Genomic and transcriptomic analyses reveal metabolic complementarity between whiteflies and their symbionts

Nutritional mutualism between insects and symbiotic bacteria is widespread. The various sap-feeding whitefly species within the Bemisia tabaci complex associate with the same obligate symbiont (Portiera) and multiple secondary symbionts. It is often assumed that some of the symbionts residing in the whiteflies play crucial roles in the nutritional physiology of their insect hosts. Although effort has been made to understand the functions of the whitefly symbionts, the metabolic complementarity offered by these symbionts to the hosts is not yet well understood. We examined two secondary symbionts, Arsenophonus and Wolbachia, in two species of the B. tabaci whitefly complex, provisionally named as Asia II 3 and China 1. Genomic sequence analyses revealed that Arsenophonus and Wolbachia retained genes responsible for the biosynthesis of B vitamins. We then conducted transcriptomic surveys of the bacteriomes in these two species of whiteflies together with that in another species named MED of this whitefly complex previously reported. The analyses indicated that several key genes in B vitamin syntheses from the three whitefly species were identical. Our findings suggest that, similar to another secondary symbiont Hamiltonella, Arsenophonus and Wolbachia function in the nutrient provision of host whiteflies. Although phylogenetically distant species of symbionts are associated with their respective hosts, they have evolved and retained similar functions in biosynthesis of some B vitamins. Such metabolic complementarity between whiteflies and symbionts represents an important feature of their coevolution.

A novel microRNA regulates cooperation between symbiont and a laterally acquired gene in the regulation of pantothenate biosynthesis within Bemisia tabaci whiteflies

Horizontally transferred genes (HTGs) play a key role in animal symbiosis, and some horizontally transferred genes or proteins are highly expressed in specialized host cells (bacteriocytes). However, it is not clear how HTGs are regulated, but miRNAs are prime candidates given their previously demonstrated roles in symbiosis and impacts on the expression of host genes. A horizontally acquired PanBC that is highly expressed in whitefly bacteriocytes can cooperate with an obligate symbiont Portiera for pantothenate production, facilitating whitefly performance and Portiera titer. Here, we found that a whitefly miRNA novel-m0780-5p was up-regulated and its target panBC was down-regulated in Portiera-eliminated whiteflies. This miRNA was located to the cytoplasmic region of whitefly bacteriocytes. A novel-m0780-5p agomir injection reduced the expression of PanBC in whitefly bacteriocytes, while a novel-m0780-5p antagomir injection enhanced PanBC expression. Agomir injection also reduced the pantothenate level, Portiera titer and whitefly performance. Supplementation with pantothenate restored Portiera titer and the fitness of agomir-injected whiteflies. Thus, we demonstrated that a whitefly miRNA regulates panBC-mediated host-symbiont collaboration required for pantothenate synthesis, benefiting the whitefly-Portiera symbiosis. Both panBC and novel-m0780-5p are present in the genomes of six B. tabaci species. The expression of a novel miRNA in multiple B. tabaci species suggests that the miRNA evolved after panBC acquisition, and allowed this gene to be more tightly regulated. Our discovery provides the first account of a HTG being regulated by a miRNA from the host genome, and suggests key roles for interactions between miRNAs and HTGs in the functioning of symbiosis.

Bacteriocyte development is sexually differentiated in Bemesia tabaci

Some symbiotic microbes are restricted to specialized host cells called bacteriocytes. However, the molecular and cellular mechanisms underlying the development of bacteriocytes are largely obscure. We find that maternally inherited bacteriocytes proliferate in adult females but degenerate in adult males of the whitefly Bemisia tabaci. Single-cell transcriptomics and immunohistochemistry reveal that cell division only occurs in the bacteriocytes of adult females, whereas autophagy and apoptosis are induced in the bacteriocytes of adult males. A transcription factor, Adf-1, enriched in bacteriocytes, is highly expressed in female bacteriocytes relative to male bacteriocytes. Silencing Adf-1 reduces the bacteriocyte number and Portiera titer and activates autophagy and apoptosis in females. The differential dynamics of both cell division and death in bacteriocytes and distinct expression of Adf-1 in bacteriocytes between whitefly sexes underlie the sexual differentiation of bacteriocyte development. Our study reveals that insect sex affects the development of bacteriocytes by cellular and molecular remodeling.

Whitefly-Mediated Transmission and Subsequent Acquisition of Highly Similar and Naturally Occurring Tomato Yellow Leaf Curl Virus Variants

