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.

Biology bacteriocyte-associated endosymbionts of plant sap-sucking insects

Psyllids, whiteflies, aphids, and mealybugs are members of the suborder Sternorrhyncha and share a common property, namely the utilization of plant sap as their food source. Each of these insect groups has an obligatory association with a different prokaryotic endosymbiont, and the association is the result of a single infection followed by maternal, vertical transmission of the endosymbionts. The result of this association is the domestication of the free-living bacterium to serve the purposes of the host, namely the synthesis of essential amino acids. This domestication is probably in all cases accompanied by a major reduction in genome size. The different properties of the genomes and fragments of the genomes of these endosymbionts suggest that there are different constraints on the permissible evolutionary changes that are probably a function of the gene repertoire of the endosymbiont ancestor and the gene losses that occurred during the reduction of genome size.

The 160-kilobase genome of the bacterial endosymbiont Carsonella

Previous studies have suggested that the minimal cellular genome could be as small as 400 kilobases. Here, we report the complete genome sequence of the psyllid symbiont Carsonella ruddii, which consists of a circular chromosome of 159,662 base pairs, averaging 16.5% GC content. It is by far the smallest and most AT-rich bacterial genome yet characterized. The genome has a high coding density (97%) with many overlapping genes and reduced gene length. Genes for translation and amino acid biosynthesis are relatively well represented, but numerous genes considered essential for life are missing, suggesting that Carsonella may have achieved organelle-like status.

A nicotinic acetylcholine receptor mutation conferring target-site resistance to imidacloprid in Nilaparvata lugens (brown planthopper)

Neonicotinoids, such as imidacloprid, are nicotinic acetylcholine receptor (nAChR) agonists with potent insecticidal activity. Since its introduction in the early 1990s, imidacloprid has become one of the most extensively used insecticides for both crop protection and animal health applications. As with other classes of insecticides, resistance to neonicotinoids is a significant threat and has been identified in several pest species, including the brown planthopper, Nilaparvata lugens, a major rice pest in many parts of Asia. In this study, radioligand binding experiments have been conducted with whole-body membranes prepared from imidacloprid-susceptible and imidacloprid-resistant strains of N. lugens. The results reveal a much higher level of [3H]imidacloprid-specific binding to the susceptible strain than to the resistant strain (16.7 +/- 1.0 and 0.34 +/- 0.21 fmol/mg of protein, respectively). With the aim of understanding the molecular basis of imidacloprid resistance, five nAChR subunits (Nlalpha1-Nlalpha4 and Nlbeta1) have been cloned from N. lugens.A comparison of nAChR subunit genes from imidacloprid-sensitive and imidacloprid-resistant populations has identified a single point mutation at a conserved position (Y151S) in two nAChR subunits, Nlalpha1 and Nlalpha3. A strong correlation between the frequency of the Y151S point mutation and the level of resistance to imidacloprid has been demonstrated by allele-specific PCR. By expression of hybrid nAChRs containing N. lugens alpha and rat beta2 subunits, evidence was obtained that demonstrates that mutation Y151S is responsible for a substantial reduction in specific [3H]imidacloprid binding. This study provides direct evidence for the occurrence of target-site resistance to a neonicotinoid insecticide.

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.

Knockdown of midgut genes by dsRNA-transgenic plant-mediated RNA interference in the hemipteran insect Nilaparvata lugens

Background

RNA interference (RNAi) is a powerful technique for functional genomics research in insects. Transgenic plants producing double-stranded RNA (dsRNA) directed against insect genes have been reported for lepidopteran and coleopteran insects, showing potential for field-level control of insect pests, but this has not been reported for other insect orders.

Methodology/Principal Findings

The Hemipteran insect brown planthopper (Nilaparvata lugens Stål) is a typical phloem sap feeder specific to rice (Oryza sativa L.). To analyze the potential of exploiting RNAi-mediated effects in this insect, we identified genes (Nlsid-1 and Nlaub) encoding proteins that might be involved in the RNAi pathway in N. lugens. Both genes are expressed ubiquitously in nymphs and adult insects. Three genes (the hexose transporter gene NlHT1, the carboxypeptidase gene Nlcar and the trypsin-like serine protease gene Nltry) that are highly expressed in the N. lugens midgut were isolated and used to develop dsRNA constructs for transforming rice. RNA blot analysis showed that the dsRNAs were transcribed and some of them were processed to siRNAs in the transgenic lines. When nymphs were fed on rice plants expressing dsRNA, levels of transcripts of the targeted genes in the midgut were reduced; however, lethal phenotypic effects after dsRNA feeding were not observed.

