velocity zone and mantle flow

SUMMARY We use an axisymmetric, spherical Earth finite difference algorithm to model SH-wave propagation through cross-sections of laterally varying lower mantle models beneath the Cocos Plate derived from recent data analyses. Synthetic seismograms with dominant periods as short as 4 s are computed for several models: (1) a D �� reflector 264 km above the core‐mantle boundary with laterally varying S-wave velocity increases of 0.9‐2.6 per cent, based on localized structures from a 1-D double-array stacking method; (2) an undulating D �� reflector with large topography and uniform velocity increase obtained using a 3-D migration method and (3) cross-sections through the 3-D mantle S-wave velocity tomography model TXBW. We apply double-array stacking to assess model predictions of data. Of the models explored, the S-wave tomography model TXBW displays the best overall agreement with data. The undulating reflector produces a double Scd arrival that may be useful in future studies for distinguishing between D �� volumetric heterogeneity and D �� discontinuity topography. Synthetics for the laterally varying models show waveform variability not observed in 1-D model predictions. It is challenging to predict 3-D structure based on localized 1-D models when lateral structural variations are on the order of a few wavelengths of the energy used, particularly for the grazing geometry of our data. Iterative approaches of computing synthetic seismograms and adjusting model characteristics by considering path integral effects are necessary to accurately model fine-scale D �� structure.

/pdf/seismic-imaging-of-the-laterally-varying-d-region-beneath-1u7pcp7rln.pdf

Michael S. Thorne

Papers

A Post-Perovskite Lens and D'' Heat Flux Beneath the Central Pacific

Inferences on ultralow-velocity zone structure from a global analysis of SPdKS waves

Geographic correlation between hot spots and deep mantle lateral shear-wave velocity gradients

Mega ultra low velocity zone and mantle flow

Seismic imaging of the laterally varying D″ region beneath the Cocos Plate