Resistance of some olive (Olea europaea) cultivars and hybrids to leaf spot disease analyzed by microsatellites
Abstract
In order to investigate the resistance of some olive (Olea europaea L.) cultivars and hybrids to leaf spot disease caused by Venturia oleaginea, this study was conducted on high susceptible cultivar Meski and nine hybrids. Samples were collected from a field site located in Nabeul (North East of Tunisia) and evaluated for their susceptibility to leaf spot dis ease by means of visible and latent infection. Therefore , the studied plants were classified into three categories: very susceptible, intermediate and resistant. Meski cultivar and three hybrids (MxA) obtained through controlled crosses between Meski and Arbequina were the most susceptible to the disease. The hybrids MxC resulting from the crosses between Meski and Chétoui olive cultivars presented less severity. However, the hybrids obtained through crosses between Meski and Picholine cultivars showed the lowest incidence of infection. Microsatellites were used as markers to analyze the genetic relationships between parental olive cultivars and hybrids and the effects of crossing on the disease resistance. Cluster analyses, using the SSR data, showed that olive cultivars and hybrids obtained by controlled cross between MeskixPicholine, Meski x Arbequina and Meski ×Picholine were related to Picholine cultivar. The hybrid Meski x Chétoui was more related to cultivar Meski. Data analyses revealed that the GAPU101 showed the highest number of alleles (8) followed by the tow loci UDO99 and GAPU71 with 6 alleles. The DCA18 locus showed 5 alleles. Genetic variability was wide as indicated by the values of observed heterozygosity as noted 1.00 at locus of the four studied loci. Polymorphic information content (PIC) varied from 0.669 to 0.776. The gene diversity values were higher than 0.53. Genetic distances were determined based on the SSR genotype data and component principal analysis were used for finding possible correlation between severity disease, Meski cultivar and hybrids.
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Introduction
Olive tree ( Olea europaea L.) is the most important tree cultivated in Tunisia. Therefore, it contributes by 11% of the total value of a gricultural production and by 33% of the wholesale value of agricultural exports. More than 1.76 million hectares planted with 74 million olives. The olive production rate over the last five -year was more than 792 thousand tons but the export rate during t he same period was about 145 thousand tons of oil worth 590 million dinars (Tunisian Ministry of Agriculture, 2014). The great damage induced by fungal diseases and especially the olive leaf spot (OLS) disease caused by the fungi Venturia oleaginea has an important incidence (Rhouma et al., 2013).
In recent years, alternative techniques have been applied for the control of the disease. Genetic resistance represents an effective, economically feasible and ecologically sustainable mean to control OLS (Rhouma et al., 2013; Sanei and Razavi, 2011; Zine El Aabidine et al., 2010). However, the level of susceptibility of olives to OLS is widely variable (Graniti, 1993; Sutter, 1994). Several varieties (eg. Leccino and Valatolina) are resistant to V. oleaginea (Mac Donald et al., 2000; Sanei and Razavi, 2011).
In order to exploit genetic diversity, many cross breeding programs were carried out and several Tunisian research teams have used PCR -based markers which include SSR for basic and applied research to assess t he genetic diversity of Tunisian olive cultivars (Taamalli et al., 2007; Hannachi et al., 2008; Rekik et al., 2008) and SNP (Rekik et al., 2010).
The microsatellite technique is one of the most reliable methods used in olive cultivar characterization. It h as revealed a high informative level because the markers are polymorphic, multiallelic, and codominant. Moreover, it consists of a relatively simple methodology that permits an easy interpretation of results (Rafalski et al., 1996). SSR markers have been successfully used in germplasm bank classification and contributed to better management of several olive collections around the world (Khadari et al., 2007; Muzzalupo et al., 2010).
OLS of olive trees is quite prevalent in Tunisia. However, no reports on V. oleaginea fungal phytopathogens causing this disease on olive cultivars and hybrids obtained through cross breeding program.
The aim of this work was to characterize the resistance of a susceptible olive cultivar and hybrids to OLS disease, grown in Tunisia and to contribute in the study and development of OLS-resistant hybrids. SSR markers were used for the establishment of a relationship between Meski cultivar and hybrids in order to produce superior progeny. Genetic distances might be used to revealed differences between hybrids and cultivars in the resistance to OLS disease. So, that resistant cultivars or hybrids can be identified and thus used for replanting, or as sources for resistance in future breeding programs. This study was conducted to characterize the genetic resistance of some hybrids and olive cultivars with disease of OLS based on four SSR markers referring to their polymorphism and reproducibility.
Conclusion
Multi-factorial analysis carried out in this work has been conducted to characterize clustering tendencies among the identified cultivars according to their resistance to the disease (Fig. 2). The analysis revealed 36.96 and 22% of clustering according to two principal components. Such percentages allow considering the groups as clustering tendencies and not as a clear separations. The test ended with the clustering of the MxP1, MxP2, MxP3 and Picholine1 olive accessions which are resistant, MxA1, MxA2, MxA 3 and Meski1 olive accessions which are not resistant and MxC1, MxC2, MxC3, Arbequina1 and Chétoui1 accessions with no clear separation. Thus, it seems that clustering tendencies correspond to the resistance to OLS disease, but resistant and susceptible pa rents might be in the same separation groups. This is probably a result of grower’s selections during the olive cultivation history considering the resistance to OLS disease which must be confirmed by further analysis using a wider collection of olive accession.