Begomoviruses are whitefly-transmitted viruses that infect many agricultural crops. Numerous reports exist on individual host plants harboring two or more begomoviruses. Mixed infection allows recombination events to occur among begomoviruses. However, very few studies have examined mixed infection of different isolates/variants/strains of a Begomovirus species in hosts. In this study, the frequency of mixed infection of tomato yellow leaf curl virus (TYLCV) variants in field-grown tomato was evaluated. At least 60% of symptomatic field samples were infected with more than one TYLCV variant. These variants differed by a few nucleotides and amino acids, resembling a quasispecies. Subsequently, in the greenhouse, single and mixed infection of two TYLCV variants (variant #2 and variant #4) that shared 99.5% nucleotide identity and differed by a few amino acids was examined. Plant-virus variant-whitefly interactions including transmission of one and/or two variants, variants' concentrations, competition between variants in inoculated tomato plants, and whitefly acquisition of one and/or two variants were assessed. Whiteflies transmitted both variants to tomato plants at similar frequencies; however, the accumulation of variant #4 was greater than that of variant #2 in tomato plants. Despite differences in variants' accumulation in inoculated tomato plants, whiteflies acquired variant #2 and variant #4 at similar frequencies. Also, whiteflies acquired greater amounts of TYLCV from singly infected plants than from mixed-infected plants. These results demonstrated that even highly similar TYLCV variants could differentially influence component (whitefly-variant-plant) interactions.

Transmission and Pathogenicity of Papaya Virus E: Insights from an Experimental Papaya Orchard

A study was conducted to investigate epidemiological aspects of papaya virus E (PpVE), a cytorhabdovirus commonly found in papaya (Carica papaya L.) plantings in Ecuador. Besides papaya, PpVE was found in three Fabaceae weeds, including Rhynchosia minima, Centrosema plumieri, and Macroptilium lathyroides, the latter being the species with the highest virus prevalence. Greenhouse experiments showed that in M. lathyroides, single infections of PpVE induce only mild leaf mosaic, whereas in mixed infections with cowpea severe mosaic virus, PpVE contributes to severe mosaic. In papaya, PpVE did not induce noticeable symptoms in single or mixed infections with papaya ringspot virus. Transmission experiments confirmed that whiteflies (Bemisia tabaci) transmit PpVE in a semipersistent, nonpropagative manner.

Earlier than expected introductions of the Bemisia tabaci B mitotype in Brazil reveal an unprecedented, rapid invasion history

During 1991, in Brazil, the presence of the exotic Bemisia tabaci B mitotype was reported in São Paulo state. However, the duration from the time of initial introduction to population upsurges is not known. To investigate whether the 1991 B mitotype outbreaks in Brazil originated in São Paulo or from migrating populations from neighboring introduction sites, country-wide field samples of B. tabaci archived from 1989-2005 collections were subjected to analysis of mitochondrial cytochrome oxidase I (mtCOI) and nuclear RNA-binding protein 15 (RP-15) sequences. The results of mtCOI sequence analysis identified all B. tabaci as the NAFME 8 haplotype of the B mitotype. Phylogenetic analyses of RP-15 sequences revealed that the B mitotype was likely a hybrid between a B type parent related to a haplotype Ethiopian endemism (NAFME 1-3), and an unidentified parent from the North Africa-Middle East (NAF-ME) region. Results provide the first evidence that this widely invasive B mitotype has evolved from a previously undocumented hybridization event. Samples from Rio de Janeiro (1989) and Ceará state (1990), respectively, are the earliest known B mitotype records in Brazil. A simulated migration for the 1989 introduction predicted a dispersal rate of 200-500 km/year, indicating that the population was unlikely to have reached Ceará by 1990. Results implicated two independent introductions of the B mitotype in Brazil in 1989 and 1990, that together were predicted to have contributed to the complete invasion of Brazil in only 30 generations.

Effect of sublethal Spirotetramat on host locating and parasitic behavior of Encarsia formosa Gahan

BACKGROUND

The use of chemical insecticides to control Bemisia tabaci Gennadius (Hemiptera: Aleyrodidae) is widespread, although it might exert a sublethal effect on its dominant parasitoid, Encarsia formosa Gahan (Hymenoptera: Aphelinidae). To investigate the sublethal effect of spirotetramat on E. formosa, we observed the ability of E. formosa to locate and handle the host, oviposit and preen after exposure to sublethal concentrations of spirotetramat.

RESULTS

After exposure to spirotetramat at LC₅₀ , the response time of E. formosa to the volatile reached 223.40 s and was significantly prolonged. Only 56.44% of the wasps were attracted by the volatile and the insect crawled the slowest among all of the treatments. The averages of oviposition posture adopted and host handled by each E. formosa in 1 h decreased significantly to 1.79 and 1.27, respectively. At the sublethal concentration of LC₁₀ , 94.59% of the wasps were attracted by the volatile and the insect crawled the fastest. The average of host handled by each E. formosa was 3.92, and the frequency of drumming while walking and drumming the host was 12.34 times per second and 12.30 times per second, respectively, demonstrating a significant acceleration in these abilities.

CONCLUSION

These findings demonstrate that spirotetramat induced hormesis in E. formosa on exposure to its LC₁₀ concentration and accelerated its host locating, host handling and frequency of antennae drumming. These findings could assist in balancing the chemical and biological control of B. tabaci and enhancing the efficacy of E. formosa as a biocontrol agent. © 2021 Society of Chemical Industry.

