The oriental fruit fly, Bactrocera dorsalis, in China: origin and gradual inland range expansion associated with population growth

The Global Epidemic of Bactrocera Pests: Mixed-Species Invasions and Risk Assessment

Throughout the past century, the global spread of Bactrocera pests has continued to pose a significant threat to the commercial fruit and vegetable industry, resulting in substantial costs associated with both control measures and quarantine restrictions. The increasing volume of transcontinental trade has contributed to an escalating rate of Bactrocera pest introductions to new regions. To address the worldwide threat posed by this group of pests, we first provide an overview of Bactrocera. We then describe the global epidemic, including border interceptions, species diagnosis, population genetics, geographical expansion, and invasion tracing of Bactrocera pests. We further consider the literature concerning the invasion co-occurrences, life-history flexibility, risk assessment, bridgehead effects, and ongoing implications of invasion recurrences, as well as a case study of Bactrocera invasions of California. Finally, we call for global collaboration to effectively monitor, prevent, and control the ongoing spread of Bactrocera pests and to share experience and knowledge to combat it.

Genomes of the cosmopolitan fruit pest Bactrocera dorsalis (Diptera: Tephritidae) reveal its global invasion history and thermal adaptation

INTRODUCTION

The oriental fruit fly Bactrocera dorsalis is one of the most destructive agricultural pests worldwide, with highly debated species delimitation, origin, and global spread routes.

OBJECTIVES

Our study intended to (i) resolve the taxonomic uncertainties between B. dorsalis and B. carambolae, (ii) reveal the population structure and global invasion routes of B. dorsalis across Asia, Africa, and Oceania, and (iii) identify genomic regions that are responsible for the thermal adaptation of B. dorsalis.

METHODS

Based on a high-quality chromosome-level reference genome assembly, we explored the population relationship using a genome-scale single nucleotide polymorphism dataset generated from the resequencing data of 487 B. dorsalis genomes and 25 B. carambolae genomes. Genome-wide association studies and silencing using RNA interference were used to identify and verify the candidate genes associated with extreme thermal stress.

RESULTS

We showed that B. dorsalis originates from the Southern India region with three independent invasion and spread routes worldwide: (i) from Northern India to Northern Southeast Asia, then to Southern Southeast Asia; (ii) from Northern India to Northern Southeast Asian, then to China and Hawaii; and (iii) from Southern India toward the African mainland, then to Madagascar, which is mainly facilitated by human activities including trade and immigration. Twenty-seven genes were identified by a genome-wide association study to be associated with 11 temperature bioclimatic variables. The Cyp6a9 gene may enhance the thermal adaptation of B. dorsalis and thus boost its invasion, which tended to be upregulated at a hardening temperature of 38 °C. Functional verification using RNA interference silencing against Cyp6a9, led to the specific decrease in Cyp6a9 expression, reducing the survival rate of dsRNA-feeding larvae exposed to extreme thermal stress of 45 °C after heat hardening treatments in B. dorsalis.

CONCLUSION

This study provides insights into the evolutionary history and genetic basis of temperature adaptation in B. dorsalis.

HCN-induced embryo arrest: passion fruit as an ecological trap for fruit flies

BACKGROUND

Fruit flies are important economic pests of fruits, vegetables, and nuts all over the world. In this study, a permanent ecological trap, which was created by the ovicidal effect of phytogenic hydrogen cyanide (HCN) liberated from passion fruits due to oviposition by fruit flies and can be used in the pest management, were determined.

RESULTS

Observation of fruit fly eggs in Passiflora within the passion fruit cultivation region in southern China, from Aug 2019 to Oct 2020 showed that the exotic Passiflora attracted the native fruit flies to oviposit, but the eggs could not hatch. Using classical staging to categorize embryonic development and fumigation assays, we show that oviposition by fruit fly on passion fruits, release HCN from the cyanogenic mesocarp. Exposure of the eggs to HCN causes arrest of embryonic development and finally the death of eggs.

CONCLUSION

Our results reveal that the life cycle of fruit fly in Passiflora is interrupted at the egg stage. Consequently, we predict that this ecological trap may be permanent. Extensive cultivation of the Passiflora vine as a dead-end trap crop may be an effective avenue to reduce populations of fruit fly pests. © 2023 Society of Chemical Industry.

Yeast mediates the interspecific interaction between introduced Bactrocera dorsalis and indigenous Bactrocera minax

BACKGROUND

Host plant-microbe associations mediate interspecific interactions amongst herbivorous insects. However, this theory has rarely been ecologically verified in tephritid fruit flies. Research on this subject can not only help predict tephritid species invasion and occurrence patterns, but also develop potential novel lures for the control of the tephritid fruit fly pests. Recently, we observed mixed infestation of Bactrocera minax and Bactrocera dorsalis larvae in citrus orchards, which prompted us to explore the underlying mechanism.

RESULTS

Following oviposition by B. minax, the yeast Pichia kluyveri translocated to and proliferated inside the citrus fruit. The level of d-limonene released from citrus fruits containing P. kluyveri was 27 times higher than that released from healthy fruits. Mature B. dorsalis females were attracted to d-limonene and oviposited into fruits previously infested by B. minax. Furthermore, the interspecific interaction between B. dorsalis and B. minax within the same fruit significantly decreased the number of surviving larvae and pupal weight in B. dorsalis, but its effect on B. minax was weaker.

