A gene-for-gene model to explain interactions between cultivars of strawberry and races of Phytophthora fragariae var. fragariae

 A gene-for-gene model is postulated to explain the observed interactions between cultivars of strawberry and races of Phytophthora fragariae. Five interacting resistance (R1–R5) and avirulence (Avr1–Avr5) factors explain all the available data involving 15 host genotypes, including the USA and Canadian differential series, and 12 pathogen isolates from North America. Interactions between pathogen isolates and UK and German differentials are also explained by the proposed model. The model makes it possible to develop a universally applicable differential series, to present a systematic, unequivocal nomenclature of races, and to increase the efficiency of breeding programs. Received: 26 April 1996 / Accepted: 19 July 1996     

Resistance to Phytophthora fragariae var. fragariae in strawberry: the Rpf2 gene

  Phytophthora fragariae var. fragariae is the causal agent of red stele (red core) root rot in strawberry (Fragaria spp.). The inheritance of resistance to one isolate of this fungus was studied in 12 segregating populations of F.×ananassa derived from crosses between four resistant cultivars (‘Climax’, ‘Redgauntlet’, ‘Siletz’, and ‘Sparkle’) and three susceptible cultivars (‘Blakemore’, ‘Glasa’, and ‘Senga’ Sengana’). The analysis clearly supports the hypothesis of a single segregating dominant resistance gene. It is proposed that this gene be designated Rpf2. Received 12 November 1996 / Accepted: 22 November 1996     

Assessment of the Resistance to Phytophthora fragariae var. fragariae of the USA and Canadian Differential Series of Strawberry Genotypes

The resistance of the USA and Canadian differential series of strawberry (Fragaria sp.) genotypes was assessed against nine USA isolates (A1-A4, A6-A10) and three Canadian isolates (NS2-NS4) of Phytophthora fragariae var. fragariae. The resistance of part of the UK series was also examined. Of the 157 host genotype-isolate combinations tested, 13 were classified differently from earlier reports. These differences can be explained by the existence of different clones under the same cultivar name (Aberdeen and Perle de Prague), the substitution of Aberdeen by Sparkle, which differ in their resistance, and by the application of different criteria for resistance. Here, incomplete resistance was considered as resistance. The discernment of the US differential series can be improved by including the UK differential Perle de Prague and the Canadian differential Sparkle. The Canadian series can be improved by including Perle de Prague and the US differential Aberdeen. This leads to one unified differential set for North America.The results also allow the proposition of a formal gene-for-gene model which, in turn, provides for a universal differential series for this pathosystem.     

Analysis of Cross-Reactive Antigens of Phytophthora fragariae and Strawberry and their Relation to Resistance and Susceptibility

The presence of cross-reactive antigens between five isolates of P, fragariae (Pf 1, Pf 2, Pf 3, Pf 10 and Pf 11) belonging to five physiological races of the fungus and five strawberry cultivars (Cambridge Favourite, Hapil, Red Gauntlet, Siletz and 52AC18) exhibiting different disease responses to the five isolates was demonstrated by Western blotting. Antiserum anti-H, raised against extracts of healthy Cambridge Favourite roots, detected two antigens which were common to all isolates. Concentration of one of these antigens might be related to the pathogenicity of P. fragariae isolates. Antiserum anti-PfM, raised against mycelial suspensions from the five isolates, detected a doublet of 64 and 61 kDa in the soluble extracts of healthy roots from the five cultivars tested. The corresponding root suspensions revealed numerous other antigens which reacted with anti-PfM. In susceptible interactions, the doublet described above and two additional polypeptides of 38 and 31 kda were detected in large concentrations in both the soluble extracts and root suspensions. These four polypeptides were shown to be present in the healthy roots as well as in the mycelial extracts. In resistant interactions, a 116 kDa polypeptide, present in the root suspensions of the healthy host, was detected in the soluble extracts of the infected roots. It is suggested that these antigens might have a role in resistance and susceptibility.     

Sensitivity of the ELISA Test to Detect Phytophthora fragariae var. rubi in Raspberry Roots

A commercial serological, multiwell assay kit was used to assess the detection limits of Phytophthora fragariae var. rubi in raspberry roots. Detection limits in time lapse after inoculation, were assessed after inoculation of root systems of raspberry plants by zoospores of P. fragariae var. rubi. In extracts taken 3-9 days after inoculation, the pathogen was detected from the fourth day after inoculation. In a test series of simulated P. fragariae var. rubi infection where 0.25, 0.5, 1.0 and 1.5% of infected root tissue respectively, were mixed with healthy tissue (w/w), it was possible to detect the pathogen at 0.25% of simulated infection level. The results obtained show the possibility of an early detection of small amounts of antigen by the ELISA test procedure used. This enhance possibilities for early diagnosis and thereby more effective prevention of Phytophthora diseases in raspberry.     

