Dominant Variance Has an Important Role in Downy Mildew Resistance in Cucumber

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1 Hort. Environ. Biotechnol. 52(4): DOI /s Research Report Dominant Variance Has an Important Role in Downy Mildew Resistance in Cucumber Jamal-Ali Olfati 1*, Habibollah Samizadeh 2, Gholam-Ali Peyvast 1, S. Akbar Khodaparast 3, and Babak Rabiei 2 1 Department of Horticultural Science, University of Guilan, Rasht, Iran 2 Department of Agronomy, University of Guilan, Rasht, Iran 3 Department of Plant Pathology, University of Guilan, Rasht, Iran *Corresponding author: jamalaliolfati@gmail.com Received February 23, 2010 / Accepted April 15, 2011 Korean Society for Horticultural Science and Springer 2011 Abstract. Downy mildew inflicts severe damage on cucurbits in humid areas of production throughout the world. The genetics of resistance to downy mildew (Pseudoperonospora cubensis) in the cucumber (Cucumis sativus L.) was studied by the means of a half diallel table between 6 inbred lines. Decomposition of dominance variation indicated that the dominance effect was not unidirectional and that dominant genes were not uniformly distributed among the parents. These facts were confirmed and using a further detailed Hayman s graphical analysis. Narrow-sense heritability estimates were 0.42 so the breeders are able to use selection for this trait. In other hands the heterosis in some crosses was high so breeders are able to use specific cross to produce hybrid with high level resistance. Additional key words: Cucumis sativus, diallel, hymen analysis, inheritance, Pseudoperonospora cubensis Cucurbit downy mildew is an obligate parasite, with the rare exception of oospore production that can only survive and reproduce on living host tissue. To minimize losses from Pseudoperonospora cubensis it is advisable to plant resistant varieties, usage of biocontrol agents or use of protective fungicides. Determining the inheritance of cucumber (Cucumis sativus L.) resistance to downy mildew (Pseudoperonospora cubensis) has been the subject of research for the past 70 years. There are several proposed inheritance patterns for resistance to downy mildew as follows: three recessive genes (Doruchowski and Lakowska-Ryk, 1992; Shimizu et al., 1963); three partially dominant genes (Pershin et al., 1988); interaction between dominant susceptible and recessive resistance genes (Badr and Mohamed, 1998; El-Hafaz et al., 1990); one or two incompletely dominant genes (Petrov et al., 2000); and finally, a single recessive gene (Angelov, 1994; Fanourakis and Simon, 1987; Van Vliet and Meysing, 1974, 1976). Conflicting results regarding the expression and inheritance of downy mildew resistance in cucumber is likely due to four main factors. First, the pathogen is highly variable and populations have not been sufficiently studied to have a full understanding of virulence factors (Lebeda and Urban, 2004). Multiple pathotypes and races have been identified (Lebeda and Widrlechner, 2003) in some cases more than one pathotype in a geographical region has been determined (Lebeda and Urban, 2004). Different races have been reported (Angelov et al., 2000; Epps and Barnes, 1952; Hughes and Van Haltern, 1952; Shetty et al., 2002) and there are likely different genes involved in resistance to different races, if a gene for gene interaction exists. Environment is the second factor that plays a role in pathogen virulence. Temperature, humidity, rainfall and inoculums concentration all influence the severity of cucumber downy mildew (Cohen, 1977). Interactions among pathogen, host and environment are complex and not easily elucidated. A third factor is the differing mechanisms of resistance. Different mechanisms of resistance have been proposed (Angelov and Krasteva, 2000; Baines, 1991; Barnes and Epps, 1950, 1954; Palti and Cohen, 1980; Tarakanov et al., 1988). Through previously mentioned inheritance studies, a number of mechanisms of resistance for cucumber downy mildew were examined. For rating the resistance of cucumber to downy mildew pathogen, Doruchowski and Lakowska-Ryk (1992) used necrotic lesions, Van Vliet and Meysing (1974, 1976) and El Hafaz et al. (1990) used sporulation intensity, Fanourakis and Simon (1987) used incidence of chlorotic

