Bacterial Endosymbionts of the Psyllid Cacopsylla pyricola (Hemiptera: Psyllidae) in the Pacific Northwestern United States

Gene expression profiling of Cacopsylla pyricola (Hemiptera: Psyllidae) infected with Ca. Phytoplasma pyri (Acholeplasmatales: Acholeplasmataceae) reveals candidate effectors and mechanisms of infection

Phytoplasmas can negatively or positively alter vector host fitness. "Candidatus Phytoplasma pyri," is the causal agent of pear decline in commercial pear (Pyrus communis L.; Rosales: Rosaceae) and peach yellow leafroll in peach [Prunus persica (L.); Rosaceae]. This plant pathogen is transmitted by several species of pear psyllids (Cacopsylla spp. Hemiptera: Psyllidae). We sought to explore the relationship between the pear decline phytoplasma and its US vector, Cacopsylla pyricola (Förster), at the molecular genetic level through transcriptomic analysis using RNA-sequencing methodology. We also focused on phytoplasma and insect effectors, which are secreted proteins that can modulate interactions within a pathosystem. In this study, we identified 30 differentially expressed genes, 14 candidate insect effector genes, and 8 Ca. Phytoplasma pyri candidate effectors. Two strains of Ca. Phytoplasma pyri were identified based on immunodominant membrane protein sequence analysis from C. pyricola collected in the Pacific Northwest agricultural region. Here, we present a first genetic look at the pear decline pathosystem and report gene candidates for further exploration of infection mechanisms and potential tools for integrated pest management.

Seasonal variation in attraction to plant volatiles by Cacopsylla pyricola (Foerster) (Hemiptera: Psyllidae)

Pear psylla, Cacopsylla pyricola (Foerster) (Hemiptera: Psyllidae), occurs as 2 seasonal morphotypes. Summerforms occur on pear (Pyrus communis L.; Rosales: Rosaceae) where they are a significant pest. The larger and darker winterform morphotype develops in response to shortening daylengths and begins winter in reproductive diapause characterized by the absence of ovarian development. Diapausing winterforms often leave pear to overwinter on coniferous shelter plants and then return to pear in late winter and early spring to begin depositing the eggs that produce the first summerform generation. Cacopsylla pyricola adults are attracted to the color of foliage most of the year, but little is known about the role of plant volatiles in host finding and in seasonal dispersal between host and shelter plants by the psyllid. We used a Y-tube olfactometer and choice assays to investigate the response by C. pyricola adults to volatiles emitted by pear and an evergreen tree (cypress) often used as a shelter plant by wintering C. pyricola. Attraction to pear and cypress volatiles varied by season, tree phenology, and psyllid physiology. Cacopsylla pyricola were attracted to cypress volatiles and preferred to settle on cypress shoots during winter and early spring but then shifted to a marked preference for the pear developmental host in late spring and summer. Female C. pyricola exhibited stronger responses to pear volatiles than did males. Our study is the first to show that plant volatiles have a role in host finding by C. pyricola and provides a foundation for research on chemical ecology and management of C. pyricola.

Phytoplasma: A plant pathogen that cannot be ignored in agricultural production-Research progress and outlook

Phytoplasmas are phloem-restricted plant-pathogenic bacteria transmitted by insects. They cause diseases in a wide range of host plants, resulting in significant economic and ecological losses worldwide. Research on phytoplasmas has a long history, with significant progress being made in the past 30 years. Notably, with the rapid development of phytoplasma research, scientists have identified the primary agents involved in phytoplasma transmission, established classification and detection systems for phytoplasmas, and 243 genomes have been sequenced and assembled completely or to draft quality. Multiple possible phytoplasma effectors have been investigated, elucidating the molecular mechanisms by which phytoplasmas manipulate their hosts. This review summarizes recent advances in phytoplasma research, including identification techniques, host range studies, whole- or draft-genome sequencing, effector pathogenesis and disease control methods. Additionally, future research directions in the field of phytoplasma research are discussed.

