Mekki Ben Khalifa

The structural pattern of infectivity matrices, which contains infection data resulting from inoculations of a set of hosts by a set of parasites, is a key parameter for our understanding of biological interactions and their evolution. This pattern determines the evolution of parasite pathogenicity and host resistance, the spatiotemporal distribution of host and parasite genotypes, and the efficiency of disease control strategies. Two major patterns have been proposed for plant-virus genotype infectivity matrices. In the gene-for-gene model, infectivity matrices show a nested pattern, where the host ranges of specialist virus genotypes are subsets of the host ranges of less specialized viruses. In contrast, in the matching-allele (MA) model, each virus genotype is specialized to infect one (or a small set of) host genotype(s). The corresponding infectivity matrix shows a modular pattern where infection is frequent for plants and viruses belonging to the same module but rare for those belonging to different modules. We analyzed the structure of infectivity matrices betweenPotato virus Y(PVY) and plant genotypes in the familySolanaceaecarrying different eukaryotic initiation factor 4E (eIF4E)-coding alleles conferring recessive resistance. Whereas this system corresponds mechanistically to an MA model, the expected modular pattern was rejected based on our experimental data. This was mostly because PVY mutations involved in adaptation to a particular plant genotype displayed frequent pleiotropic effects, conferring simultaneously an adaptation to additional plant genotypes with different eIF4E alleles. Such effects should be taken into account for the design of strategies of sustainable control of PVY through plant varietal mixtures or rotations.IMPORTANCEThe interaction pattern between host and virus genotypes has important consequences on their respective evolution and on issues regarding the application of disease control strategies. We found that the structure of the interaction betweenPotato virus Y(PVY) variants and host plants in the familySolanaceaedeparts significantly from the current model of interaction considered for these organisms because of frequent pleiotropic effects of virus mutations. These mutational effects allow the virus to expand rapidly its range of host plant genotypes, make it very difficult to predict the effects of mutations in PVY infectivity factors, and raise concerns about strategies of sustainable management of plant genetic resistance to viruses.

The pathogenic properties of Potato virus Y (PVY; genus Potyvirus , family Potyviridae ) isolates collected in naturally infected pepper ( Capsicum annuum ) fields in Tunisia were evaluated against recessive resistance alleles at the pvr2 locus of pepper. Two pathotypes were observed. Pathotype (0,1,3) isolates were able to infect plants carrying the susceptibility allele pvr2 ⁺ , together with pvr2 ¹ / pvr2 ¹ and pvr2 ³ / pvr2 ³ plants, but not pvr2 ² / pvr2 ² plants. Pathotype (0) isolates were only able to infect pvr2 ⁺ / pvr2 ⁺ plants. On the other hand, sequence data and phylogenetic analyses revealed three major groups of isolates, each characterized by particular amino acid residues in the central part of the VPg, the pathogenicity factor towards pvr2 . Correspondence between pathogenic properties and phylogeny suggested a single evolutionary step for pathogenicity towards the pvr2 ¹ and pvr2 ³ resistances, possibly under the selective pressure of pvr2 ¹ . Indeed, 23% of the pepper plants in this area were shown to carry the pvr2 ¹ resistance, while pvr2 ³ was not detected. The data suggested that pathogenicity towards pvr2 ¹ and pvr2 ³ were not costly for PVY to infect susceptible pepper genotypes and supported the matching allele model for pepper–PVY interactions.

A survey was conducted during 2009-2010 seasons to identify the distribution of Bemisia tabaci (Gennadius) biotypes in Tunisia. The genetic affiliation of collected populations was determined by polymerase chain reaction (PCR)-restriction fragment-length polymorphism (TaqI) of the mitochondrial cytochrom oxidase I (mtCOI) gene. Results, validated by sequencing and phylogenetic analysis, allowed the clustering of sampled sweetpotato whiteflies into B and Q biotypes. As B. tabaci harbors the obligatory bacterium Portiera aleyrodidarum, and a diverse array of secondary symbionts including Rickettsia, Hamiltonella, Wolbachia, Cardinium, Arsenophonus, and Fritschea, we report here the infectious status of Tunisian populations by secondary symbionts to find out a correlation between bacterial composition to biotype. The genetic variability and structure of B. tabaci populations in Tunisia was driven by analysis of molecular variance (AMOVA) and the hypothesis of isolation by distance was explored. Selective neutrality and genetic haplotype network tests suggested that Tunisian sweetpotato whiteflies have been undergoing a potential expansion followed by gene flow restriction.

‘Candidatus Phytoplasma mali’ was detected, for the first time, in plum (Prunus domestica L.) trees, with symptoms of plum decline in Tunisia. Infected trees showed leaf browning, leaf and fruit wilting followed by tree death in a few weeks. Phytoplasma was detected by polymerase chain reaction (PCR) using universal phytoplasma primer pairs P1/P7. A band with expected size was observed in samples collected from symptomatic, but not symptomless, plum trees. PCR using group-specific primers yielded a band of 1050 bp with 16SrX group specific primers fO1/rO1. Universal PCR products (1.8 kb) were used for restriction fragment length polymorphism analysis (RFLP) after digestion with endonuclease SspI. RFLP patterns obtained were similar to those previously reported for the Apple Proliferation phytoplasma (AP, 16SrX-A), the causal agent of apple proliferation disease. Blast analysis revealed that universal PCR sequences obtained from the infected Tunisian plums were identical and had the highest identity (99%) with the sequence of ‘Ca. Phytoplasma mali’. Phylogenetic analysis based on 16S rDNA gene sequences showed that the plum decline phytoplasma strain from Tunisia clustered with ‘Candidatus Phytoplasma mali’ strains. This is the first report on detection of 16S rDNA of ‘Ca. Phytoplasma mali’ in plum decline in Tunisia.

The genetic structure of Potato virus Y (PVY) populations was investigated in naturally‐infected pepper ( Capsicum annuum ) fields, collected at eight different localities in northern Tunisia, where 23% of the sampled plants were homozygous for the pvr2 ¹ recessive resistance allele, while the other plants carried the dominant susceptibility allele pvr2 ⁺ . Restriction fragment length polymorphism analysis at three PVY genome segments revealed a high level of viral diversity, with a majority of cases showing co‐infection of individual plants by several PVY haplotypes and a strong genetic differentiation of viral populations collected in the different localities. Geographic distances affected the differentiation of PVY populations and isolation by distance among these populations was significant. However, the occurrence of the pvr2 ¹ resistance allele did not contribute to the structure of viral populations, suggesting that the virulence properties of the virus did not significantly affect its fitness. Consequently, greater deployment of the pvr2 ¹ gene would probably not be a suitable strategy to control PVY, and other resistance genes should be preferred.