Role of host specificity in the speciation of <Emphasis Type="Italic">Ascochyta</Emphasis> pathogens of cool season food legumes

Ascochyta/legume interactions are attractive systems for addressing evolutionary questions about the role of host specificity in fungal speciation because many wild and cultivated cool season food legumes are infected by Ascochyta spp. and most of these fungi have described teleomorphs (Didymella spp.) that can be induced in the laboratory. Recent multilocus phylogenetic analyses of a worldwide sample of Ascochyta fungi causing ascochyta blights of chickpea (Cicer arietinum), faba bean (Vicia faba), lentil (Lens culinaris), and pea (Pisum sativum) have revealed that fungi causing disease on each host formed a monophyletic group. Host inoculations of these fungi demonstrated that they were host-specific, causing disease only on the host species from which they were isolated. In contrast to the strict association between monophyletic group and host observed for pathogens of cultivated legumes, Ascochyta fungi causing disease on wild bigflower vetch (Vicia grandiflora) were polyphyletic. Genetic crosses between several pairs of closely related, host-specific, and phylogenetically distinct Ascochyta fungi were fully sexually compatible. Progeny from these crosses had normal cultural morphology and segregation of molecular markers indicating a lack of intrinsic, post-zygotic mating barriers between the parental taxa. However, when progeny from a cross between a faba bean-adapted isolate (A. fabae) and a pea-adapted isolate (A. pisi) were assessed for their pathogenicity to the parental hosts, almost all progeny were non-pathogenic to either faba bean or pea. These results suggest that although these fungi have retained the ability to mate and produce progeny with normal saprophytic fitness, progeny are severely compromised in parasitic fitness. The host specificity of these fungi, coupled with the inability of hybrid progeny to colonize and reproduce on a host, may constitute strong extrinsic, pre-zygotic and post-zygotic mating barriers in these fungi and promote the genetic isolation and speciation of host-specific taxa. A phylogeny of the host plants is also being developed, and with more extensive sampling of pathogens and hosts from sympatric populations in the centre of origin, the hypothesis of cospeciation of pathogens and hosts will be tested. The objectives of this review are: (1) to summarize recent phylogenetic, host specificity and speciation studies of Ascochyta fungi, and (2) to suggest how current and future research using these pathosystems may lead to a better understanding of the role of host specificity in the speciation of plant-pathogenic fungi and the cospeciation of pathogens and their hosts.     

Research progress in the interactions of fungal pathogens and insect pests during host plant colonization

Journal of Plant Diseases and Protection - Microbial pathogens and insect pests frequently share the same host plant, and both can cause severe damage. When surviving on the same host, the fungal...     

Evolution of sibling fungal plant pathogens in relation to host specialization

ABSTRACT Sibling plant pathogens can be grouped according to their host rangesthe following groups: group 1, sibling pathogens with nonoverlapping host ranges; group 2, sibling pathogens with both overlapping and nonoverlapping host ranges; and group 3, sibling pathogens with overlapping host ranges. Using the adaptive dynamics methodology, we investigated the evolution of sibling pathogens in relation to host specialization for groups 1 to 3. In particular, we focused on the role of multiple host niches and a trade-off in infectivity of pathogens to these hosts on the evolutionary outcome. We have shown that this ecological mechanism can explain only the evolution of sibling pathogens in group 1 and that other ecological and epidemiological mechanisms must be responsible for the evolution of sibling pathogens in the other two groups.     

Importance of pH in assessing the potential for nitrosocarbamate formation in the stomach

