Virulence cost in Plasmopara halstedii (sunflower downy mildew)

Virulence cost (trade-off between virulence and aggressiveness) was studied in seven Plasmopara halstedii (sunflower downy mildew) isolates of races 100, 300, 304, 314, 710, 704 and 714. The seven isolates were divided, according to their virulence and aggressiveness, into two main groups as more aggressive isolates of the 100 and 3xx races that do not overcome the sunflower differential host D3, and less aggressive isolates of 7xx races that can overcome D3. Consequently, the 100 and 3xx avirulent races had a virulence cost measured by differences in aggressiveness (from 58.3 to 78.2%) compared to 7xx virulent races carrying unnecessary virulence gene.     

Relationship between virulence and aggressiveness in Plasmopara halstedii (sunflower downy mildew)

Relationship between virulence and aggressiveness was studied in seven Plasmopara halstedii (sunflower downy mildew) pathotypes including five progeny pathotypes of races 300, 304, 314, 704 and 714 arising from two parental pathotypes of races 100 and 710. Aggressiveness criteria including percentage infection, latent period, sporulation density and reduction of hypocotyl length were analysed in one sunflower inbred line showing a high level of quantitative resistance. There were significant differences between P. halstedii pathotypes for all aggressiveness criteria. Pathogenicity of progeny pathotypes as compared with parental ones (relationship between virulence and aggressiveness) seems to be positive, negative or uncorrelated. Hypothesis explaining these cases are discussed.     

Evolution of pathogenicity in Plasmopara halstedii (sunflower downy mildew)

Comprehension of the processes of co-evolution between the pathogen and its host plant is very important, particularly in the case of obligate pathogen as Plasmopara halstedii which cannot develop only on sunflower. The influence of selection pressure exercised by qualitative resistance in sunflower plants on evolution of pathogenicity was analysed in pathogenic populations of P. halstedii. This selection pressure led a new virulence to appear in P. halstedii isolates carrying several levels of aggressiveness. It seems that the qualitative resistance selection pressure plays an important role in the evolution of this pathogen, and these changes on the level of pathogenicity may help to a better adaptation of P. halstedii in the presence of intensive use of qualitative resistance.     

Relationship between aggressiveness and zoosporangia viability in Plasmopara halstedii (sunflower downy mildew)

Relationship between aggressiveness and zoosporangia viability was studied in seven Plasmopara halstedii (sunflower downy mildew) isolates of races 100, 300, 304, 314, 710, 704 and 714. Aggressiveness criteria including latent period and sporulation density were analysed on sunflower inbred line showing a high level of quantitative resistance. There were significant differences between pathogen isolates for the two aggressiveness criteria. Viability analyses were performed on oval and spheric zoosporangia. The number of zoospores released from oval zoosporangia was significantly higher than those released from spheric ones. The oval zoosporangia for more aggressive isolates of races 100 and 3xx produced more zoospores than the oval ones for less aggressive isolates of races 7xx. There was a significant correlation between aggressiveness criteria and the number of zoospores released from oval zoosporangia and vice versa for zoospores released from spheric ones. It is concluded that the relationship between aggressiveness and oval zoosporangia viability may be established in P. halstedii.     

Aggressiveness variation in Plasmopara halstedii and its alternation with non-race-specific resistance in sunflower

Aggressiveness variation and its alternation with non-race specific resistance in sunflower were studied in 19 Plasmopara halstedii isolates belonging to several races. Regarding aggressiveness criteria, percentage infection, latent period, sporulation density and dwarfing, on two sunflower inbred lines showing different levels of non-race specific resistance resistance FU and BT, there were significant differences in aggressiveness for P. halstedii isolates. The index of aggressiveness varied between 9.4 and 31.4. The inbred line BT, rather susceptible in the field, showed a higher percentage infection, a higher sporulation density, a shorter latent period and less reduced hypocotyl length than inbred line FU, which showed a greater resistance in the field. Percentage infection on FU was 1.4% less than BT, latent period on BT was 12.4% less than FU, sporulation density on FU was 22.3% less than BT and reduced hypocotyl length on BT was 15.3% less than FU. Consequently, it seems that the criteria as latent period, sporulation density and reduction of hypocotyl length could be used to measure non-race specific resistance in sunflower to P. halstedii under controlled conditions.     