CONCLUSION

In the studied interspecific interaction, B. minax occupies the dominant position, implying ecological significance for this species in terms of consolidating its own niche and inhibiting the invasion of exotic species. To our best knowledge, this is the first report from both ecological and physiological perspectives on a symbiotic yeast mediating the interaction between B. minax and B. dorsalis through altering fruit volatiles. © 2022 Society of Chemical Industry.

An odorant binding protein mediates Bactrocera dorsalis olfactory sensitivity to host plant volatiles and male attractant compounds

Odorant-binding proteins (OBPs) are believed to play critical roles in host-seeking behavior. However, little attention was paid to its different functions in male and female. The antenna-specific OBP gene from Bactrocera dorsalis, BdorOBP13, was cloned and its expression profile was examined. The results showed that BdorOBP13 was exclusively expressed in male and female adults, which exhibited a high transcript level in antennae. After injection of BdorOBP13 dsRNA, its transcript level in males and females decreased significantly. Electrophysiological responses of RNAi-injected flies to, methyl eugenol (male attractant) and γ-octalactone (female attractant) decreased significantly. However, no significant changes in the electrophysiological response were observed in RNAi-injected flies to benzothiazole, (+),dipentene, and ethyl tiglate. The behavioral bioassay showed that males treated with RNAi significantly reduced their preference to methyl eugenol, while RNAi-injected females showed a significantly lower preference to γ-octalactone, suggesting that BdorOBP13 may have different functions between males and females: it may be involved in the detection of methyl eugenol in males but is involved in the detection of γ-octalactone in females. These findings improve our understanding of insect OBPs and their roles in insect chemosensation, which may provide us with new molecular targets in the management of B. dorsalis.

Population Genetics for Inferring Introduction Sources of the Oriental Fruit Fly, Bactrocera dorsalis: A Test for Quarantine Use in Korea

To infer the introduction sources of the oriental fruit fly, Bactrocera dorsalis, we used a mitochondrial marker to reconstruct the haplotype network and 15 microsatellite loci to reveal genetic structure and relationships between the geographically or temporally different collections from Asia. We performed Approximate Bayesian computations to infer a global origin and a source of the quarantine collections found in Korea. As a result, the 40 populations were divided into three groups, of which genetic similarity is not related to the geographic vicinity. Korean samples had a similar genetic structure to Taiwan and Thailand ones. Our results suggest that the place of origin of the B. dorsalis specimens found in Korea's border quarantine is likely to be Taiwan or Thailand. As the global origin of B. dorsalis, we estimated that Taiwan and Thailand were most likely the global origins of Southeast Asian populations by testing hypothetical scenarios by the approximate Bayesian computation analyses. Our results will allow easier identification of the source region of the forthcoming invasion of quarantined B. dorsalis specimens.

Comparison of mitochondrial genome and development of specific PCR primers for identifying two scuticociliates, Pseudocohnilembus persalinus and Uronema marinum

Background

Pseudocohnilembus persalinus and Uronema marinum (Ciliophora, Scuticociliatia), as parasitic scuticociliatid ciliates, were isolated from Scophthalmus maximus and Takifugu rubripes, respectively, in our previous studies. These ciliates are morphologically very similar; hence, it is difficult to identify specific scuticociliate species using traditional classification methods for performing taxonomic research and disease control studies.

Methods

We annotated the mitochondrial genomes of these two scuticociliates on the basis of previous sequencing, including analyses of nucleotide composition, codon usage, Ka/Ks, and p-distance. We also compared the nucleotide and amino acid similarity of the mitochondrial genomes of P. persalinus, U. marinum, and other 12 related ciliates, and a phylogenetic tree was constructed using 16 common genes. We chose the nad4 and nad7 genes to design specific PCR primers for identification.

Results

P. persalinus and U. marinum were found to have a close evolutionary relationship. Although codon preferences were similar, differences were observed in the usage of codons such as CGA, CGC, and GTC. Both Ka/Ks and p-distance were less than 1. Except for yejR, ymf57, ymf67, and ymf75, the amino acid sequence similarity between P. persalinus and U. marinum was greater than 50%.

Conclusions

The mitochondrial genomes of P. persalinus and U. marinum were thoroughly compared to provide a reference for disease prevention and control. The specific PCR primers enabled us to identify P. persalinus and U. marinum rapidly and accurately at the molecular level, thus providing a basis for classification and identification.

Graphic abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s13071-021-04821-3.

Effects of male lure dispensers and trap types for monitoring of Ceratitis capitata and Bactrocera dorsalis (Diptera: Tephritidae)

BACKGROUND

Attractant-based trapping is used in the establishment of pest-free areas and areas of low pest prevalence for fruit flies (Diptera: Tephritidae). Male lures are commonly used attractants in fruit fly trapping. In this study, the effects of male lure dispensers, traps and combinations of dispensers and traps on monitoring of two fruit fly pests, Ceratitis capitata (Wiedemann) and Bactrocera dorsalis (Hendel), were investigated in South Africa.

RESULTS

In the male-lure-based trapping systems evaluated, trimedlure (TML) for C. capitata and methyl eugenol (ME) for B. dorsalis, the type of dispenser affected catches for both species. Higher catches of B. dorsalis males were recorded in bucket traps baited with a dispenser containing 15 g ME compared with traps baited with dispensers containing either 4 g ME or 2 g ME. Catches of C. capitata males were higher with dispensers containing TML than those with TML plus extender (Capilure®). The type of trap used with TML also influenced catches of C. capitata with higher numbers recorded in yellow Delta trap compared with the Sensus bucket trap.