Microbiome diversity and reproductive incompatibility induced by the prevalent endosymbiont Arsenophonus in two species of African cassava Bemisia tabaci whiteflies

A minimum of 13 diverse whitefly species belonging to the Bemisia tabaci (B. tabaci) species complex are known to infest cassava crops in sub-Saharan Africa (SSA), designated as SSA1-13. Of these, the SSA1 and SSA2 are the predominant species colonizing cassava crops in East Africa. The SSA species of B. tabaci harbor diverse bacterial endosymbionts, many of which are known to manipulate insect reproduction. One such symbiont, Arsenophonus, is known to drive its spread by inducing reproductive incompatibility in its insect host and are abundant in SSA species of B. tabaci. However, whether Arsenophonus affects the reproduction of SSA species is unknown. In this study, we investigated both the reproductive compatibility between Arsenophonus infected and uninfected whiteflies by inter-/intraspecific crossing experiments involving the sub-group three haplotypes of the SSA1 (SSA1-SG3), SSA2 species, and their microbial diversity. The number of eggs, nymphs, progenies produced, hatching rate, and survival rate were recorded for each cross. In intra-specific crossing trials, both male and female progenies were produced and thus demonstrated no reproductive incompatibility. However, the total number of eggs laid, nymphs hatched, and the emerged females were low in the intra-species crosses of SSA1-SG3A+, indicating the negative effect of Arsenophonus on whitefly fitness. In contrast, the inter-species crosses between the SSA1-SG3 and SSA2 produced no female progeny and thus demonstrated reproductive incompatibility. The relative frequency of other bacteria colonizing the whiteflies was also investigated using Illumina sequencing of 16S rDNA and diversity indices were recorded. Overall, SSA1-SG3 and SSA2 harbored high microbial diversity with more than 137 bacteria discovered. These results described for the first time the microbiome diversity and the reproductive behaviors of intra-/inter-species of Arsenophonus in whitefly reproduction, which is crucial for understanding the invasion abilities of cassava whiteflies.

Entomopathogenic Potential of Simplicillium lanosoniveum Native Strain in Suppressing Invasive Whitefly, Aleurodicusrugioperculatus Martin (Hemiptera: Aleyrodidae), Infesting Coconut

In 2016, infestation of an exotic polyphagous pest, the rugose spiraling whitefly (RSW), Aleurodicus rugioperculatus Martin (Hemiptera: Aleyrodidae), was documented on coconut for the first time in India. Instantaneously, RSW has garnered wide attention owing to its damage severity and rapid spread across the coconut-growing regions of the country. Hence, an attempt was made to devise a sustainable integrated pest management (IPM) module using biological control agents as a mainstay component. The present study documented the identification and characterization of a potential entomopathogenic fungal isolate for the management of RSW. An entomopathogenic fungus isolated from nymphal cadavers of RSW was identified as Simplicillium lanosoniveum based on morphological and phylogenetic analyses. A gradient of five conidial concentrations (1 × 10⁴, 1 × 10⁵, 1 × 10⁶, 1 × 10⁷ and 1 × 10⁸ conidia/mL) of the S.lanosoniveum were tested against eggs, first instars, second to third instars and pupae of RSW. Results revealed that S.lanosoniveum is highly virulent to all developmental stages of RSW by causing mortality rates of 95.20%, 87.33%, 85.38% and 72.85%, in eggs, initial, middle and later instar nymphs of RSW, respectively, at the highest tested concentration (1 × 10⁸ conidia/mL) at seven days after exposure. The LC₅₀ and LT₅₀ values of S.lanosoniveum were 4.72 × 10⁴, 4.94 × 10⁴, 5.11 × 10⁵, 5.92 × 10⁵ conidia/mL and 4.27, 4.86, 4.56, 5.89 days against eggs, initial, middle and later instar nymphs of RSW, respectively. Further, preliminary field trials with S.lanosoniveum strain at 1 × 10⁸ conidia/mL exhibited a significant reduction in the egg and nymphal population by 57.8% and 56.3%, respectively. This report thus demonstrated that the newly isolated S.lanosoniveum is an effective pathogen at suppressing all the developmental stages of RSW. This is the first record of S.lanosoniveum infecting RSW, and it has a great potential to be developed as a mycoinsecticide.

Insect herbivores benefit from horizontal gene transfer

Evidence suggests that horizontal gene transfer (HGT) is relatively common in eukaryotes, contrary to what was previously believed. For example, insects that feed on complex sugars and neutralize, degrade, and sequester toxic secondary metabolites have recently been shown to benefit by acquiring genes through HGT.

The Costs and Benefits of Two Secondary Symbionts in a Whitefly Host Shape Their Differential Prevalence in the Field

Insects commonly harbor maternally inherited intracellular symbionts in nature, and the microbial partners often exert influence on host reproduction and fitness to promote their prevalence. Here, we investigated composition of symbionts and their biological effects in the invasive Bemisia tabaci MED species of a whitefly complex. Our field surveys revealed that populations of the MED whitefly, in addition to the primary symbiont Portiera, mainly contain two secondary symbionts Hamiltonella, which is nearly fixed in the host populations, and Cardinium with infection frequencies ranging from 0 to 86%. We isolated and established Cardinium-positive and Cardinium-free whitefly lines with a similar nuclear genetic background from a field population, and compared performance of the two whitefly lines. The infection of Cardinium incurred significant fitness costs on the MED whitefly, including reduction of fecundity and egg viability as well as delay in development. We then selectively removed Hamiltonella from the Cardinium-free whitefly line and compared performance of two whitefly lines, one harboring both Portiera and Hamiltonella and the other harboring only Portiera. While depletion of Hamiltonella had little or only marginal effects on the fecundity, developmental rate, and offspring survival, the Hamiltonella-free whitefly line produced very few female offspring, often reducing the progeny female ratio from about 50% to less than 1%. Our findings indicate that the varying costs and benefits of the association between these two symbionts and the MED whitefly may play an important role in shaping their differential prevalence in the field.

