The worldwide spread of Aedes albopictus: New insights from mitogenomes

The tiger mosquito (Aedes albopictus) is one of the most invasive species in the world and a competent vector for numerous arboviruses, thus the study and monitoring of its fast worldwide spread is crucial for global public health. The small extra-nuclear and maternally-inherited mitochondrial DNA represents a key tool for reconstructing phylogenetic and phylogeographic relationships within a species, especially when analyzed at the mitogenome level. Here the mitogenome variation of 76 tiger mosquitoes, 37 of which new and collected from both wild adventive populations and laboratory strains, was investigated. This analysis significantly improved the global mtDNA phylogeny of Ae. albopictus, uncovering new branches and sub-branches within haplogroup A1, the one involved in its recent worldwide spread. Our phylogeographic approach shows that the current distribution of tiger mosquito mitogenome variation has been strongly affected by clonal and sub-clonal founder events, sometimes involving wide geographic areas, even across continents, thus shedding light on the Asian sources of worldwide adventive populations. In particular, different starting points for the two major clades within A1 are suggested, with A1a spreading mainly along temperate areas from Japanese and Chinese sources, and A1b arising and mainly diffusing in tropical areas from a South Asian source.

Behavioural Responses of Male Aedes albopictus to Different Volatile Chemical Compounds

Simple Summary

Many studies have been performed to assess the effects of chemical compounds on mosquito behaviour. These studies almost exclusively involve only female mosquitoes as they can transmit disease pathogens, or at least, cause biting nuisance. Few studies have considered male mosquitoes. The identification of chemical substances that attract males can be very useful for trapping purposes, especially for monitoring the makeup of the male population during control programmes, such as those involving the release of sterile male mosquitoes. Twenty-eight chemical compounds from different chemical classes were evaluated using a dual-port olfactometer assay with at least three serial hexane dilutions against a hexane control. The compounds included known animal, plant and fungal volatiles, and the components of a putative Aedes aegypti pheromone. Many of the compounds were repellent for male mosquitoes, especially at the highest concentration. One compound, decanoic acid, acted as an attractant for males at an intermediate concentration. Decanoic acid did not elicit a significant response from female mosquitoes.

Abstract

The Asian tiger mosquito, Aedes albopictus, has become one of the most important invasive vectors for disease pathogens such as the viruses that cause chikungunya and dengue. Given the medical importance of this disease vector, a number of control programmes involving the use of the sterile insect technique (SIT) have been proposed. The identification of chemical compounds that attract males can be very useful for trapping purposes, especially for monitoring the makeup of the male population during control programmes, such as those involving the use of the SIT. Twenty-eight chemical compounds from different chemical classes were evaluated using a dual-port olfactometer assay. The compounds included known animal, fungal and plant host volatiles, and components of a putative Aedes aegypti pheromone. Many of the compounds were repellent for male mosquitoes, especially at the highest concentration. One compound, decanoic acid, acted as an attractant for males at an intermediate concentration. Decanoic acid did not elicit a significant response from female mosquitoes.

Viviparity and habitat restrictions may influence the evolution of male reproductive genes in tsetse fly (Glossina) species

BACKGROUND

Glossina species (tsetse flies), the sole vectors of African trypanosomes, maintained along their long evolutionary history a unique reproductive strategy, adenotrophic viviparity. Viviparity reduces their reproductive rate and, as such, imposes strong selective pressures on males for reproductive success. These species live in sub-Saharan Africa, where the distributions of the main sub-genera Fusca, Morsitans, and Palpalis are restricted to forest, savannah, and riverine habitats, respectively. Here we aim at identifying the evolutionary patterns of the male reproductive genes of six species belonging to these three main sub-genera. We then interpreted the different patterns we found across the species in the light of viviparity and the specific habitat restrictions, which are known to shape reproductive behavior.

RESULTS

We used a comparative genomic approach to build consensus evolutionary trees that portray the selective pressure acting on the male reproductive genes in these lineages. Such trees reflect the long and divergent demographic history that led to an allopatric distribution of the Fusca, Morsitans, and Palpalis species groups. A dataset of over 1700 male reproductive genes remained conserved over the long evolutionary time scale (estimated at 26.7 million years) across the genomes of the six species. We suggest that this conservation may result from strong functional selective pressure on the male imposed by viviparity. It is noteworthy that more than half of these conserved genes are novel sequences that are unique to the Glossina genus and are candidates for selection in the different lineages.