Variation in two components of pathogenicity and its relationship with genetic diversity in Plasmopara halstedii (sunflower downy mildew)

The specificity of the two components of pathogenicity: virulence and aggressiveness and its relationship with genetic variability were analysed in a local Plasmopara halstedii (sunflower downy mildew) population. Pathogenic and molecular analyses were carried out on seven isolates including five progeny isolates of five races arising from two parental races 100 and 710. P. halstedii isolates showed significant differences for all aggressiveness criteria and important genetic variations. Three cases of relationship between virulence and aggressiveness for progeny isolates as compared with parental ones were found as positive, negative or uncorrelated. For solving the specificity of these cases, relationship between the two components of pathogenicity among the isolates of three different races localised in the same genetic clade was positive. The hypothesis explaining these cases is discussed.     

Pathogenicity and its alternation with both morphological and genetic variation in Plasmopara halstedii (sunflower downy mildew)

Morphological, pathogenic and genetic variation was studied in seven Plasmopara halstedii (sunflower downy mildew) isolates of several races using five singlezoosporangium isolates per pathogen isolate. Aggressiveness criteria were analysed in one sunflower inbred line showing a high level of quantitative resistance. Genetic relationships were detected between the single zoosporangium isolates using 12 expressed sequence tags (EST)-derived markers. Analysis of the five single zoosporangium isolates for P. halstedii isolates showed variability within pathogen isolates for all aggressiveness criteria, but not for all pathogen isolates. Isolates of races 100 and 3xx were characterised with shorter latent period and higher sporulation density than the isolate of races 7xx. All pathogen isolates showed high percentage infection values and caused a large reduction in seedling size except for one isolate involved in dwarfing. There was no relation between zoosporangia form or size and race virulence profiles or aggressiveness criteria. There was no intra-genetic variation for all pathogen isolates, but it was observed an important genetic variation between single zoosporangium isolates of all races. No correlation was detected between pathogenicity traits and EST genotypes.     

Variation in form and size of Plasmopara halstedii (sunflower downy mildew) zoosporangia

Zoosporangia form and size were studied on a collection of 94 strains of Plasmopara halstedii (sunflower downy mildew). Both oval and round forms were present in all strains analysed. The proportion of two forms varied significantly according to strain and plant age but more especially to host plant genotype. Whatever the strain or host genotype, oval zoosporangia were larger than round ones, but there was no relation between the proportion of the oval form and mean zoosporangia size. There was no relation between zoosporangia form or size and race virulence profiles or aggressiveness criteria, with the possible exception of zoosporangia size and sporulation density. It is concluded that, for this obligate parasite, although form and size of zoosporangia depend on pathogen strain, these characters also vary according to growth conditions of Plasmopara halstedii, in particular to the genotype of the plant host.     

The genetics of resistance to five races of downy mildew (Plasmopara halstedii) in sunflower (Helianthus annuus L.)

 These studies were undertaken to determine whether downy mildew resistance genes in sunflower were independent as first reported, or linked as suggested by more recent hypotheses. The segregations for downy mildew reaction of 111 F₃ progenies from a cross between a susceptible line and a line with Pl2 were used to locate this gene on the sunflower consensus RFLP linkage map. It was shown that Pl2 was linked to the same RFLP markers on linkage group 1 as Pl1 and Pl6, mapped earlier, and at a very similar distance. The F₃ progenies showed exactly the same segregation patterns when tested with race 1 and race D. One hundred and fifty four progenies from a cross between a susceptible line and HA335, containing Pl6 (considered as giving resistance to all Plasmopara halstedii races), were tested with the five French downy mildew races, 1, A, B, C and D. Two progenies were observed to show segregation for races 1 and D, while appearing homozygous-resistant to races A , B and C. Tests on F₄ progenies confirmed this separation of resistances with fixation of susceptibility to races 1 and D and resistance to races A, B and C. It is concluded that the Pl6 gene is not a “strong” gene, giving resistance to all downy mildew races, but rather a cluster of genes, each providing resistance to one, or a few, downy mildew races. The genes giving resistance to races 1 and D, on one hand, and to races A, B and C, on the other hand, must be very closely linked, with about 0.6 cM between the two groups. Received: 23 December 1996 / Accepted: 18 April 1997     

Identification of a second linkage group carrying genes controlling resistance to downy mildew (Plasmopara halstedii) in sunflower (Helianthus annuus L.)