Phylogeographic and SNPs Analyses of Bemisia tabaci B Mitotype Populations Reveal Only Two of Eight Haplotypes Are Invasive

Simple Summary

The whitefly Bemisia tabaci taxon consists of an undefined number of morphologically identical genetic variants of which only a few, including the B, harbor invasive haplotypes. These haplotypes have potential to upsurge and become important pests and plant virus vectors in irrigated agroecosystems worldwide. In the 1980s, unprecedented outbreaks associated with the B variant were reported worldwide, however, the precise origin(s) of the invasive haplotypes has not been determined. In this study, available B. tabaci mitochondrial gene sequences were examined for patterns of conserved single nucleotide changes (SNPs). The whitefly sequence records represented North Africa-Middle Eastern habitats, the proposed B variant center of origin, and distant locales recently invaded by haplotype(s) of the B variant. Unexpectedly, the analysis revealed eight SNPs groups (haplotypes) demonstrating that the genetic architecture of the B mitoype is more complex than previously recognized. Also, the distribution patterns of the eight B haplotypes were tightly linked to well-defined eco-geographic regions, suggesting the different groups have diversified by geographic isolation. Contrary to claims that collectively, the B variant is invasive, only two of the eight haplotypic groups have established in geographical locations outside of their zone of endemism.

Abstract

The Bemisia tabaci cryptic species contains 39 known mitotypes of which the B and Q are best recognized for having established outside their extant endemic range. In the 1980s, previously uncharacterized haplotype(s) of the B mitotype rapidly established in tropical and subtropical locales distant from their presumed center of origin, leading to displacement of several native mitotypes and extreme damage to crops and other vegetation particularly in irrigated agroecosystems. To trace the natural and evolutionary history of the invasive B haplotypes, a phylo-biogeographic study was undertaken. Patterns of single nucleotide polymorphisms (SNPs) and signatures potentially indicative of geographic isolation were investigated using a globally representative mitochondrial cytochrome oxidase I gene (mtCOI) sequence database. Eight haplotype groups within the North Africa-Middle East (NAFME) region were differentiated, NAFME 1–8. The NAFME 1–3 haplotypes were members of the same population that is associated with warm desert climate niches of the Arabian Peninsula and east coastal Africa-Ethiopia. The NAFME 4 and 5 haplotypes are endemic to warm and cold semi-arid niches delimited by the Irano-Turanian floristic region, itself harboring extensive biodiversity. Haplotypes 6 and 7 co-occurred in the Middle East along eastern Mediterranean Sea landmasses, while NAFME 8 was found to be endemic to Cyprus, Turkey, and desert micro-niches throughout Egypt and Israel. Contrary to claims that collectively, the B mitotype is invasive, NAFME 6 and 8 are the only haplotypes to have established in geographical locations outside of their zone of endemism.

Identification and Virulence of Cordyceps javanica Strain wf GA17 Isolated From a Natural Fungal Population in Sweetpotato Whiteflies (Hemiptera: Aleyrodidae)

A new strain of the entomopathogenic fungus, identified as Cordyceps javanica (Frieder. & Bally) Kepler, B. Shrestha & Spatafora (Hypocreales: Cordycipitaceae) wf GA17, was found naturally infecting the sweetpotato whitefly, Bemisia tabaci (Gennadius) MEAM1 in southern Georgia, US, in September 2017. The fungus was tested for pathogenicity and virulence in comparison with commercially available entomopathogenic fungal strains against several insect species in the laboratory. In specific, it was compared with Cordyceps fumosorosea (Wize) Kepler, B. Shrestha & Spatafora (Hypocreales: Cordycipitaceae) Apopka 97, Beauveria bassiana (Bals.-Criv.) Vuill. (Hypocreales: Cordycipitaceae) strain GHA, and Metarhizium brunneum Petch (Hypocreales: Clavicipitaceae) strain F52 for virulence against B. tabaci (4th instars) and cotton aphid, Aphis gossypii Glover (Hemiptera: Aphididae) (4th instars with or without wingbuds), on leaf-discs, and against last instars of pecan weevil Curculio caryae (Horn) (Coleoptera: Curculionidae) and citrus root weevil Diaprepes abbreviatus (L.) (Coleoptera: Curculionidae) in soil cups. Against B. tabaci, C. javanica exhibited higher mortality and mycosis development at 5 d post inoculation than other fungi. In assays against A. gossypii with and without wingbuds, C. javanica and C. fumosorosea had the highest mortality and mycosis levels and B. bassiana had the lowest; nymphs with wingbuds were more susceptible to some fungal infection than those without. Against C. caryae, B. bassiana was more effective than other fungi. For D. abbreviatus, B. bassiana also caused the highest mortality while M. brunneum had the lowest, with Cordyceps spp. being intermediate. Overall, the findings suggest high potential of the new strain, C. javanica wf GA17, for managing whiteflies and aphids, while it was not as effective as B. bassiana against the curculionids.

