Fungal effector proteins: past, present and future

The pioneering research of Harold Flor on flax and the flax rust fungus culminated in his gene-for-gene hypothesis. It took nearly 50 years before the first fungal avirulence ( Avr ) gene in support of his hypothesis was cloned. Initially, fungal Avr genes were identified by reverse genetics and map-based cloning from model organisms, but, currently, the availability of many sequenced fungal genomes allows their cloning from additional fungi by a combination of comparative and functional genomics. It is believed that most Avr genes encode effectors that facilitate virulence by suppressing pathogen-associated molecular pattern-triggered immunity and induce effector-triggered immunity in plants containing cognate resistance proteins. In resistant plants, effectors are directly or indirectly recognized by cognate resistance proteins that reside either on the plasma membrane or inside the plant cell. Indirect recognition of an effector (also known as the guard model) implies that the virulence target of an effector in the host (the guardee) is guarded by the resistance protein (the guard) that senses manipulation of the guardee, leading to activation of effector-triggered immunity. In this article, we review the literature on fungal effectors and some pathogen-associated molecular patterns, including those of some fungi for which no gene-for-gene relationship has been established.     

COMPARATIVE SPERMATOLOGY OF THREE SPECIES OF DONAX (BIVALVIA) FROM SOUTH AFRICA

The fine structure of the sperm and spermatogenesis in threespecies of Donax (D. madagascariensis, D. sordidus and D. serra)are described. Although the morphology of the sperm of all speciesis very similar, each has unique features. Donax madagascariensisand D. sordidus reportedly hybridize in regions of sympatryand their spermatozoa are morphologically closer to one anotherthan to D. serra. All sperm are of the primitive type with ahead(about 2 µmu; long), mid-piece of four mitochondria andtail. The head comprises a barrel-shaped nucleus which is cappedby a small, complex acrosome. The structure of the acrosomeis typical of heterodont bivalves. During spermatogenesis thepattern of nuclear chromatin condensation is granular. Glycogenfirst appears in the cytoplasm of spermatids, and in the maturesperm is sited in the mid-piece and base of the acrosome. (Received 15 May 1989; accepted 25 June 1989)     

猪水泡病病毒VP1基因的克隆和表达

以猪水泡病病毒RNA为模板,应用反转录聚合酶链式反应(RT-PCR)技术,扩增了849bp的VP1基因,通过T-A克隆技术,将VP1基因片段克隆至pMD18-T克隆载体质粒中,构建SVDVVP1基因克隆重组质粒,进行核苷酸序列分析。然后亚克隆插入pBAD/ThioTOPO表达载体,经测序鉴定,筛选获得VP1基因正向插入、有正确读码框的阳性克隆,成功构建了猪水泡病病毒VP1基因重组表达载体。经L-Arabinose诱导表达,可稳定、高效地表达VP1蛋白抗原。SDS-PAGE结果表明,以终浓度为0.002%的L-阿拉伯醛糖进行诱导,5h后表达可达到高峰,表达蛋白为融合蛋白,质量约47.13kDa,表达产量约占菌体总蛋白的16%。Westernblotting检测表明,诱导的蛋白能与猪水泡病阳性血清发生特异性反应。融合蛋白中含有猪水泡病病毒VP1蛋白抗原,为应用该表达蛋白抗原制备SVD免疫血清学诊断试剂和新型疫苗构建奠定基础。     

Effects of invasion of fire‐free arid shrublands by a fire‐promoting invasive alien grass (Pennisetum setaceum) in South Africa