Conclusions

Our study shows that genes for the RNAi pathway (Nlsid-1 and Nlaub) are present in N. lugens. When insects were fed on rice plant materials expressing dsRNAs, RNA interference was triggered and the target genes transcript levels were suppressed. The gene knockdown technique described here may prove to be a valuable tool for further investigations in N. lugens. The results demonstrate the potential of dsRNA-mediated RNAi for field-level control of planthoppers, but appropriate target genes must be selected when designing the dsRNA-transgenic plants.

An extensive field survey combined with a phylogenetic analysis reveals rapid and widespread invasion of two alien whiteflies in China

BACKGROUND

To understand the processes of invasions by alien insects is a pre-requisite for improving management. The whitefly Bemisia tabaci is a cryptic species complex that contains some of the most invasive pests worldwide. However, extensive field data to show the geographic distribution of the members of this species complex as well as the invasion by some of its members are scarce.

METHODOLOGY/PRINCIPAL FINDINGS

We used field surveys and published data to assess the current diversity and distribution of B. tabaci cryptic species in China and relate the indigenous members to other Asian and Australian members of the complex. The survey covered the 16 provinces where indigenous B. tabaci occur and extends this with published data for the whole of China. We used molecular markers to identify cryptic species. The evolutionary relationships between the different Asian B. tabaci were reconstructed using Bayesian methods. We show that whereas in the past the exotic invader Middle East-Asia Minor 1 was predominant across China, another newer invader Mediterranean is now the dominant species in the Yangtze River Valley and eastern coastal areas, and Middle East-Asia Minor 1 is now predominant only in the south and south eastern coastal areas. Based on mtCO1 we identified four new cryptic species, and in total we have recorded 13 indigenous and two invasive species from China. Diversity was highest in the southern and southeastern provinces and declined to north and west. Only the two invasive species were found in the northern part of the country where they occur primarily in protected cropping. By 2009, indigenous species were mainly found in remote mountainous areas and were mostly absent from extensive agricultural areas.

CONCLUSIONS/SIGNIFICANCE

Invasions by some members of the whitefly B. tabaci species complex can be rapid and widespread, and indigenous species closely related to the invaders are replaced.

Global relationships of Bemisia tabaci (Hemiptera: Aleyrodidae) revealed using Bayesian analysis of mitochondrial COI DNA sequences

Bemisia tabaci (Gennadius) (Hemiptera: Aleyrodidae) is a species complex that is one of the most devastating agricultural pests worldwide and affects a broad range of food, fiber and ornamental crops. Unfortunately, using parsimony and neighbor joining methods, global phylogenetic relationships of the major races/biotypes of B. tabaci remain unresolved. Aside from the limitations of these methods, phylogenetic analyses have been limited to only small subsets of the global collection of B. tabaci, and thus limited taxon sampling has confounded the analyses. To improve our understanding of global B. tabaci phylogenetic relationships, a Bayesian phylogenetic technique was utilized to elucidate the relationships among all COI DNA sequence data available in GenBank for B. tabaci worldwide (366 specimens). As a result, the first well-resolved phylogeny for the B. tabaci species complex was produced showing 12 major well-resolved (0.70 posterior probability or above) genetic groups: B. tabaci (Mediterranean/Asia Minor/Africa), B. tabaci (Mediterranean), B. tabaci (Indian Ocean), B. tabaci (sub-Saharan Africa silverleafing), B. tabaci (Asia I), B. tabaci (Australia), B. tabaci (China), B. tabaci (Asia II), B. tabaci (Italy), B. tabaci (New World), B. tabaci (sub-Saharan Africa non-silverleafing) and B. tabaci (Uganda sweet potato). Further analysis of this phylogeny shows a close relationship of the New World B. tabaci with Asian biotypes, and characteristics of the major sub-Saharan Africa non-silverleafing clade strongly supports an African origin of B. tabaci due to its position at the base of the global phylogeny, and the diversity of well-resolved sub-clades within this group. Bayesian re-analyses of B. tabaci ITS, COI, and a combined dataset from a previous study resulted in seven major well-resolved races with high posterior probabilities, also showing the utility of the Bayesian method. Relationships of the 12 major B. tabaci genetic groups are discussed herein.