Formation of the nitroso derivatives of ¹⁴C-labeled carbaryl, carbofuran, and p-chlorophenyl methylcarbamate (PCMC), and their subsequent stabilities were investigated using low levels of carbamate (0.5 μmol) and aqueous HCl concentrations encompassing those considered maximum for the gastric contents of humans (pH 1–2) and of rats (pH 3–4). Reacting the carbamates with excess sodium nitrite for 10 min at 37°C in a pH 1.0 HCl solution gave nitrosocarbamate yields of 42 to 64%, while only trace amounts were formed at pH 2 and above. The nitrosocarbamates were most stable at pH 3–5 with half-lives ranging from 114 to 470 min. Stability of all three nitroso derivatives was considerably less at pH 1.5 ($\text{t}\text{1}\text{2}$, 25–34 min), but at pH 7 the stability varied: nitroso-PCMC $\text{t}\text{1}\text{2}$, 6 min; nitrosocarbofuran $\text{t}\text{1}\text{2}$, 70 min; nitrosocarbaryl $\text{t}\text{1}\text{2}$, 139 min. Denitrosation to the parent carbamate was the predominant degradation pathway at pH 1.5, but at pH 3–7 degradation was primarily by hydrolysis of the carbamate ester linkage. Each of the nitrosocarbamates was directly mutagenic in the S. typhimurium assay system. Since the data show that nitrosation of residue levels of carbamate pesticides occurs readily at pH 1 but not at pH 2 and above, it is critical that gastric contents of any animal model used for assessing nitrosocarbamate formation have a pH approaching 1 as may occur in the human stomach.     

Determination of cycle cut off in real-time PCR for the detection of regulated plant pathogens

Given the high importance of an accurate detection for quarantine plant pathogens, and the risk of false positive or false negative results, a detection method must be correctly validated. A statistical procedure has been developed to determine the cycle cut off and the corresponding limit of detection in real-time PCR, considering a risk of false positive results of 1% and a probability of detection of 95%. The procedure presented in this paper is easy to develop and can be applied to any plant pathogen to be detected by real-time PCR.     

The significance of the zoosporangial stage in the life cycle of the plasmodiophorales

Samenvatting Bij de Plasmodiophorales onderscheidt men het zoösporangium- en het rustsporenstadium. Het zoösporangiumstadium wordt gekenmerkt door de aanwezigheid van zoösporangia, vaak in wortelharen en andere schorsepidermiscellen; het rustsporenstadium door het tot ontwikkeling komen van rustsporen, dikwijls in gehypertrofieerde delen van de waardplant. Sinds de ontdekking van het zoösporangiumstadium is nimmer zekerheid verkregen over de vraag of zoösporen uit de zoösporangia bij herinfectie wederom zoösporangia kunnen geven. Het alternatief zou zijn dat de zoösporen uit de zoösporangia de levenscyclus vervolgen en tot de ontwikkeling van rustsporen leiden.Een infectieproef metSpongospora subterranea bij tomateplanten heeft nu uitgewezen dat de eerste veronderstelling juist is; zoösporen uit de zoösporangia geven bij herinfectie inderdaad weer zoösporangia. Het rustsporenstadium komt waarschijnlijk tot ontwikkeling na herinfectie door een zygote, welke ontstaat indien twee als gameten fungerende zoösporen versmelten. Deze opbouw van de levenscyclus (fig. 1) geeft de schimmel de mogelijkheid tot een sterke vegetatieve vermeerdering in het zoösporangiumstadium.     

Evolution of a pathogen population in host mixtures: simple race–complex race competition

The evolution of the genetic structure of a pathogen population was studied in a varietal mixture with an epidemic simulator based on the model EPIMUL. The pathogen population was composed of simple races able to develop on only one component of the mixture and a complex race which developed on all mixture components. The effects on the simple race–complex race competition of a cost of virulence, of density dependence and of differential adaptation were studied. The selection for simple or complex races in the pathogen population did not depend on initial race frequencies. For a given multiplication rate, complex race frequency increased faster when the spore dispersal gradient was shallow, when distribution of initial disease was generalized, when amount of initial disease was reduced and when the number of mixture components was increased. This was attributed to a better efficacy of the mixture in controlling simple races, resulting in a higher relative fitness of the complex race. For measured values of density dependence or differential adaptation effects, the complex race was at a higher frequency after a mean number of pathogen cycles between 2.5 and 5. The effect of the cost of virulence was stronger and, in certain situations, could result in selection for simple races. In the conditions of our simulations and with the effects tested, stabilization of the pathogen population in host mixtures was unlikely to occur. However, more information is needed concerning the rate at which complex races could evolve and how quickly mixture resistance could be eroded.     