Microbiome of pear psyllids: A tale about closely related species sharing their endosymbionts

Psyllids are phloem-feeding insects that can transmit plant pathogens such as phytoplasmas, intracellular bacteria causing numerous plant diseases worldwide. Their microbiomes are essential for insect physiology and may also influence the capacity of vectors to transmit pathogens. Using 16S rRNA gene metabarcoding, we compared the microbiomes of three sympatric psyllid species associated with pear trees in Central Europe. All three species are able to transmit 'Candidatus Phytoplasma pyri', albeit with different efficiencies. Our results revealed potential relationships between insect biology and microbiome composition that varied during psyllid ontogeny and between generations in Cacopsylla pyri and C. pyricola, as well as between localities in C. pyri. In contrast, no variations related to psyllid life cycle and geography were detected in C. pyrisuga. In addition to the primary endosymbiont Carsonella ruddii, we detected another highly abundant endosymbiont (unclassified Enterobacteriaceae). C. pyri and C. pyricola shared the same taxon of Enterobacteriaceae which is related to endosymbionts harboured by other psyllid species from various families. In contrast, C. pyrisuga carried a different Enterobacteriaceae taxon related to the genus Sodalis. Our study provides new insights into host-symbiont interactions in psyllids and highlights the importance of host biology and geography in shaping microbiome structure.

Investigating the microbial community of Cacopsylla spp. as potential factor in vector competence of phytoplasma

Phytoplasmas are obligatory intracellular bacteria that colonize the phloem of many plant species and cause hundreds of plant diseases worldwide. In nature, phytoplasmas are primarily transmitted by hemipteran vectors. While all phloem-feeding insects could in principle transmit phytoplasmas, only a limited number of species have been confirmed as vectors. Knowledge about factors that might determine the vector capacity is currently scarce. Here, we characterized the microbiomes of vector and non-vector species of apple proliferation (AP) phytoplasma 'Candidatus Phytoplasma mali' to investigate their potential role in the vector capacity of the host. We performed high-throughput 16S rRNA metabarcoding of the two principal AP-vectors Cacopsylla picta and Cacopsylla melanoneura and eight Cacopsylla species, which are not AP-vectors but co-occur in apple orchards. The microbiomes of all species are dominated by Carsonella, the primary endosymbiont of psyllids and a second uncharacterized Enterobacteriaceae endosymbiont. Each Cacopsylla species harboured a species-specific phylotype of both symbionts. Moreover, we investigated differences between the microbiomes of AP-vector versus non-vector species and identified the predominant endosymbionts but also Wolbachia and several minor taxa as potential indicator species. Our study highlights the importance of considering the microbiome in future investigations of potential factors influencing host vector competence. We investigated the potential role of symbiotic bacteria in the acquisition and transmission of phytoplasma. By comparing the two main psyillid vector species of Apple proliferation (AP) phytoplasma and eight co-occurring species, which are not able to vector AP-phytoplasma, we found differences in the microbial communities of AP-vector and non-vector species, which appear to be driven by the predominant symbionts in both vector species and Wolbachia and several minor taxa in the non-vector species. In contrast, infection with AP-phytoplasma did not affect microbiome composition in both vector species. Our study provides new insights into the endosymbiont diversity of Cacopsylla spp. and highlights the importance of considering the microbiome when investigating potential factors influencing host vector competence.

Bacterial Endosymbionts of Bactericera maculipennis and Three Mitochondrial Haplotypes of B. cockerelli (Hemiptera: Psylloidea: Triozidae)