Performance of two species of whiteflies is unaffected by glucosinolate profile in Brassica plants

BACKGROUND

While plant glucosinolates are known to impart resistance to many insects, their role in the interactions between plants and many phloem-feeding insects such as whiteflies are poorly understood. The whitefly Bemisia tabaci complex comprises many cryptic species that differ in the ability to utilize Brassica plants. However, whether Brassica plants-specific traits such as glucosinolates determine differences of whiteflies in colonizing Brassica plants remains in question.

RESULTS

We first observed performance of two whitefly species MEAM1 and Asia II 3, which differ obviously in their ability to colonize Brassica plants, on four cultivars of three Brassica species that vary in glucosinolate profile. We found that the life history characteristics of each of the two whitefly species seems to be only marginally affected by cultivar. We next used wild-type Arabidopsis plants and mutants defective in glucosinolate biosynthesis or hydrolysis to explore the effects of glucosinolates on the whitefly. We found that fecundity and development of immature stages of neither of the two whitefly species differ significantly between wild-type and mutants.

CONCLUSION

The data suggest that glucosinolates may have little effect on the oviposition by adults and the survival and development of immature stages of MEAM1 and Asia II 3 whiteflies. The marked differences in colonizing Brassica crops between the two whitefly species are likely due to plant traits other than glucosinolates. © 2021 Society of Chemical Industry.

The Association between New World Alphasatellites and Bipartite Begomoviruses: Effects on Infection and Vector Transmission

Begomoviruses can be found in association with alphasatellites, which are capable of autonomous replication but are dependent on the helper begomovirus for systemic infection, encapsidation and vector transmission. Previous studies suggest that the presence of NW alphasatellites (genus Clecrusatellite) is associated with more severe symptoms. To better understand this interaction, we investigated the effects of two alphasatellites on infectivity, symptom development, viral DNA accumulation and vector transmission of three begomoviruses in three hosts. In tomato and Nicotiana benthamiana, all combinations were infectious. In Leonurus sibiricus, only the ToYSV/ToYSA combination was infectious. The presence of EuYMA increased symptom severity of EuYMV and ToYSV in N. benthamiana, and the presence of ToYSA was associated with more severe symptoms of ToYSV in N. benthamiana and L. sibiricus. EuYMA increased the accumulation of ToYSV in N. benthamiana but reduced the accumulation of EuYMV in tomato and of ToSRV in N. benthamiana. The presence of ToYSA decreased the accumulation of ToYSV in N. benthamiana and L. sibiricus. ToYSA negatively affected transmission of ToSRV by Bemisia tabaci MEAM1. Together, our results indicate that NW alphasatellites can interact with different begomoviruses, increasing symptom severity and interfering in the transmission of the helper begomovirus. Understanding this interaction is important as it may affect the emergence of diseases caused by begomovirus-alphasatellite complexes in the field.

Plasticity of the lettuce infectious yellows virus minor coat protein (CPm) in mediating the foregut retention and transmission of a chimeric CPm mutant by whitefly vectors

Transmission of the crinivirus, lettuce infectious yellows virus (LIYV), is determined by a minor coat protein (CPm)-mediated virion retention mechanism located in the foregut of its whitefly vector. To better understand the functions of LIYV CPm, chimeric CPm mutants engineered with different lengths of the LIYV CPm amino acid sequence and that of the crinivirus, lettuce chlorosis virus (LCV), were constructed based on bioinformatics and sequence alignment data. The 485 amino acid-long chimeric CPm of LIYV mutant, CPmP-1, contains 60 % (from position 3 to 294) of LCV CPm amino acids. The chimeric CPm of mutants CPmP-2, CPmP-3 and CPmP-4 contains 46 (position 3 to 208), 51 (position 3 to 238) and 41 % (position 261 to 442) of LCV CPm amino acids, respectively. All four mutants moved systemically, expressed the chimeric CPm and formed virus particles. However, following acquisition feeding of the virus preparations, only CPmP-1 was retained in the foreguts of a significant number of vectors and transmitted. In immuno-gold labelling transmission electron microscopy (IGL-TEM) analysis, CPmP-1 particles were distinctly labelled by antibodies directed against the LCV but not LIYV CPm. In contrast, CPmP-4 particles were not labelled by antibodies directed against the LCV or LIYV CPm, while CPmP-2 and -3 particles were weakly labelled by anti-LIYV CPm but not anti-LCV CPm antibodies. The unique antibody recognition and binding pattern of CPmP-1 was also displayed in the foreguts of whitefly vectors that fed on CPmP-1 virions. These results are consistent with the hypothesis that the chimeric CPm of CPmP-1 is incorporated into functional virions, with the LCV CPm region being potentially exposed on the surface and accessible to anti-LCV CPm antibodies.

