Transmission efficiency of Cotton leaf curl Multan virus by three cryptic species of Bemisia tabaci complex in cotton cultivars

Differential insecticide resistance in Bemisia tabaci (Hemiptera: Aleyrodidae) field populations in the Punjab Province of Pakistan

The cotton whitefly Bemisia tabaci (Gennadius) (Hemiptera: Aleyrodidae) has a propensity for developing high-level resistance to insecticides. Management of B. tabaci in cotton grown in Pakistan depends on insecticide use, resistance monitoring has become essential to minimize the development of resistance. In this study, resistance was monitored in adult whiteflies collected from cotton fields in the Bahawalpur, Faisalabad, Lodhran, Multan, and Vehari districts of the Punjab Province, Pakistan during 2017, 2018, and 2019. Resistance monitoring was carried out for two insect growth regulators (pyriproxyfen and buprofezin) four neonicotinoids acetamiprid, imidacloprid, thiamethoxam, thiacloprid, and the historically used pyrethroid, bifenthrin and organophosphate, chlorpyrifos. Results based on resistance ratio (RR) showed that moderate to high level of resistance against noenicitinoids insecticides have been observed in all four districts while whiteflies exhibited very low to low resistance to pyriproxyfen and buprofezin. The RRs for acetamiprid, imidacloprid, thiamethoxam, thiacloprid varied from 7.60 to 50.99, 19.32 to 65.72, 17.18 to 54.65 and 6.49-47.49-fold, respectively. Bifenthrin and chlorpyrifos showed very low toxicity against whiteflies in all districts except Faisalabad, with RRs of 12.28-50.56-fold and 7.94-26.24-fold, respectively. The results will facilitate 'smart' selection and guide rates of insecticide applications for whitefly management in cotton for effective whitefly management while also delaying the development of resistance.

Cotton leaf curl Multan virus differentially regulates innate antiviral immunity of whitefly (Bemisia tabaci) vector to promote cryptic species-dependent virus acquisition

Begomoviruses represent the largest group of economically important, highly pathogenic, DNA plant viruses that contribute a substantial amount of global crop disease burden. The exclusive transmission of begomoviruses by whiteflies (Bemisia tabaci) requires them to interact and efficiently manipulate host responses at physiological, biological and molecular scales. However, the molecular mechanisms underlying complex begomovirus-whitefly interactions that consequently substantiate efficient virus transmission largely remain unknown. Previously, we found that whitefly Asia II 7 cryptic species can efficiently transmit cotton leaf curl Multan virus (CLCuMuV) while MEAM1 cryptic species is a poor carrier and incompetent vector of CLCuMuV. To investigate the potential mechanism/s that facilitate the higher acquisition of CLCuMuV by its whitefly vector (Asia II 7) and to identify novel whitefly proteins that putatively interact with CLCuMuV-AV1 (coat protein), we employed yeast two-hybrid system, bioinformatics, bimolecular fluorescence complementation, RNA interference, RT-qPCR and bioassays. We identified a total of 21 Asia II 7 proteins putatively interacting with CLCuMuV-AV1. Further analyses by molecular docking, Y2H and BiFC experiments validated the interaction between a whitefly innate immunity-related protein (BTB/POZ) and viral AV1 (coat protein). Gene transcription analysis showed that the viral infection significantly suppressed the transcription of BTB/POZ and enhanced the accumulation of CLCuMuV in Asia II 7, but not in MEAM1 cryptic species. In contrast to MEAM1, the targeted knock-down of BTB/POZ substantially reduced the ability of Asia II 7 to acquire and accumulate CLCuMuV. Additionally, antiviral immune signaling pathways (Toll, Imd, Jnk and Jak/STAT) were significantly suppressed following viral infection of Asia II 7 whiteflies. Taken together, the begomovirus CLCuMuV potentiates efficient virus accumulation in its vector B. tabaci Asia II 7 by targeting and suppressing the transcription of an innate immunity-related BTB/POZ gene and other antiviral immune responses in a cryptic species-specific manner.

Comparative transcriptome profiling reveals a network of differentially expressed genes in Asia II 7 and MEAM1 whitefly cryptic species in response to early infection of Cotton leaf curl Multan virus