Insects harbor bacterial endosymbionts that provide their hosts with nutritional benefit or with protection against natural enemies, plant defenses, insecticides, or abiotic stresses. We used directed sequencing of 16S rDNA to identify and compare endosymbionts of Bactericera maculipennis (Crawford) and the western, central, and northwestern haplotypes of B. cockerelli (Šulc) (Hemiptera: Psylloidea: Triozidae). Both species are native to North America, are known to harbor the plant pathogen 'Candidatus Liberibacter solanacearum' and develop on shared host plants within the Convolvulaceae. The Old-World species Heterotrioza chenopodii (Reuter) (Psylloidea: Triozidae), now found in North America, was included as an outgroup. 16S sequencing confirmed that both Bactericera species harbor 'Candidatus Liberibacter solanacearum' and revealed that both species harbor unique strains of Wolbachia and Sodalis. However, the presence of Wolbachia and Sodalis varied among haplotypes of B. cockerelli. The central and western haplotypes harbored the same strains of Wolbachia, which was confirmed by Sanger sequencing of the wsp and ftsZ genes. Wolbachia was also detected in very low abundance from the northwestern haplotype by high-throughput sequencing of 16S but was not detected from this haplotype by PCR screening. The northwestern and central haplotypes also harbored Sodalis, which was not detected in the western haplotype. Heterotrioza chenopodii harbored an entirely different community of potential endosymbionts compared with the Bactericera spp. that included Rickettsia and an unidentified bacterium in the Enterobacteriaceae. Results of this study provide a foundation for further research on the interactions between psyllids and their bacterial endosymbionts.

The Incidence of Wolbachia Bacterial Endosymbiont in Bisexual and Parthenogenetic Populations of the Psyllid Genus Cacopsylla (Hemiptera, Psylloidea)

Wolbachia is one of the most common intracellular bacteria; it infects a wide variety of insects, other arthropods, and some nematodes. Wolbachia is ordinarily transmitted vertically from mother to offspring and can manipulate physiology and reproduction of their hosts in different ways, e.g., induce feminization, male killing, and parthenogenesis. Despite the great interest in Wolbachia, many aspects of its biology remain unclear and its incidence across many insect orders, including Hemiptera, is still poorly understood. In this report, we present data on Wolbachia infection in five jumping plant-lice species (Hemiptera, Psylloidea) of the genus Cacopsylla Ossiannilsson, 1970 with different reproductive strategies and test the hypothesis that Wolbachia mediates parthenogenetic and bisexual patterns observed in some Cacopsylla species. We show that the five species studied are infected with a single Wolbachia strain, belonging to the supergroup B. This strain has also been found in different insect orders (Lepidoptera, Hemiptera, Plecoptera, Orthoptera, Hymenoptera, Diptera) and even in acariform mites (Trombidiformes), suggesting extensive horizontal transmission of Wolbachia between representatives of these taxa. Our survey did not reveal significant differences in infection frequency between parthenogenetic and bisexual populations or between males and females within bisexual populations. However, infection rate varied notably in different Cacopsylla species or within distinct populations of the same species. Overall, we demonstrate that Wolbachia infects a high proportion of Cacopsylla individuals and populations, suggesting the essential role of this bacterium in their biology.

Unraveling the Host Plant Alternation of Cacopsylla pruni - Adults but Not Nymphs Can Survive on Conifers Due to Phloem/Xylem Composition

Plant sap feeding insects like psyllids are known to be vectors of phloem dwelling bacteria ('Candidatus Phytoplasma' and 'Ca. Liberibacter'), plant pathogens which cause severe diseases and economically important crop damage. Some univoltine psyllid species have a particular life cycle, within one generation they alternate two times between different host plant species. The plum psyllid Cacopsylla pruni, the vector of European Stone Fruit Yellows (ESFY), one of the most serious pests in European fruit production, migrates to stone fruit orchards (Prunus spp.) for mating and oviposition in early spring. The young adults of the new generation leave the Prunus trees in summer and emigrate to their overwintering hosts like spruce and other conifers. Very little is known about the factors responsible for the regulation of migration, reasons for host alternation, and the behavior of psyllids during their phase of life on conifers. Because insect feeding behavior and host acceptance is driven by different biotic factors, such as olfactory and gustatory cues as well as mechanical barriers, we carried out electrical penetration graph (EPG) recordings and survival bioassays with C. pruni on different conifer species as potential overwintering hosts and analyzed the chemical composition of the respective plant saps. We are the first to show that migrating psyllids do feed on overwintering hosts and that nymphs are able to ingest phloem and xylem sap of coniferous trees, but cannot develop on conifer diet. Analyses of plant saps reveal qualitative differences in the chemical composition between coniferous trees and Prunus as well as within conifer species. These differences are discussed with regard to nutritional needs of psyllid nymphs for proper development, overwintering needs of adults and restriction of 'Ca. P. prunorum' to Prunus phloem.