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.

Cospeciation of psyllids and their primary prokaryotic endosymbionts

Psyllids are plant sap-feeding insects that harbor prokaryotic endosymbionts in specialized cells within the body cavity. Four-kilobase DNA fragments containing 16S and 23S ribosomal DNA (rDNA) were amplified from the primary (P) endosymbiont of 32 species of psyllids representing three psyllid families and eight subfamilies. In addition, 0.54-kb fragments of the psyllid nuclear gene wingless were also amplified from 26 species. Phylogenetic trees derived from 16S-23S rDNA and from the host wingless gene are very similar, and tests of compatibility of the data sets show no significant conflict between host and endosymbiont phylogenies. This result is consistent with a single infection of a shared psyllid ancestor and subsequent cospeciation of the host and the endosymbiont. In addition, the phylogenies based on DNA sequences generally agreed with psyllid taxonomy based on morphology. The 3' end of the 16S rDNA of the P endosymbionts differs from that of other members of the domain Bacteria in the lack of a sequence complementary to the mRNA ribosome binding site. The rate of sequence change in the 16S-23S rDNA of the psyllid P endosymbiont was considerably higher than that of other bacteria, including other fast-evolving insect endosymbionts. The lineage consisting of the P endosymbionts of psyllids was given the designation Candidatus Carsonella (gen. nov.) with a single species, Candidatus Carsonella ruddii (sp. nov.).

Feeding-based RNA interference of a trehalose phosphate synthase gene in the brown planthopper, Nilaparvata lugens

The brown planthopper, Nilaparvata lugens, is the most devastating rice insect pest to have given rise to an outbreak in recent years. RNA interference (RNAi) is a technological breakthrough that has been developed as a powerful tool for studying gene function and for the highly targeted control of insect pests. Here, we examined the effects of using a feeding-based RNAi technique to target the gene trehalose phosphate synthase (TPS) in N. lugens. The full-length cDNA of N. lugens TPS (NlTPS) is 3235 bp and has an open reading frame of 2424 bp, encoding a protein of 807 amino acids. NlTPS was expressed in the fat body, midgut and ovary. Quantitative real-time PCR (qRT-PCR) analysis revealed that NlTPS mRNA is expressed continuously with little change during the life of the insect. Efficient silencing of the TPS gene through double-stranded RNA (dsRNA) feeding led to rapid and significant reduction levels of TPS mRNA and enzymatic activity. Additionally, the development of N. lugens larvae that had been fed with the dsRNA was disturbed, resulting in lethality, and the cumulative survival rates dropped to 75.56, 64.44, 55.56 and 40.00% after continuous ingestion of 0.5 µg/µl dsRNA for 2, 4, 7 and 10 days, respectively. These values were significantly lower than those of the insects in the control group, suggesting that NlTPS dsRNA may be useful as a means of insect pest control.

Biotypes B and Q of Bemisia tabaci and their relevance to neonicotinoid and pyriproxyfen resistance

Resistance monitoring for Bemisia tabaci field populations to the juvenile hormone mimic, pyriproxyfen, was conducted from 1996 to 2003 in commercial cotton fields in two areas of Israel: the Ayalon Valley (central Israel) and the Carmel Coast (northwestern Israel). Although the use of pyriproxyfen ceased in these areas in 1996-1997 (because of the resistance), resistance levels to pyriproxyfen declined to some extent in the fields but remained quite stable, and the susceptibility has not been totally restored. Two strains of B. tabaci collected from the Ayalon Valley in the late 1999 and 2002 cotton seasons (AV99L, AV02L) were assayed for their susceptibility to pyriproxyfen at F1, and subsequently a line of each strain was kept under controlled conditions without exposure to insecticides. After maintenance of more than 20 generations under laboratory conditions, the resistance to pyriproxyfen in the untreated strains substantially declined. This decline was concurrent with a replacement of Q biotype by B-type under non-insecticidal regimes; apparently B biotype was more competitive than the pyriproxyfen-resistant Q-type. Selection under controlled conditions with neonicotinoids on these B. tabaci strains resulted in continued pyriproxyfen resistance, predominantly of Q biotype. Based on our data, applications of either pyriproxyfen or neonicotinoids may select for biotype Q, which would survive to a greater degree where these insecticides are applied.