A sunflower line, XRQ, carrying the gene Pl5, which gives resistance to all French downy mildew races shows cotyledon-limited sporulation in seedling immersion tests; consequently, segregations in crosses with other downy mildew resistance sources were tested both by this method and by a secondary infection on leaves. Pl5 was found to segregate independently of Pl7 (HA338) but to be closely linked, or allelic, with Pl8 (RHA340). F₃ and F₄ progenies from a cross with a line containing Pl2 showed that Pl5 carries resistance to race 100 which segregates independently of Pl2. The Pl5 gene was mapped on linkage group 6 of the Cartisol RFLP map, linked to two RFLP markers, ten AFLP markers and the restorer gene Rf1. Tests with downy mildew race 330 distinguished Pl5 and Pl8, the first being susceptible, the second resistant, whereas both these genes were active against race 304 to which Pl6 (HA335) and Pl7 gave susceptibility. It is concluded that Pl5 and Pl8 are closely linked on linkage group 6 and form a separate resistance gene group from Pl6/Pl7 on linkage group 1. The origins of these groups of downy mildew resistance genes and their use in breeding are discussed. Received: 10 November 2000 / Accepted: 8 February 2001     

A QTL analysis of sunflower partial resistance to downy mildew ( Plasmopara halstedii) and black stem ( Phoma macdonaldii) by the use of recombinant inbred lines (RILs)

Partial resistance to downy mildew (Plasmopara halstedii) and to black stem (Phoma macdonaldii) in sunflower were investigated under natural field infection and a controlled growth chamber respectively. Genetic control for resistance to the diseases was determined in recombinant inbred lines (RILs) and their two parents, ’PAC-2’ and ’RHA-266.’ The experiments were undertaken in a randomized complete block design with two replications, in a field severely infected by downy mildew and in a controlled growth chamber with plants inoculated with an agressive French isolate of P. macdonaldii. Each replication consisted of three rows, 4.6-m long, giving 48 plants per RIL or parent in the field and 15 plants in the growth chamber. Genetic variability was observed among the RILs for resistance to both diseases. When 10% of the selected RILs were compared with the mean of the two parents genetic gain was significant for partial resistance to the diseases. Four putative QTLs for resistance to downy mildew on linkage groups 1, 9 and 17 were detected using composite interval mapping. The QTLs explained 54.9% of the total phenotypic variance. Major QTLs (dmr1–1 and dmr1–2) for resistance were found on linkage group 1 with up to 31% of the phenotypic variability explained by two peaks. QTL analysis of resistance to black stem showed seven QTLs on linkage groups 3, 6, 8, 9, 11, 15 and 17. The detected QTLs together explain 92% of the phenotypic variation of the trait. Crosses between RILs contrasted for their resistance to downy mildew and black stem, and exhibiting molecular polymorphism in detected QTLs, will be made in order to focus more-precisely on the genomic region of interest. Received: 28 February 2001 / Accepted: 14 June 2001     

Host specificity,pathogenicity and the presence of virulence genes in Iranian strains of Pseudomonas syringae pv. syringae from different hosts

Pseudomonas syringae pv. syringae (Pss) strains were isolated from almond, apricot, peach, pear, sweet cheery and wheat in Kohgiluye and Boyer-Ahmad, Kordestan, Fras and Chaharmahal and Bakhtiari provinces of Iran. The strains were examined for host specificity, the presence of virulence genes and pathogenicity on different hosts. After inoculation of isolates, in compatible reactions bacterial populations increased within six days of inoculation and final cell numbers increased several-fold over initial inoculum levels, but in incompatible reactions, bacterial populations declined within four days of inoculation. Almond, sweet cherry and wheat isolates induced progressive necrotic symptoms on almond leaves and stems. Apricot, peach and sweet cherry isolates induced necrotic lesions when inoculated on apricot leaves. On pear leaves and stems, only the pear isolate incited pathogenic reaction and isolates from other hosts did not. The syrB gene was detected in all of the tested isolates. Almond and pear isolates did not have the syrD gene. The sypA gene was detected in the almond, peach, pear and sweet cherry isolates while the sypB gene was detected in the apricot, peach, sweet cherry and wheat isolates. Almond, apricot, pear and wheat isolates gave negative results for the detection of nit gene. The gene Ach, was detected only in the peach isolate and gene hrmA, was detected only in the wheat isolate. This study indicates that host specificity exists among different Pss strains, and genes responsible for syringomycin and syringopeptin production contribute to the virulence of Pss strains.     

Cheating, trade-offs and the evolution of aggressiveness in a natural pathogen population

The evolutionary dynamics of pathogens are critically important for disease outcomes, prevalence and emergence. In this study we investigate ecological conditions that may promote the long-term maintenance of virulence polymorphisms in pathogen populations. Recent theory predicts that evolution towards increased virulence can be reversed if less-aggressive social 'cheats' exploit more aggressive 'cooperator' pathogens. However, there is no evidence that social exploitation operates within natural pathogen populations. We show that for the bacterium Pseudomonas syringae, major polymorphisms for pathogenicity are maintained at unexpectedly high frequencies in populations infecting the host Arabidopsis thaliana. Experiments reveal that less-aggressive strains substantially increase their growth potential in mixed infections and have a fitness advantage in non-host environments. These results suggest that niche differentiation can contribute to the maintenance of virulence polymorphisms, and that both within-host and between-host growth rates modulate cheating and cooperation in P. syringae populations.     

