DNA markers for identifying biotypes B and Q of Bemisia tabaci (Hemiptera: Aleyrodidae) and studying population dynamics

Insights into the transcriptomics of polyphagy: Bemisia tabaci adaptability to phenylpropanoids involves coordinated expression of defense and metabolic genes

The whitefly Bemisia tabaci is a major generalist agricultural pest of field and horticultural crops world-wide. Despite its importance, the molecular bases of defense mechanisms in B. tabaci against major plant secondary defense compounds, such as the phenylpropanoids, remain unknown. Our experimental system utilized transgenic Nicotiana tabacum plants constitutively expressing the PAP1/AtMYB75 transcription factor which activates relatively specifically the phenylpropanoid/flavonoids biosynthetic pathway. Our study used suppression subtractive hybridization (SSH) and cDNA microarray approaches to compare gene expression between B. tabaci adults subjected to wild-type or transgenic plants for 6 h. A total of 2880 clones from the SSH libraries were sequenced. Both the SSH and cDNA microarray analyses indicated a complex interaction between B. tabaci and secondary defense metabolites produced by the phenylpropanoids/flavonoids pathway, involving enhanced expression of detoxification, immunity, oxidative stress and general stress related genes as well as general metabolism and ribosomal genes. Quantitative real-time PCR revealed significant changes in the expression of several of these genes in response to feeding on artificial diet containing the flavonoids quercetin. The elevated transcriptional activity was not accompanied by reduced reproductive performance, indicating high adaptability of B. tabaci to this large group of plant secondary defense metabolites.

Use of mitochondrial cytochrome oxidase I polymerase chain reaction-restriction fragment length polymorphism for identifying subclades of Bemisia tabaci Mediterranean group

The Mediterranean group (commonly known as Q biotype; hereafter MED) of the sweetpotato whitefly, Bemisia tabaci (Gennadius), originated in the Mediterranean region, but it now has been found in at least 10 countries outside the Mediterranean. Collections of B. tabaci from some of these countries exhibit different pest behaviors and pesticide resistance characteristics, yet all may be classified as MED. A phylogenetic analysis of 120 mitochondrial cytochrome oxidase I (mtCOI) sequences (JN966761-JN966880) of MED whiteflies collected in Arizona and of 417 retrieved from the GenBank database resolves the MED into five subclades, designated as Q1-Q5. Only subclades Q1 and Q2 have been detected in the United States. Q1 and the other four subclades (Q2-Q5) differ in the number or position of the AluI recognition sites. Based on the differences in the AluI recognition sites reported here and the previously reported differences in VspI recognition sites, we developed a simple diagnostic technique to identify subclades Q1-Q5 by using mtCOI polymerase chain reaction (PCR)-restriction fragment-length polymorphism (RFLP). A test of a worldwide collection of whiteflies demonstrates that this combination mtCOIPCR-RFLP technique can reliably distinguish not only the MED from the Middle East-Asia Minor 1 group but also the Q1 from any of the other four MED subclades.

Molecular characterization of Bemisia tabaci populations in Tunisia: genetic structure and evidence for multiple acquisition of secondary symbionts

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.

Reproductive incompatibility between the B and Q biotypes of the whitefly Bemisia tabaci in China: genetic and behavioural evidence

The B and Q 'biotypes' of Bemisia tabaci (Gennadius) (Hemiptera: Aleyrodidae) have been invading many parts of the world and causing severe damage to a range of crops. Recent phylogenetic analyses indicate that B and Q are cryptic species within the B. tabaci species complex. Although various attempts have been made to examine the reproductive compatibility between B and Q, few studies have tested the fertility of the F1 females and so the extent of possible gene flow remains unclear. In this study, we conducted a series of crossing experiments and behavioural observations to examine in detail the reproductive compatibility between the B and Q biotypes collected from Zhejiang, China, a region recently invaded by these whiteflies. Crossing experiments between the two biotypes using either single-pairs or small groups demonstrated that proportions of females in the F1 progeny were only 0-2% in the inter-biotype crosses compared to 58-68% in the intra-biotype treatments. Furthermore, all inter-biotype F1 females were sterile. Continuous video observations showed that B and Q adults very rarely copulated, and copulation occurred only when adults of opposite sex from different biotypes were enclosed in dense cohorts for a relatively long period of time. These data show that the B and Q biotypes examined in this study are completely isolated in reproduction. The isolation was due to mainly a copulation barrier, but post-copulation barriers were also involved.