CONCLUSIONS

Dispensers with higher ME loadings are more effective for monitoring of B. dorsalis. The yellow Delta trap baited with TML (without extender) would be an effective monitoring system for C. capitata. Fruit fly prevalence levels as determined by specific trapping systems should be related to their efficiency in terms of catches of the target pests. © 2020 Society of Chemical Industry.

Influence of inoculated gut bacteria on the development of Bactrocera dorsalis and on its susceptibility to the entomopathogenic fungus, Metarhizium anisopliae

Background

Symbiotic interactions between insects and bacteria have been associated with a vast variety of physiological, ecological and evolutionary consequences for the host. A wide range of bacterial communities have been found in association with the oriental fruit fly, Bactrocera dorsalis (Hendel) (Diptera: Tephritidae), an important pest of cultivated fruit in most regions of the world. We evaluated the diversity of gut bacteria in B. dorsalis specimens from several populations in Kenya and investigated the roles of individual bacterial isolates in the development of axenic (germ-free) B. dorsalis fly lines and their responses to the entomopathogenic fungus, Metarhizium anisopliae.

Results

We sequenced 16S rRNA to evaluate microbiomes and coupled this with bacterial culturing. Bacterial isolates were mono-associated with axenic B. dorsalis embryos. The shortest embryonic development period was recorded in flies with an intact gut microbiome while the longest period was recorded in axenic fly lines. Similarly, larval development was shortest in flies with an intact gut microbiome, in addition to flies inoculated with Providencia alcalifaciens. Adult B. dorsalis flies emerging from embryos that had been mono-associated with a strain of Lactococcus lactis had decreased survival when challenged with a standard dosage of M. anisopliae ICIPE69 conidia. However, there were no differences in survival between the germ-free lines and flies with an intact microbiome.

Conclusions

These findings will contribute to the selection of probiotics used in artificial diets for B. dorsalis rearing and the development of improved integrated pest management strategies based on entomopathogenic fungi.

Supplementary information

Supplementary information accompanies this paper at 10.1186/s12866-020-02015-y.

Gut microbiota promotes host resistance to low-temperature stress by stimulating its arginine and proline metabolism pathway in adult Bactrocera dorsalis

Gut symbiotic bacteria have a substantial impact on host physiology and ecology. However, the contribution of gut microbes to host fitness during long-term low-temperature stress is still unclear. This study examined the role of gut microbiota in host low-temperature stress resistance at molecular and biochemical levels in the oriental fruit fly Bactrocera dorsalis. The results showed that after the gut bacteria of flies were removed via antibiotic treatment, the median survival time was significantly decreased to approximately 68% of that in conventional flies following exposure to a temperature stress of 10°C. Furthermore, we found that Klebsiella michiganensis BD177 is a key symbiotic bacterium, whose recolonization in antibiotic treated (ABX) flies significantly extended the median survival time to 160% of that in the ABX control, and restored their lifespan to the level of conventional flies. Notably, the relative levels of proline and arginine metabolites were significantly downregulated by 34- and 10-fold, respectively, in ABX flies compared with those in the hemolymph of conventional flies after exposure to a temperature stress of 10°C whereas recolonization of ABX flies by K. michiganensis BD177 significantly upregulated the levels of proline and arginine by 13- and 10- fold, respectively, compared with those found in the hemolymph of ABX flies. qPCR analysis also confirmed that K. michiganensis-recolonized flies significantly stimulated the expression of transcripts from the arginine and proline metabolism pathway compared with the ABX controls, and RNAi mediated silencing of two key genes Pro-C and ASS significantly reduced the survival time of conventional flies, postexposure low-temperature stress. We show that microinjection of L-arginine and L-proline into ABX flies significantly increased their survival time following exposure to temperature stress of 10°C. Transmission electron microscopy (TEM) analysis further revealed that low-temperature stress caused severe destruction in cristae structures and thus resulted in abnormal circular shapes of mitochondria in ABX flies gut, while the recolonization of live K. michiganensis helped the ABX flies to maintain mitochondrial functionality to a normal status, which is important for the arginine and proline induction. Our results suggest that gut microbiota plays a vital role in promoting the host resistance to low-temperature stress in B. dorsalis by stimulating its arginine and proline metabolism pathway.

Assessment of Bactrocera dorsalis (Diptera: Tephritidae) Diets on Adult Fecundity and Larval Development: Insights Into Employing the Sterile Insect Technique

Bactrocera dorsalis (Hendel) is a notorious insect pest that attacks diverse vegetables and fruits worldwide. The sterile insect technique has been developed as an environmentally friendly and effective control method that depends on the mass production of target flies. Because dietary yeast (protein) and sucrose (carbohydrate) are important in adult diets, yeast:sucrose (Y:S) mixtures are crucial for the mass-rearing of B. dorsalis. In this study, we found adult diets with different ratios of yeast to sucrose-influenced fecundity, and an extremely high or low Y:S ratios significantly decreased egg production of B. dorsalis. Additionally, the maximum oviposition efficiency was realized at dietary yeast to sucrose ratios of 1:1 and 1:3, suggesting their potential use to produce more eggs for the mass production of B. dorsalis. Here, new gel diets having different yeast concentrations (g/L water) were also assessed for rearing B. dorsalis larvae. Gel diets containing 20 g/L yeast led to a higher pupation, pupal weight and adult eclosion rate, and a shorter developmental time than other yeast concentrations. Moreover, the present gel diet also resulted in greater pupal production and adult emergence rates than previously used liquid and solid artificial diets, revealing that it is suitable for rearing B. dorsalis larvae. This research provides a useful reference on artificial diets mixtures for mass rearing B. dorsalis, which is critical for employing the sterile insect technique.