Optimizing Efficient RNAi-Mediated Control of Hemipteran Pests (Psyllids, Leafhoppers, Whitefly): Modified Pyrimidines in dsRNA Triggers

The advantages from exogenously applied RNAi biopesticides have yet to be realized in through commercialization due to inconsistent activity of the dsRNA trigger, and the activity level of RNAi suppression. This has prompted research on improving delivery methods for applying exogenous dsRNA into plants and insects for the management of pests and pathogens. Another aspect to improve RNAi activity is the incorporation of modified 2′-F pyrimidine nucleotides into the dsRNA trigger. Modified dsRNA incorporating 32–55% of the 2′-F- nucleotides produced improved RNAi activity that increased insect mortality by 12–35% greater than non-modified dsRNA triggers of the same sequence. These results were repeatable across multiple Hemiptera: the Asian citrus psyllid (Diaphorina citri, Liviidae); whitefly (Bemisia tabaci, Aleyroididae); and the glassy-winged sharpshooter (Homalodisca vitripennis, Cicadellidae). Studies using siRNA with modified 2′-F- pyrimidines in mammalian cells show they improved resistance to degradation from nucleases, plus result in greater RNAi activity, due to increase concentrations and improved binding affinity to the mRNA target. Successful RNAi biopesticides of the future will be able to increase RNAi repeatability in the field, by incorporating modifications of the dsRNA, such as 2′-F- pyrimidines, that will improve delivery after applied to fruit trees or crop plants, with increased activity after ingestion by insects. Costs of RNA modification have decreased significantly over the past few years such that biopesticides can now compete on pricing with commercial chemical products.

Integrative Techniques Confirms the Presence of Bemisia tabaci Parasitoids: Encarsia formosa, Encarsia porteri and Eretmocerus mundus (Hymenoptera: Aphelinidae) on Soybean and Tomatoes in South Brazil

Parasitoid wasps from the Aphelinidae family (Hymenoptera) are important control agents of Bemisia tabaci (Gennadius, 1889) cryptic species, both through reproduction and feeding processes. Identifying native parasitoid species within agricultural systems affected by Bemisia whitefly species is the first step to developing guidelines for the creation and release of biological control agents aiming at this highly damaging pest species complex. Taxonomic and phylogenetic analyses based on morphological and molecular characters, respectively, confirmed the occurrence of Encarsia formosa (Gahan, 1924) in greenhouse tomatoes from Santa Maria, Encarsia porteri (Mercet, 1928) in open-field soybean from Santa Maria, and Eretmocerus mundus Mercet, 1931 in greenhouse tomatoes from São José do Hortêncio, all within Rio Grande do Sul state (South Brazil). This is the first report of En. formosa, En. porteri and Er. mundus parasitising B. tabaci in South Brazil, and the first En. porteri partial mtCOI gene sequence being reported and characterised. The high temperature inside the tomato greenhouses can be a possible cause for the predominance of Er. mundus in São José do Hortêncio, and sex ratios in the surveyed populations point to female and male prevalence within Encarsia and Eretmocerus genera, respectively. The combined use of taxonomic and molecular characterisation highlights the importance of combining both morphological and molecular approaches in the assessment of previously unidentified whitefly parasitoids.

Bemisia tabaci Vesicle-Associated Membrane Protein 2 Interacts with Begomoviruses and Plays a Role in Virus Acquisition

Begomoviruses cause substantial losses to agricultural production, especially in tropical and subtropical regions, and are exclusively transmitted by members of the whitefly Bemisia tabaci species complex. However, the molecular mechanisms underlying the transmission of begomoviruses by their whitefly vector are not clear. In this study, we found that B. tabaci vesicle-associated membrane protein 2 (BtVAMP2) interacts with the coat protein (CP) of tomato yellow leaf curl virus (TYLCV), an emergent begomovirus that seriously impacts tomato production globally. After infection with TYLCV, the transcription of BtVAMP2 was increased. When the BtVAMP2 protein was blocked by feeding with a specific BtVAMP2 antibody, the quantity of TYLCV in B. tabaci whole body was significantly reduced. BtVAMP2 was found to be conserved among the B. tabaci species complex and also interacts with the CP of Sri Lankan cassava mosaic virus (SLCMV). When feeding with BtVAMP2 antibody, the acquisition quantity of SLCMV in whitefly whole body was also decreased significantly. Overall, our results demonstrate that BtVAMP2 interacts with the CP of begomoviruses and promotes their acquisition by whitefly.