Gene expression varies within and between enzootic and epizootic lineages of Batrachochytrium dendrobatidis (Bd) in the Americas

While much research focus is paid to hypervirulent fungal lineages during emerging infectious disease outbreaks, examining enzootic pathogen isolates can be equally fruitful in delineating infection dynamics and determining pathogenesis. The fungal pathogen of amphibians, Batrachochytrium dendrobatidis (Bd), exhibits markedly different patterns of disease in natural populations, where it has caused massive amphibian declines in some regions, yet persists enzootically in others. Here we compare in vitro gene expression profiles of a panel of Bd isolates representing both the enzootic Bd-Brazil lineage, and the more recently diverged, panzootic lineage, Bd-GPL. We document significantly different lineage-specific and intralineage gene expression patterns, with Bd-Brazil upregulating genes with aspartic-type peptidase activity, and Bd-GPL upregulating CBM18 chitin-binding genes, among others. We also find pronounced intralineage variation in membrane integrity and transmembrane transport ability within our Bd-GPL isolates. Finally, we highlight unexpectedly divergent expression profiles in sympatric panzootic isolates, underscoring microgeographic functional variation in a largely clonal lineage. This variation in gene expression likely plays an important role in the relative pathogenesis and host range of Bd-Brazil and Bd-GPL isolates. Together, our results demonstrate that functional genomics approaches can provide information relevant to studies of virulence evolution within the Bd clade.     

稻褐飞虱致害性遗传与性别的关联性研究

以单雌蜜露排泄量和体重增量作为致害性指标,研究了稻褐飞虱Nilaparvata lugens (Stål)的致害性遗传与性别的关联性。褐飞虱弱致害种群（TN1种群）与强致害种群（Mudgo种群或ASD7种群）（已分别用水稻品种TN1、Mudgo和ASD7强迫饲养82代）在抗虫品种水稻上的致害力差异明显,适宜作为遗传分析的亲本材料。当用蜜露排泄量作为致害性指标时,两种处理（TN1种群与Mudgo种群,TN1种群与ASD7种群）正、反杂交的F₁代雌成虫平均蜜露排泄量存在极显著差异,由此可以推断正交F₁代群体与反交F₁代群体致害性存在明显差别。用体重增量作为致害性指标进行实验,得到的结论与上述研究完全一致。以上的结果可推测稻褐飞虱致害性可能属于伴性遗传。     

The relationship between population means and variances of reproductive success differs between local populations of white stork (Ciconia ciconia)

We studied the size and productivity of white stork (Ciconia ciconia) populations in eight study sites in Poland. The number of nesting pairs and the average number of chicks fledged per pair fluctuated over time, and the studied populations differed in the variance of both breeding success and number of breeding pairs. The variance of breeding success (both for the mean number of chicks and the proportion of successful nests) and the variance of the number of breeding pairs was not correlated with the extent of stable habitats (pastures, meadows, wetlands), other habitats (farmland), or with local population trends over time. We found a non-linear symmetrical relationship between annual mean reproductive success and its variance but only when considered as the proportion of successful nests (i.e., when individual nests are coded as a binary value: 0, no success; 1, success). No such relationship existed when success was expressed as the number of fledged chicks. Although a positive significant correlation occurred between fledgling numbers (discrete data) and the proportion of successful nests (binary data), we believe that the use of only binary data will be inadequate in more detailed analyses, such as population viability analysis.     

Record of Eldana saccharina Walker (Lep,Pyralidae) in inland South Africa and its genetic relationship with the coastal population

Surveys to investigate the distribution and abundance of stem borers in natural habitats were conducted in February 2006 and January–February 2007. The surveys included eastern, northern and central parts of South Africa as well as three localities in Lesotho. During the surveys, Eldana saccharina Walker was recovered from three new localities in inland South Africa and two new indigenous hosts, Phragmites australis Cav. and Panicum maximum Jacq. from Boskop in the North‐West Province and Oribi Gorge in KwaZulu‐Natal respectively. Populations of E. saccharina in different parts of Africa are known for their differences in larval feeding behaviours, host plant choice and natural enemies. It is important to understand the origin of the newly recovered population for prevention of incursion and efficient management in case it invades crops. Molecular analysis indicated that the populations recovered in these new locations and from the new host plants are part of the southern African population of E. saccharina. With change in climate, and disturbance in wetlands the insect is expected in the future to be more abundant and problematic in inland areas of southern Africa.