Evolutionary relationships of primary prokaryotic endosymbionts of whiteflies and their hosts

Whiteflies (Hemiptera: Sternorrhyncha: Aleyrodidae) are plant sap-sucking insects that harbor prokaryotic primary endosymbionts (P-endosymbionts) within specialized cells located in their body cavity. Four-kilobase DNA fragments containing 16S-23S ribosomal DNA (rDNA) were amplified from the P-endosymbiont of 24 whiteflies from 22 different species of 2 whitefly subfamilies. In addition, 3-kb DNA fragments containing mitochondrial cytB, nd1, and large-subunit rDNA (LrDNA) were amplified from 17 whitefly species. Comparisons of the P-endosymbiont (16S-23S rDNA) and host (cytB-nd1-LrDNA) phylogenetic trees indicated overall congruence consistent with a single infection of a whitefly ancestor with a bacterium and subsequent cospeciation (cocladogenesis) of the host and the P-endosymbiont. On the basis of both the P-endosymbiont and host trees, the whiteflies could be subdivided into at least five clusters. The major subdivision was between the subfamilies Aleyrodinae and Aleurodicinae. Unlike the P-endosymbionts of may other insects, the P-endosymbionts of whiteflies were related to Pseudomonas and possibly to the P-endosymbionts of psyllids. The lineage consisting of the P-endosymbionts of whiteflies is given the designation "Candidatus Portiera" gen. nov., with a single species, "Candidatus Portiera aleyrodidarum" sp. nov.

Gene knockdown by intro-thoracic injection of double-stranded RNA in the brown planthopper, Nilaparvata lugens

RNA interference (RNAi) is a powerful strategy for gene function study in insects. Here, we described the development of a RNAi technique by microinjection of double-stranded RNA (dsRNA) in the brown planthopper Nilaparvata lugens. Based on the mortality and RNAi efficiency criteria, the conjunctive between prothorax and mesothorax was selected as the injection site and 50 nl as injection volume. Three genes with different expression patterns were selected to evaluate the RNAi efficiency. A comparable 40% decrease of gene expression was observed at the 4th day after injection for the ubiquitously expressed calreticulin and the gut specific cathepsin-B genes, but only 25% decrease at the 5th day for the central nervous system specific Nlbeta2 gene. Double injection could increase the RNAi efficiency, such as from 25% to 53% for Nlbeta2 gene. The gene knockdown technique developed in this study will be an essential post-genomic tool for further investigations in N. lugens.

Molecular markers for the identification and global tracking of whitefly vector-Begomovirus complexes