Recent research status of Bactrocera dorsalis: Insights from resistance mechanisms and population structure

Bactrocera dorsalis (Hendel) is considered to be a highly invasive and destructive agricultural pest due to its strong dispersal and adaptive capacity. Rapid development of insecticide resistance poses a serious threat to the sustainable control of this pest. Here, the resistance mechanisms and invasion pathways of this fly are outlined for a better understanding of the resistance-gene flow pattern and invasion routes. We believe this microreview will provide a glimpse of the native regions, spread and management of resistance, and guide future work on these important topics.

Population Genetics of Bactrocera minax (Diptera: Tephritidae) in China Based on nad4 Gene Sequence

Bactrocera minax (Enderlein) (Diptera: Tephritidae) is an important citrus pest in Asia with a non-uniform distribution. In some locations, it had been reported to occur but was either eradicated or disappeared itself. To understand species dispersal of B. minax, we collected and analyzed 359 individuals from 18 localities in China. One mitochondrial DNA gene fragment (nad4) was used to investigate the genetic diversity and population genetic structure of B. minax. The populations were divided by phylogenetic analyses and statistical parsimony haplotype networks into three branches: a Central China (CC) branch, a Western China (WC) branch, and a Southern China (SC) branch. A total of 93 variable sites (15.6% of the 595 bp alignment) and 91 unique haplotypes were observed in the 359 individuals scored from the nad4 gene of the 18 B. minax populations. This indicated that B. minax had a high level of genetic diversity. These populations also showed a discrete distribution in both the scatter plots of genetic versus geographical distance for pairwise population comparisons and the median-joining network of haplotypes, which revealed the strong genetic structure of B. minax.

Population structure of a global agricultural invasive pest, Bactrocera dorsalis (Diptera: Tephritidae)

Bactrocera dorsalis, the Oriental fruit fly, is one of the world's most destructive agricultural insect pests and a major impediment to international fresh commodity trade. The genetic structuring of the species across its entire geographic range has never been undertaken, because under a former taxonomy B. dorsalis was divided into four distinct taxonomic entities, each with their own, largely non-overlapping, distributions. Based on the extensive sampling of six a priori groups from 63 locations, genetic and geometric morphometric datasets were generated to detect macrogeographic population structure, and to determine prior and current invasion pathways of this species. Weak population structure and high genetic diversity were detected among Asian populations. Invasive populations in Africa and Hawaii are inferred to be the result of separate, single invasions from South Asia, while South Asia is also the likely source of other Asian populations. The current northward invasion of B. dorsalis into Central China is the result of multiple, repeated dispersal events, most likely related to fruit trade. Results are discussed in the context of global quarantine, trade, and management of this pest. The recent expansion of the fly into temperate China, with very few associated genetic changes, clearly demonstrates the threat posed by this pest to ecologically similar areas in Europe and North America.

Phylogeography of the termite Macrotermes gilvus and insight into ancient dispersal corridors in Pleistocene Southeast Asia

Dispersal of soil-dwelling organisms via the repeatedly exposed Sunda shelf through much of the Pleistocene in Southeast Asia has not been studied extensively, especially for invertebrates. Here we investigated the phylogeography of an endemic termite species, Macrotermes gilvus (Hagen), to elucidate the spatiotemporal dynamics of dispersal routes of terrestrial fauna in Pleistocene Southeast Asia. We sampled 213 termite colonies from 66 localities throughout the region. Independently inherited microsatellites and mtDNA markers were used to infer the phylogeographic framework of M. gilvus. Discrete phylogeographic analysis and molecular dating based on fossil calibration were used to infer the dynamics of M. gilvus dispersal in time and space across Southeast Asia. We found that the termite dispersal events were consistently dated within the Pleistocene time frame. The dispersal pattern was multidirectional, radiating eastwards and southwards out of Indochina, which was identified as the origin for dispersal events. We found no direct dispersal events between Sumatra and Borneo despite the presence of a terrestrial connection between them during the Pleistocene. Instead, central Java served as an important link allowing termite colonies to be established in Borneo and Sumatra. Our findings support the hypothesis of a north-south dispersal corridor in Southeast Asia and suggest the presence of alternative dispersal routes across Sundaland during the Pleistocene. For the first time, we also propose that a west-east dispersal through over-water rafting likely occurred across the Pleistocene South China Sea. We found at least two independent entry routes for terrestrial species to infiltrate Sumatra and Borneo at different times.

Population structure and patterns of geographic differentiation of Bactrocera oleae (Diptera: Tephritidae) in Eastern Mediterranean Basin

The olive fly (Bactrocera oleae) is the most destructive pest of olives in most commercial olive-growing regions worldwide. Significant economic damage to olive production is caused by the larvae of this fly, which feed on the pulp of Olea fruits. Studying the genetic structure of insect pest populations is essential for the success of pest management strategies. Our primary goal in the present study was to examine the population structures of olive flies collected over a wide geographic area from Turkey, a representative of eastern Mediterranean region, using two mitochondrial DNA sequences as genetic markers. The data revealed a high level of genetic variability in olive fly populations and a moderate level of genetic differentiation between Mediterranean and Aegean populations in Turkey. We also merged the sequences obtained in the present study with previously published sequences from across the world into the data matrix. Strong population substructure and a significant correlation between genetic and geographic distances were detected in northern Mediterranean basin populations of B. oleae, indicating the possibility of a westward expansion of the species in the continent. In addition, our results revealed a very close genetic relationship between the Aegean and Iranian populations, which suggests that B. oleae was introduced to Iran from western parts of Turkey. However, additional markers and analytical approaches are required to determine the exact colonization route of olive fly.