Bemisia tabaci: a statement of species status

Bemisia tabaci has long been considered a complex species. It rose to global prominence in the 1980s owing to the global invasion by the commonly named B biotype. Since then, the concomitant eruption of a group of plant viruses known as begomoviruses has created considerable management problems in many countries. However, an enduring set of questions remains: Is B. tabaci a complex species or a species complex, what are Bemisia biotypes, and how did all the genetic variability arise? This review considers these issues and concludes that there is now sufficient evidence to state that B. tabaci is not made up of biotypes and that the use of biotype in this context is erroneous and misleading. Instead, B. tabaci is a complex of 11 well-defined high-level groups containing at least 24 morphologically indistinguishable species.

Evidence for pre-zygotic reproductive barrier between the B and Q biotypes of Bemisia tabaci (Hemiptera: Aleyrodidae)

The degree of reproductive isolation between the B and Q biotypes of the whitefly Bemisia tabaci (Hemiptera: Aleyrodidae) is currently not clear. Laboratory experiments have shown that the two biotypes are capable of producing viable F1 hybrids but that these females are sterile as their F2 generation failed to develop, indicating, most likely, a post-zygotic reproductive barrier. Here, we confirm, by molecular and ecological tools, that the B and Q biotypes of Israel are genetically isolated and provide two independent lines of evidence that support the existence of a pre-zygotic reproductive barrier between them. Firstly, monitoring of mating behaviors in homogeneous and heterogeneous couples indicated no copulation events in heterogeneous couples compared to approximately 50% in homogeneous B and Q couples. Secondly, we could not detect the presence of sperm in the spermathecae of females from heterogeneous couples, compared to 50% detection in intra-B biotype crosses and 15% detection in intra-Q biotype crosses. The existence of pre-zygotic reproductive barriers in Israeli B and Q colonies may indicate a reinforcement process in which mating discrimination is strengthened between sympatric taxa that were formerly allopatric, to avoid maladaptive hybridization. As the two biotypes continued to perform all courtship stages prior to copulation, we also conducted mixed cultures experiments in order to test the reproductive consequences of inter-biotype courtship attempts. In mixed cultures, a significant reduction in female fecundity was observed for the Q biotype but not for the B biotype, suggesting an asymmetric reproductive interference effect in favour of the B biotype. The long-term outcome of this effect is yet to be determined since additional environmental forces may reduce the probability of demographic displacement of one biotype by the other in overlapping niches.

Mating behaviour, life history and adaptation to insecticides determine species exclusion between whiteflies

1. Negative interspecific interactions, such as resource competition or reproductive interference, can lead to the displacement of species (species exclusion). 2. Here, we investigated the effect of life history, mating behaviour and adaptation to insecticides on species exclusion between cryptic whitefly species that make up the Bemisia tabaci species complex. We conducted population cage experiments independently in China, Australia, the United States and Israel to observe patterns of species exclusion between an invasive species commonly referred to as the B biotype and three other species commonly known as biotypes ZHJ1, AN and Q. 3. Although experimental conditions and species varied between regions, we were able to predict the observed patterns of exclusion in each region using a stochastic model that incorporated data on development time, mating behaviour and resistance to insecticides. 4. Between-species variation in mating behaviour was a more significant factor affecting species exclusion than variation in development time. Specifically, the ability of B to copulate more effectively than other species resulted in a faster rate of population increase for B, as well as a reduced rate of population growth for other species, leading to species exclusion. The greater ability of B to evolve resistance to insecticides also contributed to exclusion of other species in some cases. 5. Results indicate that an integrative analysis of the consequences of variation in life-history traits, mating behaviours and adaption to insecticides could provide a robust framework for predicting species exclusion following whitefly invasions.