Recent unprecedented upsurges in populations of the whitefly Bemisia tabaci (Genn.) have drawn much attention to its worldwide importance as an insect pest and as the vector of emergent begomoviruses (Family: Geminiviridae; Genus: Begomovirus). Several begomoviruses that are considered 'new' and others previously regarded as minor pathogens have been linked to recent epidemics. Recent studies have revealed much variation in begomoviruses, despite the view that DNA-containing viruses do not rapidly accumulate mutations. Also, certain B. tabaci 'variants' are known that more effectively or selectively transmit certain begomoviruses and exhibit biotic differences that may influence their spread. Patterns of distribution and dissemination of begomoviruses transmitted by B. tabaci are poorly understood because standardized molecular-based tracking methods have not been available. Understanding virus/whitefly vector/host plant interrelationships in the context of emerging problems can be achieved only by linking predicted evolutionary histories with epidemiology using molecular phylogenetic approaches. Identification and validation of informative molecular sequences are essential initial steps in this process. Genus-wide degenerate polymerase chain reaction (PCR) primers have been developed to amplify and sequence the 'core' region of the coat protein open reading frame (ORF) (V1), permitting 'universal' detection and provisional virus identification by comparisons with described viral genotypes. In subsequent studies reported here, several potentially informative viral ORFs and a non-coding region are explored. Of particular use for expanding diversity studies are group- or virus-specific sequences that can be targeted by utilizing newly available core CP sequences, or additional conserved regions around which broad spectrum primers can be designed to target variable sequences in key ORFs or non-coding regions. Prospective markers under exploration were selected with a basis in the most highly conserved viral ORFs, CP (V1) and a portion of replication-associated protein (REP) (L1/C1), and a key non-coding sequence that contain sufficient variability and/or virus-specific sequences, and are consequently of potential epidemiological relevance. Because B. tabaci occurs as a cryptic species, or species complex, that exhibits biotic polymorphism, yet morphological invariance, traditional morphologically based identification is impossible. An overriding complication to establishing molecular markers for identifying whitefly vector variants is that whitefly sequences in general, have not been available. However, recent work has shown that a partial mitochondria cytochrome oxidase I (mt COI) sequence separates vector variants with a basis in geographical origin, suggesting it is useful for further exploring variability and the phylogenetic history of whiteflies on a large scale. Here, the utility of whitefly mt COI nucleotides (nt) sequences is illustrated for inferring relationships between B. tabaci collected from major world regions. Used collectively, these approaches permit investigations of the patterns of distribution and dissemination of begomovirus-whitefly vector complexes for the first time. Ultimately, more immediate recognition of exotic viruses and whitefly vectors and early detection of upsurges in vector populations and of emerging viruses will be possible.

Structural model and functional characterization of the Bemisia tabaci CYP6CM1vQ, a cytochrome P450 associated with high levels of imidacloprid resistance

The neonicotinoid imidacloprid is one of the most important insecticides worldwide. It is used extensively against the whitefly Bemisia tabaci (Hemiptera: Aleyrodidae), an insect pest of eminent importance globally, which was also the first pest to develop high levels of resistance against imidacloprid and other neonicotinoids in the field. Recent reports indicated that in both the B and Q biotypes of B. tabaci, the resistant phenotype is associated with over-expression of the cytochrome P450 gene CYP6CM1. In this study, molecular docking and dynamic simulations were used to analyze interactions of imidacloprid with the biotype Q variant of the CYP6CM1 enzyme (CYP6CM1vQ). The binding mode with the lowest energy in the enzyme active site, the key amino acids involved (i.e. Phe-130 and Phe-226), and the putative hydroxylation site (lowest distance to carbon 5 of the imidazolidine ring system of imidacloprid) were predicted. Heterologous expression of the CYP6CM1vQ confirmed the accuracy of our predictions and demonstrated that the enzyme catalyses the hydroxylation of imidacloprid to its less toxic 5-hydroxy form (K(cat) = 3.2 pmol/min/pmol P450, K(m) = 36 microM). The data identify CYP6CM1vQ as a principle target for inhibitor design, aimed at inactivating insecticide-metabolizing P450s in natural insect pest populations.

Phytoplasma effector SAP54 induces indeterminate leaf-like flower development in Arabidopsis plants

Phytoplasmas are insect-transmitted bacterial plant pathogens that cause considerable damage to a diverse range of agricultural crops globally. Symptoms induced in infected plants suggest that these phytopathogens may modulate developmental processes within the plant host. We report herein that Aster Yellows phytoplasma strain Witches' Broom (AY-WB) readily infects the model plant Arabidopsis (Arabidopsis thaliana) ecotype Columbia, inducing symptoms that are characteristic of phytoplasma infection, such as the production of green leaf-like flowers (virescence and phyllody) and increased formation of stems and branches (witches' broom). We found that the majority of genes encoding secreted AY-WB proteins (SAPs), which are candidate effector proteins, are expressed in Arabidopsis and the AY-WB insect vector Macrosteles quadrilineatus (Hemiptera; Cicadellidae). To identify which of these effector proteins induce symptoms of phyllody and virescence, we individually expressed the effector genes in Arabidopsis. From this screen, we have identified a novel AY-WB effector protein, SAP54, that alters floral development, resulting in the production of leaf-like flowers that are similar to those produced by plants infected with this phytoplasma. This study offers novel insight into the effector profile of an insect-transmitted plant pathogen and reports to our knowledge the first example of a microbial pathogen effector protein that targets flower development in a host.