Genetic analysis of Bactrocera zonata (Diptera: Tephritidae) populations from India based on cox1 and nad1 gene sequences

The peach fruit fly, Bactrocera zonata, is among the most serious and polyphagous insect pest of fruit crops in many parts of the world under genus Bactrocera. In the present study, the genetic structure, diversity and demographic history of B. zonata in India were inferred from mitochondrial cytochrome oxidase 1 (cox1) and NADH dehydrogenase 1 (nad1) sequences. The efficiency of DNA barcodes for identification of B. zonata was also tested. Genetic diversity indices [number of haplotypes (H), haplotype diversity (Hd), nucleotide diversity (π) and average number of nucleotide differences (k)] of B. zonata populations across India maintain high level of genetic diversity without isolation by distance among the geographic regions. Non-significant negative correlation between pairwise Fst and geographic distance suggests a high level of gene flow among studied populations of B. zonata. The possibility of sudden expansion of B. zonata revealed through mismatch distribution analysis as well as negative Tajima's D and Fu's Fs values further supported by star-like network of haplotypes. DNA barcoding analysis suggests that B. zonata specimens can be clearly differentiated from other species with 100% accuracy of identification. Therefore, cytochrome oxidase 1 (cox1) barcode sequences generated in the present study could be a valuable source for the rapid identification and global population genetic study of B. zonata.

Phylogeographic patterns of Lygus pratensis (Hemiptera: Miridae): Evidence for weak genetic structure and recent expansion in northwest China

Lygus pratensis (L.) is an important cotton pest in China, especially in the northwest region. Nymphs and adults cause serious quality and yield losses. However, the genetic structure and geographic distribution of L. pratensis is not well known. We analyzed genetic diversity, geographical structure, gene flow, and population dynamics of L. pratensis in northwest China using mitochondrial and nuclear sequence datasets to study phylogeographical patterns and demographic history. L. pratensis (n = 286) were collected at sites across an area spanning 2,180,000 km², including the Xinjiang and Gansu-Ningxia regions. Populations in the two regions could be distinguished based on mitochondrial criteria but the overall genetic structure was weak. The nuclear dataset revealed a lack of diagnostic genetic structure across sample areas. Phylogenetic analysis indicated a lack of population level monophyly that may have been caused by incomplete lineage sorting. The Mantel test showed a significant correlation between genetic and geographic distances among the populations based on the mtDNA data. However the nuclear dataset did not show significant correlation. A high level of gene flow among populations was indicated by migration analysis; human activities may have also facilitated insect movement. The availability of irrigation water and ample cotton hosts makes the Xinjiang region well suited for L. pratensis reproduction. Bayesian skyline plot analysis, star-shaped network, and neutrality tests all indicated that L. pratensis has experienced recent population expansion. Climatic changes and extensive areas occupied by host plants have led to population expansion of L. pratensis. In conclusion, the present distribution and phylogeographic pattern of L. pratensis was influenced by climate, human activities, and availability of plant hosts.

Genetic analysis of oriental fruit fly, Bactrocera dorsalis (Diptera: Tephritidae) populations based on mitochondrial cox1 and nad1 gene sequences from India and other Asian countries

The study examined the genetic diversity and demographic history of Bactrocera dorsalis, a destructive and polyphagous insect pest of fruit crops in diverse geographic regions of India. 19 widely dispersed populations of the fly from India and other Asian countries were analysed using partial sequences of mitochondrial cytochrome oxidase I (cox1) and NADH dehydrogenase 1 (nad1) genes to investigate genetic diversity, genetic structure, and demographic history in the region. Genetic diversity indices [number of haplotypes (H), haloptype diversity (Hd), nucleotide diversity (π) and average number of nucleotide difference (k)] of populations revealed that B. dorsalis maintains fairly high level of genetic diversity without isolation by distance among the geographic regions. Demographic analysis showed significant (negative) Tajimas' D and Fu's F _S with non significant sum of squared deviations (SSD) values, which indicate the possibility of recent sudden expansion of species and is further supported through distinctively star-like distribution structure of haplotypes among populations. Thus, the results indicate that both ongoing and historical factors have played important role in determining the genetic structure and diversity of the species in India. Consequently, sterile insect technique (SIT) could be a possible management strategy of species in the regions.

Population genetic structure and migration patterns of Liriomyza sativae in China: moderate subdivision and no Bridgehead effect revealed by microsatellites

While Liriomyza sativae (Diptera: Agromyzidae), an important invasive pest of ornamentals and vegetables has been found in China for the past two decades, few studies have focused on its genetics or route of invasive. In this study, we collected 288 L. sativae individuals across 12 provinces to explore its population genetic structure and migration patterns in China using seven microsatellites. We found relatively low levels of genetic diversity but moderate population genetic structure (0.05 < F ST < 0.15) in L. sativae from China. All populations deviated significantly from the Hardy-Weinberg equilibrium due to heterozygote deficiency. Molecular variance analysis revealed that more than 89% of variation was among samples within populations. A UPGMA dendrogram revealed that SH and GXNN populations formed one cluster separate from the other populations, which is in accordance with STRUCTURE and GENELAND analyses. A Mantel test indicated that genetic distance was not correlated to geographic distance (r = -0.0814, P = 0.7610), coupled with high levels of gene flow (M = 40.1-817.7), suggesting a possible anthropogenic influence on the spread of L. sativae in China and on the effect of hosts. The trend of asymmetrical gene flow was from southern to northern populations in general and did not exhibit a Bridgehead effect during the course of invasion, as can be seen by the low genetic diversity of southern populations.