棉大卷叶螟的年生活史与种群动态

经过3年田间调查及室内饲养观察,绘制了江淮棉区棉大卷叶螟年生活史图。调查结果显示,该棉区棉大卷叶螟年发生5~6代,主要以第5代老熟幼虫越冬,越冬幼虫于次年4月中下旬化蛹,棉田幼虫始见于6月中下旬,但发生量少,进入7月中下旬后虫量开始上升,8~9月上旬达到高峰。近3年的调查数据表明,该棉区棉大卷叶螟种群数量发生大,其中常规棉田3年百株平均虫量达1 656~2 324头,并对棉花造成了较大损失。转基因棉田棉大卷叶螟种群数量受到明显抑制,3年百株虫量为619~1 358头。     

RNA silencing against viruses: molecular arms race between Cucumber mosaic virus and its host

Plants have developed RNA silencing as an antiviral defense mechanism. To escape from the plant host’s defenses, viruses have countered their host’s antiviral silencing by producing RNA silencing suppressor proteins (RSSs). Although the mode of action of the majority of viral RSSs has been found to be through double-stranded RNA-binding, viruses have different strategies to counteract the host’s antiviral silencing pathways. The 2b protein of Cucumber mosaic virus, which is one of the most extensively studied viral RSSs, is reviewed here to provide insights on the molecular arms race between viruses and their host plants.     

Thermal requirements for the embryonic development and life cycle of Meloidogyne hispanica

The life cycle of a Portuguese Meloidogyne hispanica isolate on susceptible cv. Easypeel and resistant (Mi‐1.2 gene) cv. Rossol tomato plants was studied in growth chambers at constant temperatures (10–35°C). The development within the egg and hatching were compared to those of a Portuguese M. arenaria isolate. The base temperature was 10·11 and 8·31°C with 179·5 and 235·3 thermal units for M. hispanica and M. arenaria, respectively, suggesting better potential adaptation to low temperatures by M. arenaria than M. hispanica. No egg development occurred at 10 or 35°C. An increase in invasion of tomato roots by M. hispanica second‐stage juveniles (J2s) was correlated with an increase in temperature on both tomato cultivars. Tomato cv. Rossol limited M. hispanica development at 20, 25 and 30°C, but not at 35°C, indicating that these high temperatures blocked the resistance mechanism provided by the Mi‐1.2 gene. At 15°C, J2s penetrated tomato cv. Rossol roots, but failed to develop and establish feeding sites. On tomato cv. Easypeel, nematode development and reproduction occurred at 20, 25 and 30°C, but at 20°C the life cycle was 1·5 and 2·0 times longer than at 25 and 30°C, respectively. No egg production was observed at 15°C. The results of this study showed that M. hispanica is most suited to soil temperatures around 25°C. Predicted climate change might favour the spread of this nematode species into southern Europe and northwards. The thermal requirements for M. hispanica development are analysed and compared with those of M. arenaria, M. hapla, M. incognita and M. javanica.     

Studies on the life cycle of Cryptosporidium coccidia in experimentally infected chickens

Experimental infections of 7-28-day-old chickens with Cryptosporidium oocysts isolated from spontaneously infected chickens demonstrated that the endogenous development of this parasite takes place simultaneously in the organs of digestive, respiratory and excretory systems and in bursa of Fabricius. It was demonstrated for the first time that the oocysts of Cryptosporidium are shed through the respiratory tract into the beak cavity. A novel rapid and simple method has been developed for the detection of oocysts. Its principle is the rinsing of the beak cavity. This method enabled to isolate the oocysts from experimentally infected chickens and immediately use them in the dose of 6 X 10(5) for peroral infection of a 37-day-old chicken. The prepatent period was 8 days, patent period 12 days. On days 4-7 after the first detection of Cryptosporidium oocysts in chicken excrements, the oocysts were detected also by the method of beak cavity rinsing. This indicates that the oocysts released from the respiratory tract are infective. This fact is important from the epizootological viewpoint in relation with possible spreading of Cryptosporidium infections in chicken farms.     