A real-time PCR assay to differentiate the B and Q biotypes of the Bemisia tabaci complex in Cyprus

A real-time PCR assay based on TaqMan technology was developed and evaluated for the rapid detection of the B and Q biotypes of Bemisia tabaci (Gennadius) (Hemiptera: Aleyrodidae). A survey was conducted during 2005-2007 in order to identify the distribution and prevalence of B. tabaci biotypes in Cyprus using the real-time PCR assay. More than 700 adult whiteflies collected from 35 cultivated and weed plant species were individually haplotyped using TaqMan PCR, and the results of the assay were validated by restriction fragment length polymorphism analysis and DNA sequencing of the mitochondrial cytochrome oxidase I (mtCOI) gene. Two biotypes, B and Q, were identified in the collected plant species on the island. The real-time PCR and RFLP assay consistently yielded the same results, although the real-time assay was more sensitive and less time consuming. Phylogenetic analysis of the mtCOI DNA sequences corroborated the identity of the B and Q biotypes 100% of the time and by phylogenetic analysis the haplotypes grouped, as expected, in the major North African-Mediterranean-Middle Eastern clade of the B. tabaci complex.

The role of specific tomato volatiles in tomato-whitefly interaction

Bemisia tabaci (whitefly) infestations and the subsequent transfer of viruses are the cause of severe losses in crop production and horticultural practice. To improve biological control of B. tabaci, we investigated repellent properties of plant-produced semiochemicals. The mix of headspace volatiles, collected from naturally repellent wild tomato accessions, influenced B. tabaci initial choice behavior, indicating a role for plant semiochemicals in locating host plants. A collection of wild tomato accessions and introgression lines (Solanum pennellii LA716 x Solanum lycopersicum 'Moneyberg') were extensively screened for attractiveness to B. tabaci, and their headspace profiles were determined by means of gas chromatography-mass spectrometry. Correlation analysis revealed that several terpenoids were putatively involved in tomato-whitefly interactions. Several of these candidate compounds conferred repellence to otherwise attractive tomato plants when applied to the plant's branches on paper cards. The sesquiterpenes zingiberene and curcumene and the monoterpenes p-cymene, alpha-terpinene, and alpha-phellandrene had the strongest effects in free-choice bioassays. These terpenes also elicited a response of receptors on the insect's antennae as determined by electroantennography. Conversely, the monoterpene beta-myrcene showed no activity in both assays. B. tabaci apparently uses, besides visual cues, specific plant volatile cues for the initial selection of a host. Altering whitefly choice behavior by manipulation of the terpenoid composition of the host headspace may therefore be feasible.

Over-expression of cytochrome P450 CYP6CM1 is associated with high resistance to imidacloprid in the B and Q biotypes of Bemisia tabaci (Hemiptera: Aleyrodidae)

The two most damaging biotypes of Bemisia tabaci, B and Q, have both evolved strong resistance to the neonicotinoid insecticide imidacloprid. The major mechanism in all samples investigated so far appeared to be enhanced detoxification by cytochrome P450s monooxygenases (P450s). In this study, a polymerase chain reaction (PCR) technology using degenerate primers based on conserved P450 helix I and heme-binding regions was employed to identify P450 cDNA sequences in B. tabaci that might be involved in imidacloprid resistance. Eleven distinct P450 cDNA sequences were isolated and classified as members of the CYP4 or CYP6 families. The mRNA expression levels of all 11 genes were compared by real-time quantitative RT-PCR across nine B and Q field-derived strains of B. tabaci showing strong resistance, moderate resistance or susceptibility to imidacloprid. We found that constitutive over-expression (up to approximately 17-fold) of a single P450 gene, CYP6CM1, was tightly related to imidacloprid resistance in both the B and Q biotypes. Next, we identified three single-nucleotide polymorphic (SNP) markers in the intron region of CYP6CM1 that discriminate between the resistant and susceptible Q-biotype CYP6CM1 alleles (r-Q and s-Q, respectively), and used a heterogeneous strain to test for association between r-Q and resistance. While survivors of a low imidacloprid dose carried both the r-Q and s-Q alleles, approximately 95% of the survivors of a high imidacloprid dose carried only the r-Q allele. Together with previous evidence, the results reported here identify enhanced activity of P450s as the major mechanism of imidacloprid resistance in B. tabaci, and the CYP6CM1 gene as a leading target for DNA-based screening for resistance to imidacloprid and possibly other neonicotinoids in field populations.