Gene flow and genetic structure of Bactrocera carambolae (Diptera, Tephritidae) among geographical differences and sister species, B. dorsalis, inferred from microsatellite DNA data

The Carambola fruit fly, Bactroceracarambolae, is an invasive pest in Southeast Asia. It has been introduced into areas in South America such as Suriname and Brazil. Bactroceracarambolae belongs to the Bactroceradorsalis species complex, and seems to be separated from Bactroceradorsalis based on morphological and multilocus phylogenetic studies. Even though the Carambola fruit fly is an important quarantine species and has an impact on international trade, knowledge of the molecular ecology of Bactroceracarambolae, concerning species status and pest management aspects, is lacking. Seven populations sampled from the known geographical areas of Bactroceracarambolae including Southeast Asia (i.e., Indonesia, Malaysia, Thailand) and South America (i.e., Suriname), were genotyped using eight microsatellite DNA markers. Genetic variation, genetic structure, and genetic network among populations illustrated that the Suriname samples were genetically differentiated from Southeast Asian populations. The genetic network revealed that samples from West Sumatra (Pekanbaru, PK) and Java (Jakarta, JK) were presumably the source populations of Bactroceracarambolae in Suriname, which was congruent with human migration records between the two continents. Additionally, three populations of Bactroceradorsalis were included to better understand the species boundary. The genetic structure between the two species was significantly separated and approximately 11% of total individuals were detected as admixed (0.100 ≤ Q ≤ 0.900). The genetic network showed connections between Bactroceracarambolae and Bactroceradorsalis groups throughout Depok (DP), JK, and Nakhon Sri Thammarat (NT) populations. These data supported the hypothesis that the reproductive isolation between the two species may be leaky. Although the morphology and monophyly of nuclear and mitochondrial DNA sequences in previous studies showed discrete entities, the hypothesis of semipermeable boundaries may not be rejected. Alleles at microsatellite loci could be introgressed rather than other nuclear and mitochondrial DNA. Bactroceracarambolae may be an incipient rather than a distinct species of Bactroceradorsalis. Regarding the pest management aspect, the genetic sexing Salaya5 strain (SY5) was included for comparison with wild populations. The SY5 strain was genetically assigned to the Bactroceracarambolae cluster. Likewise, the genetic network showed that the strain shared greatest genetic similarity to JK, suggesting that SY5 did not divert away from its original genetic makeup. Under laboratory conditions, at least 12 generations apart, selection did not strongly affect genetic compatibility between the strain and wild populations. This knowledge further confirms the potential utilization of the Salaya5 strain in regional programs of area-wide integrated pest management using SIT.

Flight capacity of Bactrocera dorsalis (Diptera: Tephritidae) adult females based on flight mill studies and flight muscle ultrastructure

The oriental fruit fly, Bactrocera dorsalis (Hendel) (Diptera: Tephritidae), is considered a major economic threat in many regions worldwide. To better comprehend flight capacity of B. dorsalis and its physiological basis, a computer-monitored flight mill was used to study flight capacity of B. dorsalis adult females of various ages, and the changes of its flight muscle ultrastructures were studied by transmission electron microscopy. The flight capacity (both speed and distance) changed significantly with age of B. dorsalis female adults, peaking at about 15 d; the myofibril diameter of the flight muscle of test insects at 15-d old was the longest, up to 1.56 µm, the sarcomere length at 15-d old was the shortest, averaging at 1.37 µm, volume content of mitochondria of flight muscle at 15-d old reached the peak, it was 32.64%. This study provides the important scientific data for better revealing long-distance movement mechanism of B. dorsalis.

Thermal plasticity is related to the hardening response of heat shock protein expression in two Bactrocera fruit flies

It is generally believed that widely distributed species differ in their thermal plasticity from narrowly distributed species, but how differences in thermal plasticity are regulated at the molecular level remains largely unknown. Here, we conducted a comparative study of two closely related invasive fruit fly species, Bactrocera correcta and Bactroceradorsalis, in China. The two species had overlapping distributions, but B. dorsalis had a much wider range throughout the country and a longer invasive history than B. correcta. We first examined the effects of thermal acclimation on the ability of the two fruit flies to survive heat stress. The heat shock tolerance of B. dorsalis was significantly enhanced by heat hardening at 35, 37, 39 and 41°C, but that of B. correcta was only enhanced by heat hardening at 39°C and 41°C. Thus, the more widespread species has a higher thermal plasticity than the narrowly distributed species. We then determined the expression of Hsp70 and Hsp90 during different developmental stages and their responses to thermal hardening. The expression of both Hsp70 and Hsp90 in larvae was upregulated in response to heat hardening, starting at 35°C for B. dorsalis and at 39°C for B. correcta. The two species exhibited a highly consistent pattern of thermal response in terms of their heat shock survival rates and levels of Hsp gene expression. The results suggest that the difference in thermal plasticity may be responsible for the different distributions of the two species and that Hsp expression may be involved in the regulation of thermal plasticity. Our findings have important implications for the prediction of the thermal limits and ecological responses of related species in nature.