Production of hydrogen peroxide and expression of ROS‐generating genes in peach flower petals in response to host and non‐host fungal pathogens

Reactive oxygen species (ROS) play dual roles in plant–microbe interactions in that they can either stimulate host resistance or enhance pathogen virulence. Innate resistance in peach (Prunus persica) to the brown rot fungal pathogen Monilinia fructicola is very limited, and knowledge of the mechanism of virulence is rudimentary. In this study, production of hydrogen peroxide, a major component of ROS, was determined in peach flower petals in response to M. fructicola (a host pathogen) and Penicillium digitatum (a non‐host pathogen). Monilinia fructicola was able to infect flower petals while P. digitatum was not. During the host‐specific interaction, M. fructicola induced hydrogen peroxide accumulation in flower petals. Application of exogenous antioxidants significantly reduced hydrogen peroxide accumulation as well as the incidence of brown rot disease. Application of M. fructicola spores to the surface of intact flower petals induced gene expression and increased enzyme activity of NADPH oxidase and cell wall peroxidase in host tissues, resulting in the production of hydrogen peroxide. Petals inoculated with M. fructicola exhibited high levels of protein carbonylation and lipid peroxidation. No significant response in gene expression, enzyme activity or hydrogen peroxide levels was observed in peach flower petals treated with P. digitatum. These results suggest that M. fructicola, as with other necrotrophic fungi, uses the strong oxidative response as part of a virulence mechanism.     

A study of proteases throughout the life cycle of Trichinella spiralis

In the present report we study the proteolytic activity of the excretion-secretion and crude extracts of different stages of Trichinella spiralis (Owen, 1835) Railliet, 1895, (muscle-stage larvae, adult worms before and after mating, and newborn larvae) using natural substrates (structural and hematic mammalian proteins). The analysis of the results allow us to set up a certain stage-specificity, as well as an important relationship between the protease patterns throughout the parasite life cycle and how the parasite may overcome both mechanical and humoral barriers within the host. Muscle-stage larvae present a great activity against structural proteins (collagen), while newborn larvae and adult worms degrade principally hematic proteins (hemoglobin, fibrinogen and immunoglobulin G).     

甜菜孢囊线虫在我国的寄主范围及生活史研究

甜菜孢囊线虫Heterodera schachtii是严重危害甜菜生产的主要病原物,每年造成严重的经济损失,该线虫是我国对外重要的检疫性有害生物.本研究通过对十字花科、茄科、葫芦科、锦葵科、豆科、苋科和禾本科等16种作物29个品种进行人工接种,对其主要的寄主范围和生活史进行了测定.结果 表明,甜菜孢囊线虫2龄幼虫能够侵...     

The role of primary germ tubes in the life cycle of Blumeria graminis: The primary germ tube is responsible for the suppression of resistance induction of a host plant cell

When the conidia of Blumeria graminis f. sp. hordei (Bgh) are inoculated on barley coleoptile cells they produce short germ tubes called primary germ tubes (PGTs) about 2 h after inoculation. We evaluated the positive role of a PGT in inducing accessibility of the host cell under the germ tube. When an appressorium (APP) penetrated the same cell on which a PGT was present, the ratio of haustorium formation (penetration efficiency) was significantly higher than when an APP penetrated the cell adjacent to the one on which a PGT was present. When an APP penetrated the cell laterally adjacent to the one on which a PGT was present we killed the cell under the PGT by puncturing it with a microneedle and then investigated the penetration efficiency of the cell adjacent to the dead cell. As a control we killed the cell longitudinally adjacent to the one on which a PGT was present and investigated the penetration efficiency of the laterally adjacent cell. The results showed that the penetration efficiency of the former was significantly lower than that of the latter. This suggests that some accessibility factor might transfer from a cell on which a PGT is present to a laterally adjacent cell. The existence of a conidium body but not a PGT was not effective for induced accessibility of the host cell. Moreover, when a Bgh germling was removed 6 h after inoculation and another germling was transferred to the same cell, the penetration efficiency was significantly higher than that of control. As a control, a Bgh germling was transferred to a cell on which no germling was present. These results suggest that the existence of PGT is effective for induced accessibility of a host cell when penetrated by Bgh. However, it is unclear whether or not a PGT secretes some substance(s) which suppresses the resistance induction of a host cell.     