Genotype-dependence in the interaction betweenGlomus fistulosum,Phytophthora fragariae and the wild strawberry (Fragaria vesca)

Root colonisation byGlomus fistulosum BEG 31 of a plant population from the outbreeding wild strawberry (Fragaria vesca) ranged from 42 to 80%; this was correlated with a significant increase in fruit trusses, berries and leaves. The mycorrhizal plants produced significantly more primary and secondary runners but less lateral runners. One hundred randomly selected seeds were multiplied in vitro and 4 clonal populations were selected. Root colonisation in the clones varied from 16 to 75%, with significant difference in the % colonisation between some of the clones. Percentage mycorrhizal colonisation was positively correlated with effects on host growth.The positive correlation between mycorrhizal root colonisation and growth effects in the clones was not reflected in their respective susceptibility, in the non-mycorrhizal state, toPhytophthora fragariae infection. Clones showing the highest and lowest mycorrhizal root colonisation showed high disease susceptibility whereas a clone with intermediate colonisation was resistant. When the clones were colonised with mycorrhizal fungi, the two susceptible clones became resistant toP. fragariae whereas the resistant and partially-resistant clones were less affected. Only one clone showed variation in vesicle formation when challenged withP. fragariae.     

Existence of two sexual races in the planarian species switching between asexual and sexual reproduction

In certain planarian species that are able to switch between asexual and sexual reproduction, determining whether a sexual has the ability to switch to the asexual state is problematic, which renders the definition of sexuals controversial. We experimentally show the existence of two sexual races, acquired and innate, in the planarian Dugesia ryukyuensis. Acquired sexuals used in this study were experimentally switched from asexuals. Inbreeding of acquired sexuals produced both innate sexuals and asexuals, but inbreeding of innate sexuals produced innate sexuals only and no asexuals. Acquired sexuals, but not innate sexuals, were forced to become asexuals by ablation and regeneration (asexual induction). This suggests that acquired sexuals somehow retain asexual potential, while innate sexuals do not. We also found that acquired sexuals have the potential to develop hyperplastic and supernumerary ovaries, while innate sexuals do not. In this regard, acquired sexuals were more prolific than innate sexuals. The differences between acquired and innate sexuals will provide a structure for examining the mechanism underlying asexual and sexual reproduction in planarians.     

Glucanolytic Actinomycetes Antagonistic to Phytophthora fragariae var. rubi, the Causal Agent of Raspberry Root Rot

A collection of about 200 actinomycete strains was screened for the ability to grow on fragmented Phytophthora mycelium and to produce metabolites that inhibit Phytophthora growth. Thirteen strains were selected, and all produced (beta)-1,3-, (beta)-1,4-, and (beta)-1,6-glucanases. These enzymes could hydrolyze glucans from Phytophthora cell walls and cause lysis of Phytophthora cells. These enzymes also degraded other glucan substrates, such as cellulose, laminarin, pustulan, and yeast cell walls. Eleven strains significantly reduced the root rot index when inoculated on raspberry plantlets.     

Growth and reproduction of New Zealand

In New Zealand, as elsewhere, research on rare species has been dominated by autecological studies of individual threatened species. Limitations of this approach are that it involves no comparison with related common species which may have similar traits, and that the minimal sample size prevents generalisation about causes and consequences of rarity. We report on experimentally determined growth and reproductive traits of 10 rare and common Acaena(Rosaceae) species from two taxonomic sections (sect. Ancistrum and sect. Microphyllae). We examined the relationship between rarity or commonness and relative growth rate, mode of vegetative expansion, morphology/presentation of reproductive structures and reproductive allocation. Rarity and commonness were defined according to geographic range size, measured as the number of 10-km grid squares containing at least one record of the species. There were tendencies across both taxonomic sections for species with large range size to have higher relative growth rates and in section Microphyllae, faster lateral expansion. Among section Ancistrum species, common species tended to produce inflorescences for a shorter period and held their capitula higher above the canopy, but other reproductive attributes showed little association with range size. In section Microphyllae all reproductive traits tended to be positively associated with range size. This was mainly due to the single very common species having high fecundity. The lack of strong patterns among our results may reflect insufficient sample size or that the rare species represent different types of rarity.     