The oriental fruitfly Bactrocera dorsalis s.s. in East Asia: disentangling the different forces promoting the invasion and shaping the genetic make-up of populations

The Oriental fruit fly, Bactrocera dorsalis sensu stricto, is one of the most economically destructive pests of fruits and vegetables especially in East Asia. Based on its phytophagous life style, this species dispersed with the diffusion and implementation of agriculture, while globalization allowed it to establish adventive populations in different tropical and subtropical areas of the world. We used nine SSR loci over twelve samples collected across East Asia, i.e. an area that, in relatively few years, has become a theatre of intensive agriculture and a lively fruit trade. Our aim is to disentangle the different forces that have affected the invasion pattern and shaped the genetic make-up of populations of this fruit fly. Our data suggest that the considered samples probably represent well established populations in terms of genetic variability and population structuring. The human influence on the genetic shape of populations and diffusion is evident, but factors such as breeding/habitat size and life history traits of the species may have determined the post introduction phases and expansion. In East Asia the origin of diffusion can most probably be allocated in the oriental coastal provinces of China, from where this fruit fly spread into Southeast Asia. The spread of this species deserves attention for the development and implementation of risk assessment and control measures.

Sequence analysis of mitochondrial ND1 gene can reveal the genetic structure and origin of Bactrocera dorsalis s.s

BACKGROUND

The oriental fruit fly, Bactrocera dorsalis s.s., is one of the most important quarantine pests in many countries, including China. Although the oriental fruit fly has been investigated extensively, its origins and genetic structure remain disputed. In this study, the NADH dehydrogenase subunit 1 (ND1) gene was used as a genetic marker to examine the genetic diversity, population structure, and gene flow of B. dorsalis s.s. throughout its range in China and southeast Asia.

RESULTS

Haplotype networks and phylogenetic analysis indicated two distinguishable lineages of the fly population but provided no strong support for geographical subdivision in B. philippinensis. Demographic analysis revealed rapid expansion of B. dorsalis s.s. populations in China and Southeast Asia in the recent years. The greatest amount of genetic diversity was observed in Manila, Pattaya, and Bangkok, and asymmetric migration patterns were observed in different parts of China. The data collected here further show that B. dorsalis s.s. in Yunnan, Guangdong, and Fujian Provinces, and in Taiwan might have different origins within southeast Asia.

CONCLUSIONS

Using the mitochondrial ND1 gene, the results of the present study showed B. dorsalis s.s. from different parts of China to have different genetic structures and origins. B. dorsalis s.s. in China and southeast Asia was found to have experienced rapid expansion in recent years. Data further support the existence of two distinguishable lineages of B. dorsalis s.s. in China and indicate genetic diversity and gene flow from multiple origins.The sequences in this paper have been deposited in GenBank/NCBI under accession numbers KC413034-KC413367.

Global haplotype analysis of the whitefly Bemisia tabaci cryptic species Asia I in Asia

The whitefly, Bemisia tabaci (Hemiptera: Aleyrodidiae), is a cryptic species complex comprising a minimum of 24 cryptic species. Some members of this complex are important agricultural pests, causing considerable damage to vegetable as well as ornamental and horticultural crops. Asia I, one of the cryptic species of B. tabaci, is widely distributed in Asia. One hundred and sixty mitochondrial cytochrome oxidase I (COI) sequences from eight countries have been analyzed to investigate the geographic origin and current genetic structure of this cryptic species. Sixty different haplotypes were identified, with levels of genetic distances ranging from 0.001 to 0.021. A sign of possible genetic differentiation emerges from the differential distribution of dominant haplotypes in Indonesia and India compared to China. A possible ancient separation between Asia I in India and Indonesia and secondary contact in China has been hypothesized.

In silico cloning and annotation of genes involved in the digestion, detoxification and RNA interference mechanism in the midgut of Bactrocera dorsalis [Hendel (Diptera: Tephritidae)]

As the second largest organ in insects, the insect midgut is the major tissue involved in the digestion of food and detoxification of xenobiotics, such as insecticides, and the first barrier and target for oral RNA interference (RNAi). In this study, we performed a midgut-specific transcriptome analysis in the oriental fruit fly, Bactrocera dorsalis, an economically important worldwide pest, with many populations showing high levels of insecticide resistance. Using high-throughput sequencing, 52 838 060 short reads were generated and assembled to 25 236 unigenes with a mean length of 758 bp. Interestingly, 34 unique sequences encoding digestion enzymes were newly described and these included aminopeptidase and trypsin, genes associated with Bacillus thuringiensis resistance and fitness cost. Second, 41 transcripts were annotated to particular detoxification genes such as glutathione S-transferases, carboxylesterases and cytochrome P450s, and the subsequent phylogenetic analysis indicated homology with tissue-specific and insecticide resistance-related genes of Drosophila melanogaster. Third, we identified the genes involved in the mechanism of RNAi and the uptake of double-stranded RNA. The sequences encoding Dicer-2, R2D2, AGO2, and Eater were confirmed, but SID and SR-CI were absent in the midgut transcriptome. In conclusion, the results provide basic molecular information to better understand the mechanisms of food digestion, insecticide resistance and oral RNAi in this important pest insect in agriculture. Specific genes in these systems can be used in the future as potential targets for pest control, for instance, with RNAi technology.