Effect of fungicides on stages of the life cycle of Phytophthora cinnamomi

An assessment was made of the effects of several fungicides on the formation of zoosporangia, the release of zoospores from zoosporangia, zoospore motility, germ tube formation and mycelial growth, by Phytophthora cinnamomi Rands. Compounds active against mycelial growth (such as metalaxyl and furalaxyl), as well as those active against zoospores (such as captafol and zineb), could be detected by a lupin (Lupinus angustifolius L.) assay which has been developed. This assay may be useful as a screening procedure to detect new fungicides.     

中药材储藏害虫优势种药材甲形态及生活史

药材甲[Stegobium paniceum(L.)]是中药材储藏期害虫优势种,对我国储藏中药材造成极大危害。此虫卵小,椭圆形,长径296.80～380.80μm,短径224.00～313.60μm。幼虫有4个龄期,1～2龄体小,3～4龄体长迅速增加,整个幼虫期蛀食生存于药材中。蛹被蛹室,长5～10 mm,发育过程中伴随颜色变化。成虫体长2～6 mm,羽化后2～3 d交配产卵,单雌卵量(55.78±3.01)粒,寿命(22.22±1.39)d。1年发生3～4代,世代重叠明显,多以当年第4代幼虫越冬。     

The life cycle of Eimeria danailovi from ducks.

The life cycle of Eimeria danailovi Gr?fner, Graubmann et Betke in experimentally infected ducks was studied by optical microscopy. The asexual generation developed in the posterior part of jejunum and in the whole ileum. The sexual stages occurred in jejunum and ileum, and, in addition, in cecum and colon. All endogenous stages were localized in the cytoplasm of epithelial cells in the apical and basal parts of villi. Two generations of meronts were found to develop, differing from one another in the number of merozoites. The meronts of the first generation were observed 2 days post infection (DPI) and contained 10-14 (on the average 11) merozoites. The second generation of meronts, containing 12-22 (on the average 16) merozoites, developed on 3 DPI. The sexual stages were found in histological preparations on 5 and 6 DPI. They appeared in the faces of experimentally infected ducks first on 5 DPI and they were shed for three days. Oocyst sporulation at room temperature lasted 2-3 days.     

Biofilm formation and motility of Xanthomonas strains with different citrus host range

Xanthomonas citri subsp. citri (Xcc) strain A is the causal agent of citrus bacterial canker (CBC) on most Citrus spp. and close relatives. Two restricted host range strains of CBC, A^w and A*, from Florida and southwest Asia, respectively, infect Mexican lime. Several studies have linked biofilm formation by Xcc to bacterial colonization prior to and after plant ingress, but none have evaluated connections between biofilm formation and the behaviour of different strains of Xcc on citrus hosts and non‐hosts. In this study biofilm formation and swimming motility were evaluated for citrus pathogenic xanthomonads including wide and restricted host range strains of Xcc, X. alfalfae subsp. citrumelonis (Xac) (the causal agent of citrus bacterial spot) and X. campestris pv. campestris (Xc). Differential biofilm formation was observed in vitro and in planta among the Xanthomonas strains assayed. Minimal medium XVM2 increased biofilm formation, especially for those strains with a host range restricted to Mexican lime. In planta, strains produced more biofilm on leaves or fruits of their host than on non‐hosts. Scanning electron microscopy of biofilms on leaf and fruit surfaces revealed differences in structure of bacterial aggregates with respect to the strain's host range. In addition, swimming motility varied widely depending on the host range of the strain. It was concluded that biofilm formation in vitro and in planta for strains of Xcc and Xac was related to their host range, as these processes affect colonization at the early stages of the infection process.