Overexpression of a cytochrome P450 monooxygenase, CYP6ER1, is associated with resistance to imidacloprid in the brown planthopper, Nilaparvata lugens

The brown planthopper, Nilaparvata lugens, is an economically significant pest of rice throughout Asia and has evolved resistance to many insecticides including the neonicotinoid imidacloprid. The resistance of field populations of N. lugens to imidacloprid has been attributed to enhanced detoxification by cytochrome P450 monooxygenases (P450s), although, to date, the causative P450(s) has (have) not been identified. In the present study, biochemical assays using the model substrate 7-ethoxycoumarin showed enhanced P450 activity in several resistant N. lugens field strains when compared with a susceptible reference strain. Thirty three cDNA sequences encoding tentative unique P450s were identified from two recent sequencing projects and by degenerate PCR. The mRNA expression level of 32 of these was examined in susceptible, moderately resistant and highly resistant N. lugens strains using quantitative real-time PCR. A single P450 gene (CYP6ER1) was highly overexpressed in all resistant strains (up to 40-fold) and the level of expression observed in the different N. lugens strains was significantly correlated with the resistance phenotype. These results provide strong evidence for a role of CYP6ER1 in the resistance of N. lugens to imidacloprid.

Hemipterans as plant pathogens

Integration of the tools of genetics, genomics, and biochemistry has provided new approaches for identifying genes responding to herbivory. As a result, a picture of the complexity of plant-defense signaling to different herbivore feeding guilds is emerging. Plant responses to hemipteran insects have substantial overlap with responses mounted against microbial pathogens, as seen in changes in RNA profiles and emission of volatiles. Responses to known defense signals and characterization of the signaling pathways controlled by the first cloned insect R gene (Mi-1) indicate that perception and signal transduction leading to resistance may be similar to plant-pathogen interactions. Additionally, novel signaling pathways are emerging as important components of plant defense to insects. The availability of new tools and approaches will further enhance our understanding of the nature of defense in plant-hemipteran interactions.

Identification of biochemical markers linked to neonicotinoid cross resistance in Bemisia tabaci (Hemiptera: Aleyrodidae)

The whitefly, Bemisia tabaci (Hemiptera: Aleyrodidae), is a serious pest in many cropping systems world-wide and occurs in different biotypes. The most widespread one is the B-type, whereas the Q-biotype is nowadays still mostly restricted to Southern Spain. Neonicotinoid cross-resistance is known at a high level in Q-types from Spain and individual samples collected in Italy and Germany. Now we detected for the first time high neonicotinoid cross-resistance in a B-type from Israel. Target site resistance to imidacloprid using [(3)H]imidacloprid in nicotinic acetylcholine receptor (nAChR) binding assays could not be detected in any of these highly resistant strains. The impact of metabolizing enzymes such as esterases, glutathione S-transferases, and cytochrome P450-dependent monooxygenases in neonicotinoid resistance was studied biochemically with artificial substrates. Monooxygenase activity was increased 2-3-fold in moderately resistant strains (RF approximately 30) and even 5-6-fold in highly resistant strains (RF approximately 1,000). Only monooxygenase activity correlated with imidacloprid, thiamethoxam and acetamiprid resistance and, therefore, monooxygenases seem to be the only enzyme system responsible for neonicotinoid resistance in B. tabaci Q- and B-types. The oxidative degradation of imidacloprid in resistant Q-type strains could be confirmed by metabolism studies of [(14)C]imidacloprid in vivo. Five-hydroxy-imidacloprid could be detected as the only main metabolite. The insecticidal activity and binding affinity to nAChR of this compound was 10 times lower than imidacloprid itself in B. tabaci.