The genetic polymorphisms and colonization process of olive fly populations in Turkey

The olive fruit fly, Bactrocera oleae, is the most important pest of olives in olive growing regions worldwide, especially in the Mediterranean basin and North America. Despite the economic importance of the olive fly, the colonization route of this species is unclear. We used nuclear microsatellite markers and mitochondrial DNA to provide information about the population structure and invasion route of olive fly populations in Turkey, as representative of the Eastern Mediterranean region. Adult fly samples were collected from 38 sublocations covering all olive growing regions in Turkey. The simple sequence variability data revealed a significant genetic variability in olive fly populations and a certain degree of differentiation between Mediterranean and Aegean populations. Mediterranean populations harbor higher levels of microsatellite variation than Aegean populations, which points to the eastern part of the Mediterranean as the putative source of invasion. mtDNA results suggest olive flies from the western part of Turkey are closely related to Italo-Aegean flies of the Mediterranean basin and the olive fly populations have invaded the northern part of the Mediterranean basin through western Turkey. In addition, finding specific American haplotypes in high frequencies might indicate that Turkey is the possible source of American olive fly populations. In order to more precisely characterize the population structure and invasion routes of this organism, more DNA-based sequence analysis should be carried out worldwide.

Population structure of Bactrocera dorsalis s.s., B. papayae and B. philippinensis (Diptera: Tephritidae) in southeast Asia: evidence for a single species hypothesis using mitochondrial DNA and wing-shape data

Background

Bactrocera dorsalis s.s. is a pestiferous tephritid fruit fly distributed from Pakistan to the Pacific, with the Thai/Malay peninsula its southern limit. Sister pest taxa, B. papayae and B. philippinensis, occur in the southeast Asian archipelago and the Philippines, respectively. The relationship among these species is unclear due to their high molecular and morphological similarity. This study analysed population structure of these three species within a southeast Asian biogeographical context to assess potential dispersal patterns and the validity of their current taxonomic status.

Results

Geometric morphometric results generated from 15 landmarks for wings of 169 flies revealed significant differences in wing shape between almost all sites following canonical variate analysis. For the combined data set there was a greater isolation-by-distance (IBD) effect under a ‘non-Euclidean’ scenario which used geographical distances within a biogeographical ‘Sundaland context’ (r² = 0.772, P < 0.0001) as compared to a ‘Euclidean’ scenario for which direct geographic distances between sample sites was used (r² = 0.217, P < 0.01). COI sequence data were obtained for 156 individuals and yielded 83 unique haplotypes with no correlation to current taxonomic designations via a minimum spanning network. beast analysis provided a root age and location of 540kya in northern Thailand, with migration of B. dorsalis s.l. into Malaysia 470kya and Sumatra 270kya. Two migration events into the Philippines are inferred. Sequence data revealed a weak but significant IBD effect under the ‘non-Euclidean’ scenario (r² = 0.110, P < 0.05), with no historical migration evident between Taiwan and the Philippines. Results are consistent with those expected at the intra-specific level.

Conclusions

Bactrocera dorsalis s.s., B. papayae and B. philippinensis likely represent one species structured around the South China Sea, having migrated from northern Thailand into the southeast Asian archipelago and across into the Philippines. No migration is apparent between the Philippines and Taiwan. This information has implications for quarantine, trade and pest management.

Genetic structure and inferences on potential source areas for Bactrocera dorsalis (Hendel) based on mitochondrial and microsatellite markers

Bactrocera dorsalis (Diptera: Tephritidae) is mainly distributed in tropical and subtropical Asia and in the Pacific region. Despite its economic importance, very few studies have addressed the question of the wide genetic structure and potential source area of this species. This pilot study attempts to infer the native region of this pest and its colonization pathways in Asia. Combining mitochondrial and microsatellite markers, we evaluated the level of genetic diversity, genetic structure, and the gene flow among fly populations collected across Southeast Asia and China. A complex and significant genetic structure corresponding to the geographic pattern was found with both types of molecular markers. However, the genetic structure found was rather weak in both cases, and no pattern of isolation by distance was identified. Multiple long-distance dispersal events and miscellaneous host selection by this species may explain the results. These complex patterns may have been influenced by human-mediated transportation of the pest from one area to another and the complex topography of the study region. For both mitochondrial and microsatellite data, no signs of bottleneck or founder events could be identified. Nonetheless, maximal genetic diversity was observed in Myanmar, Vietnam and Guangdong (China) and asymmetric migration patterns were found. These results provide indirect evidence that the tropical regions of Southeast Asia and southern coast of China may be considered as the native range of the species and the population expansion is northward. Yunnan (China) is a contact zone that has been colonized from different sources. Regions along the southern coast of Vietnam and China probably served to colonize mainly the southern region of China. Southern coastal regions of China may also have colonized central parts of China and of central Yunnan.

Invasion history of the oriental fruit fly, Bactrocera dorsalis, in the Pacific-Asia region: two main invasion routes

The oriental fruit fly, Bactrocera dorsalis, was initially recorded in Taiwan Island in 1912, and has dispersed to many areas in the Pacific-Asia region over the last century. The area of origin of the species may be confidently placed in South-East China. However, routes of range expansion to new areas and underlying population processes remain partially unclear, despite having been the subject of several studies. To explore the invasion history of this species, a partition of the cox1 gene of mitochondrial DNA was used to investigate genetic diversity, haplotype phylogeny and demographic history of 35 populations, covering China and South-East Asia and including marginal populations from Pakistan and Hawaii. Based on neighbor-joining tree analysis and the distribution of haplotypes, two main invasion routes are inferred: one from South-East China to Central China, another from South-East China to South-East Asia, with both routes probably coinciding in Central China. Populations in Taiwan Island and Hainan Island might have originated in South-East China. The marginal populations in Pakistan and Hawaii might have undergone founding events or genetic bottlenecks. Possible strategies for the control of this species are proposed based on the invasion history and reconstructed expansion routes.