CONCLUSIONS

Tsetse flies represent a model to interpret the evolution and differentiation of male reproductive biology under different, but complementary, perspectives. In the light of viviparity, we must take into account that these genes are constrained by a post-fertilization arena for genomic conflicts created by viviparity and absent in ovipositing species. This constraint implies a continuous antagonistic co-evolution between the parental genomes, thus accelerating inter-population post-zygotic isolation and, ultimately, favoring speciation. Ecological restrictions that affect reproductive behavior may further shape such antagonistic co-evolution.

Transcriptional variation of sensory-related genes in natural populations of Aedes albopictus

BACKGROUND

The Asian tiger mosquito, Aedes albopictus, is a highly dangerous invasive vector of numerous medically important arboviruses including dengue, chikungunya and Zika. In four decades it has spread from tropical Southeast Asia to many parts of the world in both tropical and temperate climes. The rapid invasion process of this mosquito is supported by its high ecological and genetic plasticity across different life history traits. Our aim was to investigate whether wild populations, both native and adventive, also display transcriptional genetic variability for functions that may impact their biology, behaviour and ability to transmit arboviruses, such as sensory perception.

RESULTS

Antennal transcriptome data were derived from mosquitoes from a native population from Ban Rai, Thailand and from three adventive Mediterranean populations: Athens, Greece and Arco and Trento from Italy. Clear inter-population differential transcriptional activity was observed in different gene categories related to sound perception, olfaction and viral infection. The greatest differences were detected between the native Thai and the Mediterranean populations. The two Italian populations were the most similar. Nearly one million quality filtered SNP loci were identified.

CONCLUSION

The ability to express this great inter-population transcriptional variability highlights, at the functional level, the remarkable genetic flexibility of this mosquito species. We can hypothesize that the differential expression of genes, including those involved in sensory perception, in different populations may enable Ae. albopictus to exploit different environments and hosts, thus contributing to its status as a global vector of arboviruses of public health importance. The large number of SNP loci present in these transcripts represents a useful addition to the arsenal of high-resolution molecular markers and a resource that can be used to detect selective pressure and adaptive changes that may have occurred during the colonization process.

The Nix locus on the male-specific homologue of chromosome 1 in Aedes albopictus is a strong candidate for a male-determining factor

Background

Global concern over the rapid expansion of the Asian tiger mosquito, Aedes albopictus, and its vector competence has highlighted an urgent need to improve currently available population control methods, like the Sterile Insect Technique. Knowledge of the sex determination cascade is a prerequisite for the development of early-stage sexing systems. To this end, we have characterised the putative sex determination gene, Nix, in this species. In Aedes species the chromosome complement consists of three pairs of chromosomes. The sex determination alleles are linked to the smallest homomorphic chromosome.

Results

We identified the male-specific chromosome 1 of Ae. albopictus that carries the putative male-determining gene Nix. We have also characterised the complete genomic sequence of the Nix gene which is composed of two exons and a short intron. The gene displays different levels of intron retention during development. Comparison of DNA sequences covering most of the Nix gene from individuals across the species range revealed no polymorphism.

Conclusions

Our characterisation of the Nix gene in Ae. albopictus represents an initial step in the analysis of the sex determination cascade in this species. We found evidence of intron retention (IR) in Nix. IR might play a role in regulating the expression of Nix during development. Our results provide the basis for the development of new genetic control strategies.

Electronic supplementary material

The online version of this article (10.1186/s13071-018-3215-8) contains supplementary material, which is available to authorized users.

Genetic evidence for a worldwide chaotic dispersion pattern of the arbovirus vector, Aedes albopictus

BACKGROUND

Invasive species represent a global concern for their rapid spread and the possibility of infectious disease transmission. This is the case of the global invader Aedes albopictus, the Asian tiger mosquito. This species is a vector of medically important arboviruses, notably chikungunya (CHIKV), dengue (DENV) and Zika (ZIKV). The reconstruction of the complex colonization pattern of this mosquito has great potential for mitigating its spread and, consequently, disease risks.

METHODOLOGY/PRINCIPAL FINDINGS

Classical population genetics analyses and Approximate Bayesian Computation (ABC) approaches were combined to disentangle the demographic history of Aedes albopictus populations from representative countries in the Southeast Asian native range and in the recent and more recently colonized areas. In Southeast Asia, the low differentiation and the high co-ancestry values identified among China, Thailand and Japan indicate that, in the native range, these populations maintain high genetic connectivity, revealing their ancestral common origin. China appears to be the oldest population. Outside Southeast Asia, the invasion process in La Réunion, America and the Mediterranean Basin is primarily supported by a chaotic propagule distribution, which cooperates in maintaining a relatively high genetic diversity within the adventive populations.

