Marika Rossi

Symbiotic partnerships have opened new ecological niches and contributed to the remarkable diversification of insects. The leafhopper Scaphoideus titanus , a phloem‐feeding insect known to be the primary vector of Flavescence dorée phytoplasma, harbours two primary endosymbionts: the bacterium ‘ Candidatus Karelsulcia muelleri’ and a yeast‐like symbiont (YLS). While most studies on insect‐associated microorganisms have focused on obligate bacterial symbionts, fungal endosymbionts, although documented for almost a century, are only now gaining renewed attention for their evolutionary and ecological significance. In this study, we integrated genomic and proteomic data with phylogenetic analyses to elucidate the functional and evolutionary features of the YLS associated with S. titanus . Using a data‐independent proteomic approach supported by a newly sequenced symbiont genome, we defined the proteins expressed by the YLS that may contribute to host physiology. Comparative analyses across the five currently available YLS genomes enabled a proteome‐wide phylogenetic reconstruction within the genus Ophiocordyceps , refining the evolutionary placement of these symbioses. Finally, large‐scale mining of NCBI transcriptomic Sequence Read Archive datasets using a novel computational workflow, combined with an extensive literature survey, identified several new candidate insect hosts and provided a comprehensive inventory of species harbouring these fungal partners.

An infection event of the spittlebug Philaenus spumarius (Hemiptera, Aphrophoridae) has been described, for the first time, in northwest Italy. The causative agents were two entomopathogenic fungi belonging to Entomophthorales, specifically Zoophthora radicans and, more rarely, Batkoa major . The morphological description and molecular identification of fungi have been reported, in addition to recording meteorological data that may have affected the outbreak of the infection. When massive events are ongoing, entomopathogenic fungi really behave as determinant regulators of natural populations of arthropod pests and the possibility to stress their action in this direction should be deeply investigated.

Pear decline (PD), associated with ‘Candidatus Phytoplasma pyri’, is one of the most severe diseases affecting pear cultivation in Europe and the United States. Several psyllid species act as vectors of phytoplasmas belonging to the 16SrX group and play a key role in the epidemiology of the disease. This study aimed to characterize the epidemiology of pear decline in Sicily using integrated field, molecular, vector, and remote sensing approaches, four years after the first detection of PD in the region. Visual surveys and molecular analyses were conducted over two years in eight pear orchards. A total of 115 plant samples and 101 Cacopsylla spp. specimens, selected from 1435 collected individuals, were analysed, confirming the presence of ‘Ca. P. pyri’ in 69% of symptomatic plants and in 4.6% of C. pyri individuals. Genetic characterization revealed a high degree of similarity among the phytoplasma isolates analysed. Remote sensing analyses conducted since 2018, combined with vector population monitoring, confirmed the epidemic nature of PD and indicated the persistence of a risk of further pathogen spread within the region, supporting the use of remote sensing as a complementary tool for large-scale disease monitoring.

Pear decline (PD), associated with ‘Candidatus Phytoplasma pyri’, is a major disease of pear in Europe and the United States. Several psyllid species are involved in the tritrophic system of PD as vectors of phytoplasmas belonging to the 16SrX group. Four years after the first detection of PD in Sicily, an integrated approach was applied to investigate the epidemic in a major pear-growing area. Visual surveys and molecular analyses were conducted over two years in eight orchards. A total of 115 plant samples and 101 Cacopsylla spp. specimens selected from a total of 1,435 collected individuals were analysed, confirming ‘Ca. P. pyri’ in 69% of symptomatic plants and in 4.6% of C. pyri individuals. Multilocus sequence typing (MLST) revealed high genetic similarity among 16SrX isolates. Remote sensing analyses since 2018, combined with vector monitoring, confirmed the epidemic nature of PD and the persistence of a risk of further pathogen spread within the region, proving, inter alia, to be a valid method for identifying the syndrome even on a large scale.

RNA interference (RNAi) regulates gene expression in eukaryotes, and it is an emerging tool in crop protection by exogenous applications of double-stranded RNAs (dsRNAs) to silence the expression of essential pest genes. Nevertheless, delivery of dsRNAs to sap-sucking insects is a major challenge for RNAi applications. The present work aimed at verifying whether in leafhopper species, RNAi can be triggered by plant-mediated delivery, and providing a proof of concept towards field applications. Two phytoplasma vectors species, Euscelidius variegatus and Scaphoideus titanus (Hemiptera: Cicadellidae), were used as case study. Gene silencing can be achieved efficiently in both species through microinjection of dsRNAs, despite the technique being time consuming and inapplicable on large scale. This protocol was set as gold standard for the development of a higher throughput approach. Soaking of nymphs in a solution with co-adjuvant and dsRNAs as well as insect feeding on whole plants or detached leaves immersed in a dsRNA solution were assayed as alternative delivery strategies. Nymph soaking did not induce specific gene silencing, while plant absorption proved to be suitable to deliver both a coloured solution and control dsRNAs targeting green fluorescent protein gene. Insect feeding on detached leaves immersed in dsRNA solution was selected to test silencing of two gut-specific (legumain and natterin) and one ubiquitous (ATP synthase β) genes. The expression of the three genes significantly decreased in E. variegatus insects fed on dsRNA-treated plants. Similarly, a significant reduction of ATP synthase β transcript was measured in S. titanus fed on dsRNA-treated plants.