2 Hort. Environ. Biotechnol. 52(4): and necrotic lesions on cotyledons, and Petrov et al. (2000) used chlorotic lesions. Other studies did not specify how resistance was measured. Different mechanisms of resistance may show independent inheritance patterns and this should be thoroughly tested. Finally, the source of resistance genes must be considered. There are likely at least two gene sources for resistance to downy mildew (Wehner and Shetty, 1997). One resistance gene came from India (PI ), and the other from China (P.R. 40) and other countries. As opposed to qualitative resistance, quantitative or partial resistance may reduce the selection pressure for virulence in the pathogen population and could thus stabilize the hostpathogen system (Crill, 1977). The genetics of these forms of resistance have not been extensively studied. Some research groups evaluated resistance with a four reaction scale from 1 to 4 (Kenigsbuch and Cohen, 1992; Thomas et al, 1988), others measured the number of sporangia (Cohen et al, 1985), but all finally place the individuals in 3 classes: resistant; intermediate; and susceptible. They all reported monogenic or digenic control with a rough indication of the level of dominance (characterized as intermediate or partial ). In this paper, we investigate the genetics of resistance to downy mildew in a 6 6 partial diallel cross on the F 1 generation. How general combining ability effects vary from generation, incubation and scaling method. The present investigation was undertaken to study this aspect. The evaluation of resistance for cucumber to downy mildew, six lines were used in this study. Lines 08wvc c-115 and 08wvc c-118 were received from the World Vegetable Center and lines BH-502, BH-504, BH-604 and BH-502 were received from Ing. B. HOLMAN (Czech Republic). Lines and all possible crosses, except reciprocals, were evaluated in 2009 in a completely randomized design with three replications. Seeds were sown on 9 Jul in single plastic pots (12 11 cm) filled with coco peat and perlite. Seedlings were transplanted on 23 Jul into 24 L bags, at a plant density of 3.1 plants m -2 for the remainder of the experiment. Temperature inside the greenhouse was controlled using an automatic activation of the aerial heating fan with a TCL split type indoor air conditioner unit system to maintain temperature between 25 and 15 (day and night). Plants were inoculated at the one- to two-true leaf stage of cucumber with P. cubensis collected from fields that had not been sprayed with fungicides, and we repeat to inoculated plants at 5 true leaf stage. Infected leaves were collected in the morning, placed in plastic bags and stored in a cooler with ice. In the greenhouse, three heavily-infected leaves were placed on each plant and rubbed gently with a glass rod to harvest sporangia. The inoculated plants were covered with Fig. 1. The incubated plants were covered with plastic covers. plastic covers for 2 days (Fig. 1). Resistance of cucumber were evaluated after the development of downy mildew symptoms by estimating the diseased area with a planimeter. Plants were rated for amount of diseased leaf surface area on a 0 to 9 visual rating scale, where 0, 1, 2, 3, 4, 5, 6, 7, 8, and 9 indicates no visual disease symptoms, 0-3% disease (highly resistant), 3-6% disease (highly resistant), 6-12% disease (moderately resistant), 12-25% disease (intermediate), 25-50% disease (intermediate), 50-75% disease (moderately susceptible), 75-87% disease (highly susceptible), 87-99% disease (highly susceptible) and 100% disease (plant dead) area of the leaf covered by necrotic or chlorotic lesions respectively (Jenkins and Wehner, 1983). The data were subjected to analysis of variance using SAS (SAS, Inc., Cary, NC). Data transformation via root square was performed to provide a more normally distributed set of data for statistical analysis. The data were then analyzed as described by Griffing s (1956) and Hayman (1954). Significant differences (P < 0.01) among genotypes of cucumber were showed thorough the variance analysis of infection percent and disease score. Analysis of variance following Griffing s decomposition proved significance of General and Specific combining ability effects (Table 1). Combining ability analysis is used in the selection of parents in the formulations of a crossing plan. The general combining ability (GCA) of a parental clone provides an assessment of its breeding value, as judged by the mean performance of its progenies from crosses with other clones. As suggested by Baker (1978), the relative importance of GCA and SCA in determining progeny performance should be assessed by estimating the ratio of the mean squares. The

3 424 Jamal-Ali Olfati, Habibollah Samizadeh, Gholam-Ali Peyvast, S. Akbar Khodaparast, and Babak Rabiei Table 1. Analysis of variance of the half diallel following Griffing,s method 2 with a mixed model. Components of variation d.f. Mean square Downy mildew infection percent G.C.A z S.C.A M e Baker ratio h 2 n z Significant at P Table 2. General and Specific combining ability of lines (on diagonal) and hybrids (out of diagonal) for infection percent according to Griffing s method 2. Parents ** -0.47ns 1.32** -1.29* 0.34ns -0.38ns ns -0.73ns 1.85** ns -1.32** ** -2.49** 1.69** 0.54ns ** -1.07* 0.99* ** -0.28ns ** Table 3. Heterosis by cross, relative to midparents or high parent for traits. downy mildew infection percent Parent 1 Parent 2 Het. Het. Mean Midparents Maxparent baker ratio equals 0.92 for powdery mildew infection percent and confirms the high predictability of progeny performance based on GCA alone. In other hand, the baker ratio showed that these traits controlled additively, additive variance is important for this trait, and breeders are able to reach suitable materials via selection. Quantitative genetic data on partial downy mildew resistance was of practical interest to breeders. Initially, they were valuable for identification of those single crosses with high resistance and parents with high combining ability for resistance. It was observed that 118 and 115 had the higher GCA value (Table 2) but their hybrids showed low heterosis (Table 3). However, hybridization of a line with high negative GCA (118) and a line with high positive GCA (504) produce a cross with high negative heterosis. Such diallel study is also interesting for population im-