High-throughput allelic discrimination of B and Q biotypes of the whitefly, Bemisia tabaci, using TaqMan allele-selective PCR

BACKGROUND

B and Q biotypes of the whitefly, Bemisia tabaci (Gennadius), are generally regarded as the most significant given their global distribution and strong resistance to insecticides. Since these biotypes can coexist and differ markedly in their insecticide resistance profiles, a rapid but reliable means of discriminating between them would be a valuable complement to resistance monitoring and management programmes. Recently, PCR-based methods have been developed to determine the biotype status of B. tabaci populations. However, these require post-amplification procedures, which increase time and labour.

RESULTS

The authors have developed an allelic discrimination real-time PCR assay using fluorescent dye-labelled probes to distinguish the B and Q biotypes. The assay targets a single nucleotide polymorphism (SNP) in the mitochondrial cytochrome oxidase I (mtCOI) gene. To evaluate the assay, DNA was extracted from individual whiteflies of six known biotype strains, and all scored correctly as either a B or Q biotype. As further validation, 72 individuals from field samples collected in different parts of the world were also tested by the assay. No failed reactions were observed, with all 72 samples scoring clearly as either the B or Q biotype.

CONCLUSION

The development of this rapid and high-throughput assay has important potential for routine monitoring of B and Q biotypes on ornamental plants and for the screening of B. tabaci populations in countries where these biotypes are not yet established.

Development of silverleaf assay, protein and nucleic acid-based diagnostic techniques for the quick and reliable detection and monitoring of biotype B of the whitefly, Bemisia tabaci (Gennadius)

The aim of this study was to develop and optimize silverleaf bioassay, esterase analysis and PCR-based techniques to distinguish quickly and reliably biotype B of the whitefly, Bemisia tabaci (Gennadius), from Indian indigenous biotypes. Zucchini and squash readily develop silverleaf symptoms upon feeding by the B biotype, but they are not readily available in Indian markets. A local pumpkin variety 'Big' was, therefore, used in silverleaf assay, which developed symptoms similar to those on zucchini and squash and can be used reliably to detect B biotype. Analysis of non-specific esterases of B and the indigenous biotypes indicated both quantitative and qualitative differences in esterase patterns. Two high molecular weight bands were unique to B biotype and they occurred in abundance. These esterases were used to develop quick and field-based novel detection methods for differentiating B from the indigenous biotypes. Development of these simple and cost-effective protocols has wider application as they can be potentially used to identify other agricultural pests. Mitochondrial cytochrome oxidase I gene sequences and randomly amplified polymorphic DNA (RAPD) polymorphisms, generated using the primer OpB11, were also found useful for detecting B. tabaci biotypes. A B biotype-specific RAPD band of 800 bp was sequenced, which was used to a develop sequence characterized amplified region (SCAR) marker. The SCAR marker involved the development of B biotype-specific primers that amplified 550 bp PCR products only from B biotype genomic DNA. Silverleaf assay, esterases, RAPDs or a SCAR marker were used in combination to analyse whitefly samples collected from selected locations in India, and it was found that any of these techniques can be used singly or in combination to detect B biotype reliably. The B biotype was found in southern parts of India but not in the north in 2004-06.