Odorant binding protein diversity and distribution among the insect orders, as indicated by LAP, an OBP-related protein of the true bug Lygus lineolaris (Hemiptera, Heteroptera)

Insect odorant binding proteins (OBPs) are thought to deliver odors to olfactory receptors, and thus may be the first biochemical step in odor reception capable of some level of odor discrimination. OBPs have been identified from numerous species of several insect orders, including Lepidoptera, Diptera, Coleoptera and Hymenoptera; all are holometabolous insects belonging to the monophyletic division of insects known as the Endopterygota. Recently, an antennal protein with OBP-like properties was identified from Lygus lineolaris, a hemipteran insect representing the Hemipteroid Assemblage, a sister division to the Endopterygota. The full length sequence of Lygus antennal protein (LAP) is presented in this report. In situ hybridization analysis revealed LAP expression in cell clusters associating with olfactory sensilla; expression was adult-specific, initiating in developing adult tissue during the transitional period that precedes the actual adult molt. Sequence analysis confirmed that LAP is homologous with the OBP-related protein family, and most similar to the OS-E and OS-F proteins of Drosophila, the ABPX proteins of Lepidoptera and the OBPRP proteins of the Coleoptera. Assuming that the OBP-related proteins represent one homologous family, the identification of LAP significantly expands the phylogenetic depth of that family and its underlying role in odor detection to encompass all members of the Endopterygota and Hemipteroid Assemblage, which comprise >90% of all insect species.

Organization of the mitochondrial genomes of whiteflies, aphids, and psyllids (Hemiptera, Sternorrhyncha)

Background

With some exceptions, mitochondria within the class Insecta have the same gene content, and generally, a similar gene order allowing the proposal of an ancestral gene order. The principal exceptions are several orders within the Hemipteroid assemblage including the order Thysanoptera, a sister group of the order Hemiptera. Within the Hemiptera, there are available a number of completely sequenced mitochondrial genomes that have a gene order similar to that of the proposed ancestor. None, however, are available from the suborder Sternorryncha that includes whiteflies, psyllids and aphids.

Results

We have determined the complete nucleotide sequence of the mitochondrial genomes of six species of whiteflies, one psyllid and one aphid. Two species of whiteflies, one psyllid and one aphid have mitochondrial genomes with a gene order very similar to that of the proposed insect ancestor. The remaining four species of whiteflies had variations in the gene order. In all cases, there was the excision of a DNA fragment encoding for cytochrome oxidase subunit III(COIII)-tRNA^gly-NADH dehydrogenase subunit 3(ND3)-tRNA^ala-tRNA^arg-tRNA^asn from the ancestral position between genes for ATP synthase subunit 6 and NADH dehydrogenase subunit 5. Based on the position in which all or part of this fragment was inserted, the mitochondria could be subdivided into four different gene arrangement types. PCR amplification spanning from COIII to genes outside the inserted region and sequence determination of the resulting fragments, indicated that different whitefly species could be placed into one of these arrangement types. A phylogenetic analysis of 19 whitefly species based on genes for mitochondrial cytochrome b, NADH dehydrogenase subunit 1, and 16S ribosomal DNA as well as cospeciating endosymbiont 16S and 23S ribosomal DNA indicated a clustering of species that corresponded to the gene arrangement types.

Conclusions

In whiteflies, the region of the mitochondrial genome consisting of genes encoding for COIII-tRNA^gly-ND3-tRNA^ala-tRNA^arg-tRNA^asn can be transposed from its ancestral position to four different locations on the mitochondrial genome. Related species within clusters established by phylogenetic analysis of host and endosymbiont genes have the same mitochondrial gene arrangement indicating a transposition in the ancestor of these clusters.

Reference gene selection for qRT-PCR analysis in the sweetpotato whitefly, Bemisia tabaci (Hemiptera: Aleyrodidae)

BACKGROUND

Accurate evaluation of gene expression requires normalization relative to the expression of reliable reference genes. Expression levels of "classical" reference genes can differ, however, across experimental conditions. Although quantitative real-time PCR (qRT-PCR) has been used extensively to decipher gene function in the sweetpotato whitefly Bemisia tabaci, a world-wide pest in many agricultural systems, the stability of its reference genes has rarely been validated.