4 Hort. Environ. Biotechnol. 52(4): Table 4. Estimation of genetic components of variation for infection percent of downy mildew according to the method of Hayman (1954). Parameter D H 1 H 2 F h 2 H 1-H 2 H 2/4H 1 (H 1/D) 1/2 h 2 /H 2 r (P r,w r+v r) r 2 h 2 n h 2 b b t 2 t (H 0:b^=1) t (H 0:b^=0) Wr-Vr Wr+Vr Infection percent ns 0.47 ns 4.22* 0.92 ns ns that gene distribution among parents was not symmetrical. The positive sign for F confirmed that dominant alleles were more frequent than recessive ones, irrespective of whether dominant or non-dominant alleles had increasing or decreasing effects. Narrow sense heritability was high (0.42). Fig. 2. Hyman s graph representing the linear relationship between parents in an F 1 population of a 6 6 diallel infected with P. Cubensis. The limiting parabola (Wr 2 = V p V r) and the tangent to the parabola with slope one are drawn. The position of (V r, W r) on the line reveals the relative proportions of dominant and recessive genes in the parents. provement. Results that were analyzed with Hayman s variance analysis (Table 4) were consistant with the results of Griffing s analysis. The H 1 -H 2 component was highly significant and indicated that dominant genes were not uniformly distributed among parents (some parents had more dominant alleles than others). If all the assumptions underlying the analysis were met, Wr-Vr would be a constant (Hayman, 1954). Analysis of variance of (Wr-Vr) (Table 4) after removing line 504 attested the constancy of the quantity among the parents (the parental effect was non-significant). The mean regression slope of Wr on Vr was significantly different from zero and don t show significant differences from unit so all hypotheses was probable. Hayman s graph for infection percent is presented in Fig. 2. Line 115 (located in the lower part of the graph) was the most dominant parents whereas 604 (located in the higher part of the graph) was the most recessive parent. The estimation of genetic parameters for the half diallel table is presented in Table 7. D and H 1 were the estimates of additive and dominance effects variation so that H 1 /D = 1.34 estimated the mean degree of over dominance in agreement with the graphical estimation. H 2 was the estimation of dominance variation weighted by gene distribution among parents. H 1 was significantly different from H 2, indicating The diallel study provided evidence for the existence of significant additive variation through large values of GCA. As suggested by Baker (1978), the relative importance of GCA and SCA in determining progeny performance should be assessed by estimating the ratio of the mean squares. This ratio equals 0.92 in our case and confirms the high predictability of progeny performance based on GCA alone. The analysis of Wr, Vr and graphical statistics computed from F 1 generation provided detailed information on the interrelations between the parents. The negative Wr intercept in the graphic analysis suggested over dominance. Comparison of 2 dominance variation estimations (H 1 and H 2 ) showed that the gene distribution among parents was not symmetrical. Finally, the apparent differences in dominance effects and degree of resistance between our resistant parents indicated that either these are different sources of resistance located at different loci or that significant effects of their genetic backgrounds affect the final expression of a resistance controlled by a major gene. Angelov, D Inheritance of resistance to downy mildew, Pseudoperonospora cubensis (Berk. & Curt.) Rostow. Rep. 2nd Natl. Symp. Plant Immunity. (Plovdiv) 3: Angelov. D. and L. Krasteva Selecting downy mildew-resistant short-fruited cucumbers. p In: N. Katzir and H.S. Paris (eds.). Proc. Cucurbitaceae ISHS Press, Ma'ale Ha Hamisha, Israel. Angelov, D., P. Georgiev, and L. Krasteva Two races of Pseudoperonospora cubensis on cucumbers in Bulgaria. p In: N. Katzir and H.S. Paris (eds.). Proc. Cucurbitaceae ISHS Press, Ma'ale Ha Hamisha, Israel. Badr, L.A.A. and F.G. Mohamed Inheritance and nature of resistance to downy mildew disease in cucumber (Cucumis sativus L.). Ann. Agr. Sci. Moshtohor (in Arabic) 36: Bains, S.B Classification of cucurbit downy mildew lesions into distinct categories. Indian J. Mycology Plant Pathol. 21: Baker, R.J Issues in diallel analysis. Crop Sci. 18: Barnes, W.C. and W.M. Epps Some factors related to the expression of resistance of cucumbers to downy mildew. Proc. Amer. Soc. Hort. Sci. 56:

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