Phytoplasmas are plant pathogenic wall-less bacteria transmitted in a persistent propagative manner by hemipteran insects, mainly belonging to the suborder Auchenorrhyncha (Fulgoromorpha and Cicadomorpha). Flavescence dorée (FD) is a quarantine disease of grapevine, causing great damage to European viticulture and associated with phytoplasmas belonging to 16SrV-C (FD-C) and -D (FD-D) subgroups. FD-C and FD-D strains share similar pathogenicity, but mixed infections are rare in nature. To investigate the competition among FDp strains, specimens of the laboratory vector Euscelidius variegatus (Hemiptera: Cicadellidae) were forced to acquire both phytoplasma haplotypes upon feeding on FD-C- and FD-D-infected plants or after the injection of both strains. The pathogen colonization of insect bodies and heads was monitored with multiplex qPCR, and the efficiencies of phytoplasma transmission were estimated. Single infection, irrespective of strain type, was more frequent than expected, indicating that competition among FD strains occurs. Hypotheses of competition for resources and/or host active sites or the direct antibiosis of one strain against the other are discussed, based on the genetic complexity of FDp populations and on the high genome variability of the FD-D strain. As FD management still mainly relies on insecticides against vectors, the characterization of FDp haplotypes and the description of their epidemiology also have practical implications.

Scaphoideus titanus (Hemiptera: Cicadellidae) is the natural vector of Flavescence dorée phytoplasma, a quarantine pest of grapevine with severe impact on European viticulture. RNA interference (RNAi) machinery components are present in S. titanus transcriptome and injection of ATP synthase β dsRNAs into adults caused gene silencing, starting three days post injection (dpi) up to 20 dpi, leading to decrease cognate protein. Silencing of this gene in the closely related leafhopper Euscelidiusvariegatus previously showed female sterility and lack of mature eggs in ovaries. Here, alteration of developing egg morphology in S. titanus ovaries as well as overexpression of hexamerin transcript (amino acid storage protein) and cathepsin L protein (lysosome proteinase) were observed in dsATP-injected females. To evaluate RNAi-specificity, E.variegatus was used as dsRNA-receiving model-species. Different doses of two sets of dsRNA-constructs targeting distinct portions of ATP synthase β gene of both species induced silencing, lack of egg development, and female sterility in E. variegatus, indicating that off-target effects must be evaluated case by case. The effectiveness of RNAi in S. titanus provides a powerful tool for functional genomics of this non-model species and paves the way toward RNAi-based strategies to limit vector population, despite several technical and regulatory constraints that still need to be overcome to allow open field application.

Insect vectors transmit viruses and bacteria that can cause severe diseases in plants and economic losses due to a decrease in crop production. Insect vectors, like all other organisms, are colonized by a community of various microorganisms, which can influence their physiology, ecology, evolution, and also their competence as vectors. The important ecological meaning of bacteriophages in various ecosystems and their role in microbial communities has emerged in the past decade. However, only a few phages have been described so far in insect microbiomes. The leafhopper Euscelidius variegatus is a laboratory vector of the phytoplasma causing Flavescence dorée, a severe grapevine disease that threatens viticulture in Europe. Here, the presence of a temperate bacteriophage in E. variegatus (named Euscelidius variegatus phage 1, EVP-1) was revealed through both insect transcriptome analyses and electron microscopic observations. The bacterial host was isolated in axenic culture and identified as the bacterial endosymbiont of E. variegatus (BEV), recently assigned to the genus Candidatus Symbiopectobacterium. BEV harbors multiple prophages that become active in culture, suggesting that different environments can trigger different mechanisms, finely regulating the interactions among phages. Understanding the complex relationships within insect vector microbiomes may help in revealing possible microbe influences on pathogen transmission, and it is a crucial step toward innovative sustainable strategies for disease management in agriculture.

Flavescence dorée phytoplasmas (FDp, 16SrV-C and -D) are plant pathogenic non-cultivable bacteria associated with a severe grapevine disease. The incidence of the two reference strains on cultivated grapevines is unbalanced, and mixed infections are rare. To investigate the interaction between the two strains, Catharanthus roseus plants were graft-infected with both strains, either simultaneously or sequentially. Different combinations of lateral and apical grafting were applied to avoid possible benefits due to graft position. The infection was monitored for four months through a new diagnostic protocol developed for differentiation and relative quantification of the two strains. Regardless of the temporal or spatial advantage at grafting, FD-C generally outcompeted FD-D. The prevalence of FD-C increased over time and, at the end of the experiment, FD-C was the unique strain detected in the aerial part and the roots of 74% and 90% of grafted plants, respectively. These data indicate that the interaction between the two strains results in competitive exclusion. Understanding the bases of the competition between FD-C and FD-D may contribute to explain the biology of the coexistence of different FDp strains under field conditions, aiming at identifying potential suppressor strains, which can provide alternative and environmentally sustainable solutions for FD control.