Influence of Cassava Morphological Traits and Environmental Conditions on Field Populations of Bemisia tabaci

High populations of species in the whitefly complex Bemisia tabaci Gennadius (Hemiptera: Aleyrodidae) were reported to cause severe damage to cassava in East and Central Africa. However, reasons for B. tabaci population increases are not well understood. We investigated the effect of cassava morphological traits, temperature, rainfall and relative humidity (RH) on the abundance of B. tabaci. Five cassava genotypes with varying levels of resistance to cassava mosaic disease, cassava brown streak disease, and B. tabaci infestation were planted in three Ugandan agro-ecological zones. The experiment was conducted in 2016 and 2017 in a randomized complete block design. Across all locations, the tallest genotype Alado alado supported the lowest number of B. tabaci adults. In areas with high B. tabaci prevalence, leaf area, leaf lobe width, and leaf lobe number exhibited significant positive effects (p < 0.001) on B. tabaci adult count. Positive effects of relative humidity and negative effects of temperature and rainfall on B. tabaci adult and nymph counts were observed in 2016 and 2017, resulting in low populations in Lira. Evidently, temperatures of 28-30 °C, rainfall of 30-150 mm and RH of 55-70%, and deployment of cassava genotypes of low plant height, large leaf area, and lobe width significantly enhanced B. tabaci population growth.

Gut-Expressed Vitellogenin Facilitates the Movement of a Plant Virus across the Midgut Wall in Its Insect Vector

Many viral pathogens of global importance to plant and animal health are persistently transmitted by insect vectors. Midgut of insects forms the first major barrier that these viruses encounter during their entry into the vectors. However, the vector ligand(s) involved in the movement of plant viruses across the midgut barrier remains largely uncharacterized. Begomoviruses, many of which are disease agents of some major crops worldwide, are persistently transmitted by whiteflies (Bemisia tabaci). Here, in order to identify whitefly midgut proteins that interact with a devastating begomovirus, tomato yellow leaf curl virus (TYLCV), we performed midgut-specific TYLCV coat protein (CP) immunoprecipitation followed by high-throughput mass spectrometry proteomic analysis. We find that vitellogenin (Vg), a critical insect reproductive protein that has been considered to be synthesized by the fat body, is also synthesized by and interacts with TYLCV CP in the whitefly midgut. TYLCV appears to be internalized into midgut epithelial cells as a complex with Vg through endocytosis. Virus-containing vesicles then deliver the virus-Vg complexes to early endosomes for intracellular transport. Systematic silencing of Vg or midgut-specific immune blocking of Vg inhibited virus movement across the midgut wall and decreased viral acquisition and transmission by whitefly. Our findings show that a functional Vg protein is synthesized in the midgut of an insect and suggest a novel Vg mechanism that facilitates virus movement across the midgut barrier of its insect vector.

IMPORTANCE An essential step in the life cycle of many viruses is transmission to a new host by insect vectors, and one critical step in the transmission of persistently transmitted viruses is overcoming the midgut barrier to enter vectors and complete their cycle. Most viruses enter vector midgut epithelial cells via specific interaction between viral structural proteins and vector cell surface receptor complexes. Tomato yellow leaf curl virus (TYLCV) is persistently transmitted by the whitefly Bemisia tabaci between host plants. Here, we find that TYLCV coat protein interacts with vitellogenin (Vg) in the whitefly midgut. This interaction is required for the movement of the virus crossing the midgut wall and thus facilitates viral acquisition and transmission by whitefly. This study reveals a novel mechanism of virus overcoming the insect midgut barrier and provides new insights into the function of Vg beyond serving as nutrition for developing embryos in insects.

Identification of a Sulfatase that Detoxifies Glucosinolates in the Phloem-Feeding Insect Bemisia tabaci and Prefers Indolic Glucosinolates

Cruciferous plants in the order Brassicales defend themselves from herbivory using glucosinolates: sulfur-containing pro-toxic metabolites that are activated by hydrolysis to form compounds, such as isothiocyanates, which are toxic to insects and other organisms. Some herbivores are known to circumvent glucosinolate activation with glucosinolate sulfatases (GSSs), enzymes that convert glucosinolates into inactive desulfoglucosinolates. This strategy is a major glucosinolate detoxification pathway in a phloem-feeding insect, the silverleaf whitefly Bemisia tabaci, a serious agricultural pest of cruciferous vegetables. In this study, we identified and characterized an enzyme responsible for glucosinolate desulfation in the globally distributed B. tabaci species MEAM1. In in vitro assays, this sulfatase showed a clear preference for indolic glucosinolates compared with aliphatic glucosinolates, consistent with the greater representation of desulfated indolic glucosinolates in honeydew. B. tabaci might use this detoxification strategy specifically against indolic glucosinolates since plants may preferentially deploy indolic glucosinolates against phloem-feeding insects. In vivo silencing of the expression of the B. tabaci GSS gene via RNA interference led to lower levels of desulfoglucosinolates in honeydew. Our findings expand the knowledge on the biochemistry of glucosinolate detoxification in phloem-feeding insects and suggest how detoxification pathways might facilitate plant colonization in a generalist herbivore.