Evidence from molecular markers and population genetic analyses suggests recent invasions of the Western North Pacific region by biotypes B and Q of Bemisia tabaci (Gennadius)

Invasive events by Bemisia tabaci (Gennadius) biotypes in various parts of the world are of continuing interest. The most famous is biotype B that has caused great economic losses globally. In addition, biotype Q has also recently been reported to be a new invasive pest. These two biotypes have been monitored for some time in the Western North Pacific region, but the invasive events and population genetic structures of these two biotypes are still not clear in this region. In this study, the mitochondrial cytochrome oxidase I (COI) gene was used to reconstruct a phylogenetic tree for identifying biotypes B and Q and to study the relationships between invasive events and ornamental plants. Population genetic analyses of mtCOI sequences were also used to study the genetic relationships within and between populations. A combination of a phylogenetic tree and haplotype analysis suggested the recent invasion of biotype Q in this region is related to the international ornamental trade from the Mediterranean region. Low levels of haplotype diversity and nucleotide diversity indicate that the presence of biotypes B and Q in the Western North Pacific region are caused by multiple invasions. Hierarchical analysis of molecular variance supports the hypothesis of multiple invasions. In addition, high sequence identities and low genetic distances within and between populations of the two biotypes revealed that these invasive events occurred recently. The low levels of genetic differentiation revealed by pairwise F (ST) values between populations also suggests the invasions were recent. Therefore, results of this study suggested that biotypes B and Q entered this region through multiple recent invasions. A quarantine of agricultural crops may be necessary to prevent further invasions.

Reversal of resistance to pyriproxyfen in the Q biotype of Bemisia tabaci (Hemiptera: Aleyrodidae)

Pyriproxyfen, a juvenile hormone (JH) mimic, is a biorational insecticide that disrupts insect development. It is one of the principal insecticides being used to control Bemisia tabaci (Gennadius) on cotton, and has many environmentally positive attributes that make it compatible with integrated pest management (IPM) programs. In Israel, a high level of resistance to pyriproxyfen has been observed in several isolated regions. Here, tests were conducted to establish whether temporal refuges from exposure to pyriproxyfen could be useful for restoring the effectiveness of the compound. Resistance was found to decrease by a factor of 8 when exposure to pyriproxyfen was ceased for 13 generations. Reversal of resistance was accompanied with increased biotic fitness of the revertant colony. By incorporating experimental estimates of nymph survival, sex ratio, fecundity, egg hatching rate and developmental time, the seasonal cost per generation for resistant insects was estimated to be 25%. A genetic simulation model, optimized by empirical data from bioassays, predicted fitness cost per generation of 19% for resistant homozygous (RR) females and hemizygous (R) males, and produced rates of reversal similar to the experimental results. The model also predicted that, even after 5 years ( approximately 55 generations) without pyriproxyfen treatments, the frequency of the resistance allele (R) will still remain high (0.02). It is therefore concluded, on the basis of experimental and modeling results, that the effectiveness of temporal refuges for reversing development of resistance to pyriproxyfen in B. tabaci may be limited.

Biotype-dependent secondary symbiont communities in sympatric populations of Bemisia tabaci

The sweet potato whitefly, Bemisia tabaci, harbors Portiera aleyrodidarum, an obligatory symbiotic bacterium, as well as several secondary symbionts including Rickettsia, Hamiltonella, Wolbachia, Arsenophonus, Cardinium and Fritschea, the function of which is unknown. Bemisia tabaci is a species complex composed of numerous biotypes, which may differ from each other both genetically and biologically. Only the B and Q biotypes have been reported from Israel. Secondary symbiont infection frequencies of Israeli laboratory and field populations of B. tabaci from various host plants were determined by PCR, in order to test for correlation between bacterial composition to biotype and host plant. Hamiltonella was detected only in populations of the B biotype, while Wolbachia and Arsenophonus were found only in the Q biotype (33% and 87% infection, respectively). Rickettsia was abundant in both biotypes. Cardinium and Fritschea were not found in any of the populations. No differences in secondary symbionts were found among host plants within the B biotype; but within the Q biotype, all whiteflies collected from sage harboured both Rickettsia and Arsenophonus, an infection frequency which was significantly higher than those found in association with all other host plants. The association found between whitefly biotypes and secondary symbionts suggests a possible contribution of these bacteria to host characteristics such as insecticide resistance, host range, virus transmission and speciation.