Identification of RAPD markers linked to a Phytophthora fragariae resistance gene (Rpf1) in the cultivated strawberry

 Bulked segregant analysis (BSA) was used to identify seven random amplified polymorphic DNA (RAPD) markers linked to the Rpf 1 gene. Rpf 1 confers resistance to Phytophthora fragariae var. fragariae, the causal agent of red stele root rot in Fragaria spp. The bulked DNAs represented subsets of a F₁ population obtained from the cross Md683×Senga Sengana which consisted of 60 plants and segregated in a 1:1 ratio for resistance or susceptibility to race 2.3.4 isolate NS2 of P.  fragariae. Seven markers were shown to be linked to Rpf 1 and were generated from four primers; five of these markers were in coupling phase and two in repulsion phase with respect to the gene. A linkage map of this resistance gene region was generated using JoinMap 2.0^TM. The manner in which Rpf 1 and the linked markers co-segregated indicated that they are inherited in a disomic fashion. These markers could enable gene pyramiding and marker-assisted selection of resistance genes in strawberry breeding programmes. Received: 26 August 1996 / Accepted: 20 December 1996     

Effect of hymexazol on growth and reproduction of low temperature Phytophthora species

The effect of hymexazol (3-hydroxy-5-methylisoxazole or HMI) on growth and reproduction of low temperature Phytophthora species with growth maxima under 30 °C, was studied on clarified V-8 juice agar medium at 50 g/ml and 5 g/ml. HMI severely inhibited growth of P. infestons, P. phaseoli, P. porri and P. syringae, retarded the growth of P. hibernalis and P. lateralis but enhanced the growth of P. fragariae and P. ilicis. Phytophthora cactorum and P. pseudotsugae with a maximal temperature of 30 °C were moderately inhibited.HMI reduced or prevented sporangial production on agar medium by P. cactorum, P. hibernalis, P. ilicis, P. infestons, P. lateralis, P. phaseoli, P. porri and P. primulae, and suppressed chlamydospore formation in P. porri and to a lesser extent, P. lateralis. The production of sex organs was prevented by HMI in P. phaseoli, P. porri, and P. syringae, reduced in P. hibernalis and P. ilicis but unaffected in P. cactorum, P. primulae and P. psuedotsugae.     

Nachweis von Phytophthora fragariae Hickman in Wurzeln der Erdbeerkultursorte 'Tenira' mit Hilfe des enzyme-linked immunosorbent assay (ELISA)

Detection of Phytophthora fragariae Hickman in roots of strawberry cultivar ‘Tenira’ by enzyme-linked immunosorbent assay (ELISA) Phytophthora fragariae Hickman is detected by ELISA in roots of strawberry cv. ‘Tenira’. Because of the high sensitivity of ELISA presence on fungal antigen was demonstrated before symptoms are detected in microscopical investigations.     

厦门2种文昌鱼的实验室饲养与繁殖

文昌鱼属于脊索动物门头索动物亚门,由于独特的进化地位,生物学家普遍认为文昌鱼在不久的将来会成为实验室模式动物[1,2],全世界许多科学家正致力于文昌鱼的模式化工作,要使文昌鱼成为实验室模式动物,首先要实现它在实验室的连续繁育,保证它能一代又一代的繁育下去.     

A Comparison of the Resistance Against Phytophthora fragariae var. rubi, the Causal Agent of a Raspberry Root Rot

Zoospore encystment, sporulation, oospore formation and detection of Phytophthora fragariae var. rubi , the causal agent of raspberry root rot, on three Rubus genotypes with different levels of resistance were investigated. There were no qualitative or quantitative differences in encystment as determined by scanning electron and fluorescence microscopy. Susceptibility was associated with an extended period of sporulation on roots with a proportion of roots containing oospores increasing continuously until the whole root system was colonised and the fungus could be isolated from the stem bases. Quantitative resistance as found in some raspberry ( Rubus idaeus ) cultivars was associated with a shorter sporulation period and decreasing proportion of the roots containing oospores due to the strong regeneration of healthy roots and limited spread of the fungus in the root system. Stem bases were consequently not affected. Sporulation was very limited on a blackberry x raspberry hybrid that was almost totally resistant to root rot and sustained infections could not be found.