Competitive Interactions Between Whitefly- and Aphid-Transmitted Poleroviruses Within the Plant Host and the Insect Vectors

Pepper cultivation in Israel has been constrained by two sympatric poleroviruses, Pepper vein yellows virus-2 (PeVYV-2) and Pepper whitefly-borne vein yellows virus (PeWBVYV), which are transmitted specifically by aphids and whiteflies, respectively. This study investigated the interaction between PeVYV-2 and PeWBVYV inside the host plant and the insect vectors. Our results show that PeVYV-2 and PeWBVYV compete against each other inside the host plant and also inside aphids. PeWBVYV was the weaker competitor inside the host plant, with diminished transmission rates when inoculated simultaneously or successively after PeVYV-2 and could only be transmitted efficiently when inoculated first and then challenged by PeVYV-2. Successive inoculations of plants with viruliferous whiteflies with PeWBVYV followed by viruliferous aphids with PeVYV-2 led to a coinfection rate of 60%, but with severely reduced titers of PeWBVYV in the coinfected plants compared with singly infected plants. In contrast, PeVYV-2 was the weaker competitor inside the insect vector, with reduced quantities of the acquired virus and a reduced transmission rate by aphids when given prior acquisition on PeWBVYV. However, we also show that the transmission efficiency of PeVYV-2 and PeWBVYV from coinfected plants by whiteflies and aphids remained comparable to that from singly infected plants. This is likely attributable to the reduced titers of PeWBVYV inside coinfected plants causing lesser impact on transmission of PeVYV-2 by aphids and the stronger competitiveness of PeWBVYV inside the whitefly. Competitive interactions between PeVYV-2 and PeWBVYV inside the host plant and insect vector can thus be beneficial for their coexistence.

First report of cucurbit yellow stunting disorder virus and cucurbit chlorotic yellows virus in melon in the Central Valley of California

In California, the whitefly-transmitted yellowing viruses, cucurbit yellow stunting disorder virus (CYSDV) and cucurbit chlorotic yellows virus (CCYV), both genus Crinivirus, fam. Closteroviridae, have been limited to the Sonoran Desert production regions of Imperial and Riverside counties since their emergence in 2006 and 2014, respectively (Kuo et al., 2007; Wintermantel et al., 2009, 2019) where losses to these viruses have nearly eliminated fall melon production. CYSDV and CCYV have never been identified in the Central Valley, but the aphid-transmitted cucurbit aphid-borne yellows virus (CABYV; genus Polerovirus, fam. Luteoviridae) which produces symptoms nearly identical to those induced by CYSDV and CCYV (Lemaire et al. 1993) is common. As part of a larger study to monitor for whitefly-transmitted yellowing viruses in the southwestern United States, melon leaves exhibiting foliar mottling and interveinal chlorosis beginning near the crown and spreading outward along vines (e-Xtra 1), typical of symptoms caused by yellowing viruses, were collected from 106 melon plants in four commercial fields and a research plot in Fresno County, California, during October 2020. Whiteflies (B. tabaci) were present in all fields and confirmed as MEAM1 (biotype B) by PCR. Total RNA and DNA were extracted separately from the same leaf from each plant to determine the presence of RNA and DNA viruses. Total RNA was extracted as described in Tamang et al. (2021), and was used in RT-PCR with primer sets designed to amplify a 277 nt portion of the CABYV RNA dependent RNA polymerase (RdRp) gene (CABYV RdRp-F - 5' AAGAGCGGCAGCTACAATAC 3', CABYV RdRp-R - 5' TGCCACATTCCGGTTCATAG 3'), and portions of the CCYV and CYSDV RdRp genes encoded on RNA1 of the latter two viruses (Kavalappara et al., 2021). In addition, each CYSDV and CCYV infection was confirmed using a second set of primers that amplified 394 and 372 nt portions of the coat protein gene of each virus, respectively, encoded on RNA2 (Wintermantel et al., 2009; 2019). The 953 nt CCYV RdRp and 394 nt CYSDV CP amplicons were sequenced and found to share greater than 98% sequence identity to CCYV RNA1 (Accession No. MH477611.1) and CYSDV RNA2 (Accession No. LT992901.1), respectively. The CABYV infections were secondarily confirmed using a second set of primers designed to the CP gene (Kassem et al. 2007). Furthermore, four RNA samples from two separate fields that previously tested positive for CYSDV and CABYV and the only CCYV infection were confirmed using a recently developed multiplex RT-qPCR method (Mondal et al. 2021, submitted). Total DNA was extracted using methods described in Mondal et al. (2016) and was used in PCR to test for the presence of the whitefly-transmitted begomovirus, cucurbit leaf crumple virus (CuLCrV) which also occurs in the Sonoran Desert melon production region (Hagen et al, 2008), and is capable of inducing yellowing and leaf curl symptoms in melon. CABYV was by far the most prevalent virus, infecting 34/106 plants tested (32%) among the five fields. Four plants from three fields were infected singly with CYSDV (4%), and three more CYSDV infected plants from two fields were co-infected with CABYV (3%). Only one plant was found to be infected with CCYV as a single virus infection (1%). No triple infections nor any CuLCrV were detected in any of the plants sampled. This is the first report of CYSDV and CCYV in the Central Valley of California. In this survey, although CABYV was the predominant yellowing virus infecting melons in the Central Valley (32%), detection of CYSDV in fields distant from one another and the presence of CCYV even in a single field warrant more extensive monitoring of cucurbit crops and known alternate hosts of these viruses in the Central Valley.