Virulent strains of Toxoplasma gondii comprise a single clonal lineage.

The protozoan Toxoplasma gondii is a prevalent parasite in wild and domestic animals worldwide, being transmitted through the food chain by carnivorous feeding and scavenging. Toxoplasma normally divides asexually to yield a haploid form that can infect virtually any vertebrate but it also has a well defined sexual cycle that occurs exclusively in cats. Toxoplasma has become important as an often fatal opportunistic pathogen in patients with AIDS, although the 15-85% of adult human populations that are chronically infected with T. gondii are typically asymptomatic. Infections in immunocompromised hosts have variable outcomes. For example, only 30 to 50% of AIDS patients that are chronically infected with the parasite develop toxoplasmic encephalitis and only about half of acute maternal infections result in congenital disease of the newborn. T. gondii strains differ in their virulence in animals, but the extent to which different strains are related has not been determined. Here we analyse 28 strains from a variety of hosts on five continents and find that the ten virulent strains have an essentially identical genotype, whereas the nonvirulent strains are moderately polymorphic. These data strongly suggest that virulent strains of T. gondii originated from a single lineage which has remained genetically homogeneous despite being globally widespread, and despite the ability of this organism to reproduce sexually.     

An hour-glass magnetic spectrum in an insulating, hole-doped antiferromagnet

Superconductivity in layered copper oxide compounds emerges when charge carriers are added to antiferromagnetically ordered CuO(2) layers. The carriers destroy the antiferromagnetic order, but strong spin fluctuations persist throughout the superconducting phase and are intimately linked to superconductivity. Neutron scattering measurements of spin fluctuations in hole-doped copper oxides have revealed an unusual 'hour-glass' feature in the momentum-resolved magnetic spectrum that is present in a wide range of superconducting and non-superconducting materials. There is no widely accepted explanation for this feature. One possibility is that it derives from a pattern of alternating spin and charge stripes, and this idea is supported by measurements on stripe-ordered La(1.875)Ba(0.125)CuO(4) (ref. 15). Many copper oxides without stripe order, however, also exhibit an hour-glass spectrum. Here we report the observation of an hour-glass magnetic spectrum in a hole-doped antiferromagnet from outside the family of superconducting copper oxides. Our system has stripe correlations and is an insulator, which means that its magnetic dynamics can conclusively be ascribed to stripes. The results provide compelling evidence that the hour-glass spectrum in the copper oxide superconductors arises from fluctuating stripes.     

Patterning of sodium ions and the control of electrons in sodium cobaltate

Sodium cobaltate (Na(x)CoO2) has emerged as a material of exceptional scientific interest due to the potential for thermoelectric applications, and because the strong interplay between the magnetic and superconducting properties has led to close comparisons with the physics of the superconducting copper oxides. The density x of the sodium in the intercalation layers can be altered electrochemically, directly changing the number of conduction electrons on the triangular Co layers. Recent electron diffraction measurements reveal a kaleidoscope of Na+ ion patterns as a function of concentration. Here we use single-crystal neutron diffraction supported by numerical simulations to determine the long-range three-dimensional superstructures of these ions. We show that the sodium ordering and its associated distortion field are governed by pure electrostatics, and that the organizational principle is the stabilization of charge droplets that order long range at some simple fractional fillings. Our results provide a good starting point to understand the electronic properties in terms of a Hubbard hamiltonian that takes into account the electrostatic potential from the Na superstructures. The resulting depth of potential wells in the Co layer is greater than the single-particle hopping kinetic energy and as a consequence, holes preferentially occupy the lowest potential regions. Thus we conclude that the Na+ ion patterning has a decisive role in the transport and magnetic properties.     

多项式微分系统的不变直线的不同斜率的最大个数

本文证明了公式β（ｎ）＝σ（ｎ－１）＋１其中α（ｎ－１）是ｎ－１次多项式微分系统的不为直线的最多条数，βｎ）是ｎ次多项式微分系统的不变直线的不同斜率的最大个数。这里假设所讨论的多项式系统只有限条不变直线。