Cotton leaf curl Multan virus (CLCuMuV) is a whitefly-vectored begomovirus that poses ramping threat to several economically important crops worldwide. The differential transmission of CLCuMuV by its vector Bemisia tabaci mainly relies on the type of whitefly cryptic species. However, the molecular responses among different whitefly cryptic species in response to early CLCuMuV infection remain elusive. Here, we compared early-stage transcriptomic profiles of Asia II 7 and MEAM1 cryptic species infected by CLCuMuV. Results of Illumina sequencing revealed that after 6 and 12 h of CLCuMuV acquisition, 153 and 141 genes among viruliferous (VF) Asia II 7, while 445 and 347 genes among VF MEAM 1 whiteflies were differentially expressed compared with aviruliferous (AVF) whiteflies. The most abundant groups of differentially expressed genes (DEGs) among Asia II 7 and MEAM1 were associated with HTH-1 and zf-C2H2 classes of transcription factors (TFs), respectively. Notably, in contrast to Asia II 7, MEAM1 cryptic species displayed higher transcriptional variations with elevated immune-related responses following CLCuMuV infection. Among both cryptic species, we identified several highly responsive candidate DEGs associated with antiviral innate immunity (alpha glucosidase, LSM14-like protein B and phosphoenolpyruvate carboxykinase), lysosome (GPI-anchored protein 58) and autophagy/phagosome pathways (sequestosome-1, cathepsin F-like protease), spliceosome (heat shock protein 70), detoxification (cytochrome P450 4C1), cGMP-PKG signaling pathway (myosin heavy chain), carbohydrate metabolism (alpha-glucosidase), biological transport (mitochondrial phosphate carrier) and protein absorption and digestion (cuticle protein 8). Further validation of RNA-seq results showed that 23 of 28 selected genes exhibited concordant expression both in RT-qPCR and RNA-seq. Our findings provide vital mechanistic insights into begomovirus-whitefly interactions to understand the dynamics of differential begomovirus transmission by different whitefly cryptic species and reveal novel molecular targets for sustainable management of insect-transmitted plant viruses.

Differential Transmission of Old and New World Begomoviruses by Middle East-Asia Minor 1 (MEAM1) and Mediterranean (MED) Cryptic Species of Bemisia tabaci

Middle East-Asia Minor 1 (MEAM1) and Mediterranean (MED) are two of the most invasive members of the sweetpotato whitefly, Bemisia tabaci, cryptic species complexes and are efficient vectors of begomoviruses. Bemisia tabaci MEAM1 is the predominant vector of begomoviruses in open-field vegetable crops in the southeastern United States. However, recently B. tabaci MED also has been detected in the landscape outside of greenhouses in Florida and Georgia. This study compared the transmission efficiency of one Old-World (OW) and two New-World (NW) begomoviruses prevalent in the southeastern United States, viz.., tomato yellow leaf curl virus (TYLCV), cucurbit leaf crumple virus (CuLCrV), and sida golden mosaic virus (SiGMV) between B. tabaci MEAM1 and B. tabaci MED. Bemisia tabaci MEAM1 efficiently transmitted TYLCV, CuLCrV, or SiGMV, whereas B. tabaci MED only transmitted TYLCV. Percent acquisition and retention of OW TYLCV following a 72 h acquisition access period was significantly higher for B. tabaci MED than B. tabaci MEAM1. In contrast, B. tabaci MEAM1 acquired and retained significantly more NW bipartite begomoviruses, CuLCrV or SiGMV, than B. tabaci MED. Quantitative analysis (qPCR) of virus DNA in whitefly internal tissues revealed reduced accumulation of CuLCrV or SiGMV in B. tabaci MED than in B. tabaci MEAM1. Fluorescent in situ hybridization (FISH) showed localization of CuLCrV or SiGMV in the midgut of B. tabaci MED and B. tabaci MEAM1. However, localization of CuLCrV or SiGMV was only observed in the primary salivary glands of B. tabaci MEAM1 and not B. tabaci MED. TYLCV localization was observed in all internal tissues of B. tabaci MEAM1 and B. tabaci MED. Overall, results demonstrate that both B. tabaci MEAM1 and B. tabaci MED are efficient vectors of OW TYLCV. However, for the NW begomoviruses, CuLCrV and SiGMV, B. tabaci MEAM1 seems to a better vector.

Changes in Visual and Olfactory Cues in Virus-Infected Host Plants Alter the Behavior of Bemisia tabaci

The cucurbit chlorotic yellows virus (CCYV) has caused serious damage to melon crops in many countries in recent years. This plant virus is exclusively transmitted by the whitefly Bemisia tabaci (Gennadius) in a semi-persistent mode. Previous studies have shown that both persistent and non-persistent viruses can affect the orientation and performance of insect vectors, through changing host phenotype or interacting with insect vectors directly to facilitate the spread of viruses. However, how CCYV affects host-plant selection by B. tabaci has not been reported. In this study, we investigated the visual and olfactory preferences of B. tabaci between healthy and CCYV-infected host plants Cucumis sativus (Cucurbitaceae). Volatile profiles of healthy and CCYV-infected C. sativus plants were analyzed using gas chromatography-mass spectrometry (GC-MS). In the choice assay, whiteflies preferred to settle on CCYV-infected C. sativus seedlings. However, the concentrations of total volatiles and terpenes in C. sativus plants decreased after CCYV infection. Interestingly, in the Y-tube assay and vision preference test, whitefly B. tabaci adults showed significant visual preference to CCYV-infected host but showed olfactory preference to healthy plants. These results indicated that CCYV infection in plants differently affected the visual and olfactory-mediated orientation behaviors of vector whiteflies and implied that visual cues could play a more important role than olfactory cues in whiteflies in locating CCYV-infected host plants.

How many begomovirus copies are acquired and inoculated by its vector, whitefly (Bemisia tabaci) during feeding?