SCAR molecular markers of the B biotype and two non-B populations of the whitefly, Bemisia tabaci (Hemiptera: Aleyrodidae)

Random amplified polymorphic DNA (RAPD) analyses were performed with random primer H16 for the B biotype and two non-B populations of Bemisia tabaci collected from Zhejiang (China). The specific sequence fragments containing 446, 390 and 1317 nucleotides were amplified for the B biotype, ZHJ-1, ZHJ-2 populations, respectively. The three specific fragments were cloned and sequenced, and three pairs of SCAR primers were designed according to the sequences determined. With improvement of the conditions of the polymerase chain reaction (PCR), the specific fragments of B biotype, ZHJ-1 and ZHJ-2 populations, namely 439, 366 and 1238 nucleotides, respectively, were amplified with the sequence characterized amplified region (SCAR) primer of the corresponding population, while specific fragments of the other populations of B. tabaci or Trialeurodes vaporariorum could not be amplified.

Biotype status and genetic polymorphism of the whitefly Bemisia tabaci (Hemiptera: Aleyrodidae) in Greece: mitochondrial DNA and microsatellites

The genetic polymorphism and the biotype identity of the tobacco whitefly Bemisia tabaci (Gennadius) have been studied in population samples taken from different localities within Greece from cultivated plants growing in greenhouses or in open environments and from non-cultivated plants. Two different approaches were used: sequencing of the mitochondrial cytochrome oxidase I (mtCOI) gene and genotyping using microsatellite markers. Analyses of the mtCOI sequences revealed a high homogeneity between the Greek samples which clustered together with Q biotype samples that had been collected from other countries. When genetic polymorphism was examined using six microsatellite markers, the Greek samples, which were all characterized as Q biotype were significantly differentiated from each other and clustered into at least two distinct genetic populations. Moreover, based on the fixed differences revealed by the mtCOI comparison of known B. tabaci biotype sequences, two diagnostic tests for discriminating between Q and B and non-Q/non-B biotypes were developed. Implementation of these diagnostic tools allowed an absence of the B biotype and presence of the Q biotype in the Greek samples to be determined.

Multiple origins of pyrethroid resistance in sympatric biotypes of Bemisia tabaci (Hemiptera: Aleyrodidae)

The two most damaging biotypes of Bemisia tabaci, B and Q, are sympatric in the Mediterranean basin and show high resistance to pyrethroids synergized by organophosphates. Previous work showed that in the B biotype, this resistance is associated with the L925I mutation in the para-type voltage gated sodium channel. Here we identified two mutations in the para-type voltage gated sodium channel associated with resistance to pyrethroids synergized by organophosphates in the Q biotype: the L925I mutation that occurs in the B biotype, and substitution of threonine to valine, at position 929 (T929V). To determine if the L925I and T929V mutations have single or multiple origins, we sequenced the DNA regions flanking the mutations from 13 B and Q strains collected worldwide. The survey identified five resistant alleles and five susceptible alleles. In the resistant alleles, the nucleotide diversity was low within biotypes (0.001), but high between biotypes (0.033). Nucleotide diversity in susceptible alleles was high between the two biotypes (0.028). These observations are consistent with multiple independent origins of resistance. Although the B and Q biotypes coexist in several regions of the Mediterranean basin, divergence in their DNA sequences at the para-type voltage gated sodium channel locus suggests gene flow between these biotypes is low or nil.