RESULTS

In this study, 15 candidate reference genes from B. tabaci were evaluated using two Excel-based algorithms geNorm and Normfinder under a diverse set of biotic and abiotic conditions. At least two reference genes were selected to normalize gene expressions in B. tabaci under experimental conditions. Specifically, for biotic conditions including host plant, acquisition of a plant virus, developmental stage, tissue (body region of the adult), and whitefly biotype, ribosomal protein L29 was the most stable reference gene. In contrast, the expression of elongation factor 1 alpha, peptidylprolyl isomerase A, NADH dehydrogenase, succinate dehydrogenase complex subunit A and heat shock protein 40 were consistently stable across various abiotic conditions including photoperiod, temperature, and insecticide susceptibility.

CONCLUSION

Our finding is the first step toward establishing a standardized quantitative real-time PCR procedure following the MIQE (Minimum Information for publication of Quantitative real time PCR Experiments) guideline in an agriculturally important insect pest, and provides a solid foundation for future RNA interference based functional study in B. tabaci.

Combined data, Bayesian phylogenetics, and the origin of the New Zealand cicada genera

We have applied Bayesian and maximum likelihood methods of phylogenetic estimation to data from four mitochondrial genes (COI, COII, 12S, and 16S) and a single nuclear gene (EF1alpha) from several genera of New Zealand, Australian, and New Caledonian cicada taxa. We specifically focused on the heterogeneity of phylogenetic signal among the different data partitions and the biogeographic origins of the New Zealand cicada fauna. The Bayesian analyses circumvent many of the problems associated with other statistical tests for comparing data partitions. We took an information-theoretic approach to model selection based on the Akaike Information Criterion (AIC). This approach indicated that there was considerable uncertainty in identifying the best-fit model for some of the partitions. Additionally, a large amount of uncertainty was associated with many parameter estimates from the substitution model. However, a sensitivity analysis on the combined dataset indicated that the model selection uncertainty had little effect on estimates of topology because these estimates were largely insensitive to changes in the assumed model. This outcome suggests strong signal in our data. Our analyses support a New Caledonian affiliation of the New Zealand cicada genera Maoricicada, Kikihia, and Rhodopsalta and Australian affinities for the genera Amphipsalta and Notopsalta. This result was surprising, given that previous cicada biologists suspected a close relationship between Amphipsalta, Notopsalta, and Rhodopsalta based on genitalic characters. Relationships among the closely related genera Maoricicada, Kikihia, and Rhodopsalta were poorly resolved, the mitochondrial data and the EF1alpha data favoring different arrangements within this clade.

Differentiating between hypotheses of lineage sorting and introgression in New Zealand alpine cicadas (Maoricicada Dugdale)

Lineage sorting and introgression can lead to incongruence among gene phylogenies, complicating the inference of species trees for large groups of taxa that have recently and rapidly radiated. In addition, it can be difficult to determine which of these processes is responsible for this incongruence. We explore these issues with the radiation of New Zealand alpine cicadas of the genus Maoricicada Dugdale. Gene trees were estimated from four putative independent loci: mitochondrial DNA (2274 nucleotides), elongation factor 1-alpha (1275 nucleotides), period (1709 nucleotides), and calmodulin (678 nucleotides). We reconstructed phylogenies using maximum likelihood and Bayesian methods from 44 individuals representing the 19 species and subspecies of Maoricicada and two outgroups. Species-level relationships were reconstructed using a novel extension of gene tree parsimony, whereby gene trees were weighted by their Bayesian posterior probabilities. The inferred gene trees show marked incongruence in the placement of some taxa, especially the enigmatic forest and scrub dwelling species, M. iolanthe. Using the species tree estimated by gene tree parsimony, we simulated coalescent gene trees in order to test the null hypothesis that the nonrandom placement of M. iolanthe among gene trees has arisen by chance. Under the assumptions of constant population size, known generation time, and panmixia, we were able to reject this null hypothesis. Furthermore, because the two alternative placements of M. iolanthe are in each case with species that share a similar song structure, we conclude that it is more likely that an ancient introgression event rather than lineage sorting has caused this incongruence.