Begomoviruses are transmitted by whitefly (Bemisia tabaci Gennadius, Hemiptera: Aleyrodidae) in a persistent-circulative way. Once B. tabaci becomes viruliferous, it remains so throughout its life span. Not much is known about the copies of begomoviruses ingested and/or released by B. tabaci during the process of feeding. The present study reports the absolute quantification of two different begomoviruses viz. tomato leaf curl New Delhi virus (ToLCNDV, bipartite) and chilli leaf curl virus (ChiLCV, monopartite) at different exposure of active acquisition and inoculation feeding using a detached leaf assay. A million copies of both the begomoviruses were acquired by a single B. tabaci with only 5 min of active feeding and virus copy number increased in a logarithmic model with feeding exposure. Whereas, a single B. tabaci could inoculate 8.21E+09 and 4.19E+11 copies of ToLCNDV and ChiLCV, respectively in detached leaves by 5 min of active feeding. Virus copies in inoculated leaves increased with an increase in feeding duration. Comparative dynamics of these two begomoviruses indicated that B. tabaci adult acquired around 14-fold higher copies of ChiLCV than ToLCNDV 24 hrs post feeding. Whereas, the rate of inoculation of ToLCNDV by individual B. tabaci was significantly higher than ChiLCV. The study provides a better understanding of begomovirus acquisition and inoculation dynamics by individual B. tabaci and would facilitate research on virus-vector epidemiology and screening host resistance.

Molecular phylogenetics and evolutionary analysis of a highly recombinant begomovirus, Cotton leaf curl Multan virus, and associated satellites

Cotton leaf curl Multan virus (CLCuMuV) and its associated satellites are a major part of the cotton leaf curl disease (CLCuD) caused by the begomovirus species complex. Despite the implementation of potential disease management strategies, the incessant resurgence of resistance-breaking variants of CLCuMuV imposes a continuous threat to cotton production. Here, we present a focused effort to map the geographical prevalence, genomic diversity, and molecular evolutionary endpoints that enhance disease complexity by facilitating the successful adaptation of CLCuMuV populations to the diversified ecosystems. Our results demonstrate that CLCuMuV populations are predominantly distributed in China, while the majority of alphasatellites and betasatellites exist in Pakistan. We demonstrate that together with frequent recombination, an uneven genetic variation mainly drives CLCuMuV and its satellite's virulence and evolvability. However, the pattern and distribution of recombination breakpoints greatly vary among viral and satellite sequences. The CLCuMuV, Cotton leaf curl Multan alphasatellite, and Cotton leaf curl Multan betasatellite populations arising from distinct regions exhibit high mutation rates. Although evolutionarily linked, these populations are independently evolving under strong purifying selection. These findings will facilitate to comprehensively understand the standing genetic variability and evolutionary patterns existing among CLCuMuV populations across major cotton-producing regions of the world.

Implication of the Whitefly Protein Vps Twenty Associated 1 (Vta1) in the Transmission of Cotton Leaf Curl Multan Virus

Cotton leaf curl Multan virus (CLCuMuV) is one of the major casual agents of cotton leaf curl disease. Previous studies show that two indigenous whitefly species of the Bemisia tabaci complex, Asia II 1 and Asia II 7, are able to transmit CLCuMuV, but the molecular mechanisms underlying the transmission are poorly known. In this study, we attempted to identify the whitefly proteins involved in CLCuMuV transmission. First, using a yeast two-hybrid system, we identified 54 candidate proteins of Asia II 1 that putatively can interact with the coat protein of CLCuMuV. Second, we examined interactions between the CLCuMuV coat protein and several whitefly proteins, including vacuolar protein sorting-associated protein (Vps) twenty associated 1 (Vta1). Third, using RNA interference, we found that Vta1 positively regulated CLCuMuV acquisition and transmission by the Asia II 1 whitefly. In addition, we showed that the interaction between the CLCuMuV coat protein and Vta1 from the whitefly Middle East-Asia Minor (MEAM1), a poor vector of CLCuMuV, was much weaker than that between Asia II 1 Vta1 and the CLCuMuV coat protein. Silencing of Vta1 in MEAM1 did not affect the quantity of CLCuMuV acquired by the whitefly. Taken together, our results suggest that Vta1 may play an important role in the transmission of CLCuMuV by the whitefly.

Genetic engineering and bacterial pathogenesis against the vectorial capacity of mosquitoes

Mosquitoes are the main vector of multiple diseases worldwide and transmit viral (malaria, chikungunya, encephalitis, yellow fever, as well as dengue fever), as well as bacterial diseases (tularemia). To manage the outbreak of mosquito populations, various management programs include the application of chemicals, followed by biological and genetic control. Here we aimed to focus on the role of bacterial pathogenesis and molecular tactics for the management of mosquitoes and their vectorial capacity. Bacterial pathogenesis and molecular manipulations have a substantial impact on the biology of mosquitoes, and both strategies change the gene expression and regulation of disease vectors. The strategy for genetic modification is also proved to be excellent for the management of mosquitoes, which halt the development of population via incompatibility of different sex. Therefore, the purpose of the present discussion is to illustrate the impact of both approaches against the vectorial capacity of mosquitoes. Moreover, it could be helpful to understand the relationship of insect-pathogen and to manage various insect vectors as well as diseases.