Arid shrublands in the Karoo (South Africa) seldom accumulate sufficient combustible fuel to support fire. However, as a result of invasion by an alien perennial grass (Pennisetum setaceum), they could become flammable. This paper reports on an experiment to assess the effects of fire following invasion by P. setaceum. We established 10 plots (5 × 10 m) separated by 2.5 m, and added grass fuel to five plots (5 and 10 tons ha^?1 to alternate halves of the plot) leaving the remaining five plots as interspersed controls. Plots with fuel added were burnt, and fire behaviour was measured during the burns. Rates of fire spread were generally low (0.01–0.07 m s^?1) and did not differ significantly between burn treatments. Mean fireline intensities were higher in the high compared with the low fuel treatments (894 and 427 kW m^?1, respectively). We recorded plant species and their cover before and after burning on each of the plots. After 15 months of follow‐up monitoring in the burn plots, only two species, the dwarf shrub (Tripteris sinuata) and the perennial herb (Gazania krebsiana) resprouted. Most individuals of other species were killed and did not reseed during the 15‐month study. The mass of added fuel load (high or low) did not influence vegetation recovery rates after fire. Should future invasions by P. setaceum lead to similar fuel loads in these shrublands, inevitable fires could change the vegetation and may favour spread of the flammable grass. Our results have important implications for predicting the effects of invasive alien plants (especially grasses) on fire‐free ecosystems elsewhere. The predicted impacts of fire may alter species composition, ultimately affecting core natural resources that support the Karoo economy.     

Arsenic release and attenuation in low organic carbon aquifer sediments from West Bengal

High arsenic concentrations in groundwater are causing a humanitarian disaster in Southeast Asia. It is generally accepted that microbial activities play a critical role in the mobilization of arsenic from the sediments, with metal‐reducing bacteria stimulated by organic carbon implicated. However, the detailed mechanisms underpinning these processes remain poorly understood. Of particular importance is the nature of the organic carbon driving the reduction of sorbed As(V) to the more mobile As(III), and the interplay between iron and sulphide minerals that can potentially immobilize both oxidation states of arsenic. Using a multidisciplinary approach, we identified the critical factors leading to arsenic release from West Bengal sediments. The results show that a cascade of redox processes was supported in the absence of high loadings of labile organic matter. Arsenic release was associated with As(V) and Fe(III) reduction, while the removal of arsenic was concomitant with sulphate reduction. The microbial populations potentially catalysing arsenic and sulphate reduction were identified by targeting the genes arrA and dsrB, and the total bacterial and archaeal communities by 16S rRNA gene analysis. Results suggest that very low concentrations of organic matter are able to support microbial arsenic mobilization via metal reduction, and subsequent arsenic mitigation through sulphate reduction. It may therefore be possible to enhance sulphate reduction through subtle manipulations to the carbon loading in such aquifers, to minimize the concentrations of arsenic in groundwaters.     

OVARIAN DEVELOPMENT AND OOSORPTION IN AUTOGENOUS FEMALES OF THE BLOWFLY,LUCILIA CUPRINA (WIED.)

Nearly 90% of nulliparous females of a laboratory selected autogenous strain of the sheep blowly, Lucilia cuprina, developed mature oocytes when fed only sucrose and water. The mean number of oocytes matured by those females which reached maturity was about 100. In the few females which did not reach maturity, ovarian development ceased early in vitellogenesis. Females had approximately 250 ovarioles, and when given ad libitum access to sheep liver, matured virtually all their primary oocytes. Flies given limited amounts of protein-rich material matured more oocytes than individuals given only sucrose and water; the apparent efficiency of conversion of ingested protein to ovary protein was of the order of 50%.

In females fed only sucrose and water or limited amounts of protein-rich material, the reduction in the number of oocytes resulted from oosorption initiated early in vitellogenesis. Oosorption was initiated earlier in females fed only sucrose and water, which were destined to resorb of the order of 150 oocytes, than in females which resorbed about 65 oocytes after having consumed a limited amount of protein-rich material.

Mating increased slightly both the proportion of females expressing autogeny and the number of oocytes matured.     

Identification of hypervariable single locus minisatellite DNA probes in the blue tit Parus caeruleus

We report the isolation of a set of hypervariable minisatellite DNA sequences from a blue tit Parus caeruleus genomic DNA library. In our strategy, we cloned a minisatellite-rich DNA fraction into a charomid vector. The resulting cosmid library was screened with the two minisatellite DNA probes 33.6 and 33.15 for recombinants containing a minisatellite DNA insert. A total of 233 positive clones were isolated. Of 37 clones that have been analysed, nine gave polymorphic signals and can be used as single locus probes (SLPs). Four of the SLPs were investigated in more detail. The number of alleles, the heterozygosity and the mutation rate were estimated. Linkage analysis revealed that two of these loci were linked. The SLPs are of value to studies of the mating system and reproductive success in the blue tit, and may also be useful in population genetic studies.