CONCLUSIONS/SIGNIFICANCE

From our data, it appears that independent and also trans-continental introductions of Ae. albopictus may have facilitated the rapid establishment of adventive populations through admixture of unrelated genomes. As a consequence, a great amount of intra-population variability has been detected, and it is likely that this variability may extend to the genetic mechanisms controlling vector competence. Thus, in the context of the invasion process of this mosquito, it is possible that both population ancestry and admixture contribute to create the conditions for the efficient transmission of arboviruses and for outbreak establishment.

Sperm-less males modulate female behaviour in Ceratitis capitata (Diptera: Tephritidae)

In the Mediterranean fruit fly, Ceratitis capitata (Wiedemann)(Diptera: Tephritidae), mating has a strong impact on female biology, leading to a decrease in sexual receptivity and increased oviposition and fecundity. Previous studies suggest that sperm transfer may play a role in inducing these behavioural changes. Here we report the identification of a medfly innexin gene, Cc-inx5, whose expression is limited to the germ-line of both sexes. Through RNA interference of this gene, we generated males without testes and, consequently, sperm, but apparently retaining all the other reproductive organs intact. These sperm-less males were able to mate and, like their wild-type counterparts, to induce in their partners increased oviposition rates and refractoriness to remating. Interestingly, matings to sperm-less males results in oviposition rates higher than those induced by copulation with control males. In addition, the observed female post-mating behavioural changes were congruent with changes in transcript abundance of genes known to be regulated by mating in this species. Our results suggest that sperm transfer is not necessary to reduce female sexual receptivity and to increase oviposition and fecundity. These data pave the way to a better understanding of the role/s of seminal components in modulating female post-mating responses. In the long term, this knowledge will be the basis for the development of novel approaches for the manipulation of female fertility, and, consequently, innovative tools to be applied to medfly control strategies in the field.

The Worldwide Spread of the Tiger Mosquito as Revealed by Mitogenome Haplogroup Diversity

In the last 40 years, the Asian tiger mosquito Aedes albopictus, indigenous to East Asia, has colonized every continent except Antarctica. Its spread is a major public health concern, given that this species is a competent vector for numerous arboviruses, including those causing dengue, chikungunya, West Nile, and the recently emerged Zika fever. To acquire more information on the ancestral source(s) of adventive populations and the overall diffusion process from its native range, we analyzed the mitogenome variation of 27 individuals from representative populations of Asia, the Americas, and Europe. Phylogenetic analyses revealed five haplogroups in Asia, but population surveys appear to indicate that only three of these (A1a1, A1a2, and A1b) were involved in the recent worldwide spread. We also found out that a derived lineage (A1a1a1) within A1a1, which is now common in Italy, most likely arose in North America from an ancestral Japanese source. These different genetic sources now coexist in many of the recently colonized areas, thus probably creating novel genomic combinations which might be one of the causes of the apparently growing ability of A. albopictus to expand its geographical range.

The whole genome sequence of the Mediterranean fruit fly, Ceratitis capitata (Wiedemann), reveals insights into the biology and adaptive evolution of a highly invasive pest species

Background

The Mediterranean fruit fly (medfly), Ceratitis capitata, is a major destructive insect pest due to its broad host range, which includes hundreds of fruits and vegetables. It exhibits a unique ability to invade and adapt to ecological niches throughout tropical and subtropical regions of the world, though medfly infestations have been prevented and controlled by the sterile insect technique (SIT) as part of integrated pest management programs (IPMs). The genetic analysis and manipulation of medfly has been subject to intensive study in an effort to improve SIT efficacy and other aspects of IPM control.

Results

The 479 Mb medfly genome is sequenced from adult flies from lines inbred for 20 generations. A high-quality assembly is achieved having a contig N50 of 45.7 kb and scaffold N50 of 4.06 Mb. In-depth curation of more than 1800 messenger RNAs shows specific gene expansions that can be related to invasiveness and host adaptation, including gene families for chemoreception, toxin and insecticide metabolism, cuticle proteins, opsins, and aquaporins. We identify genes relevant to IPM control, including those required to improve SIT.

Conclusions

The medfly genome sequence provides critical insights into the biology of one of the most serious and widespread agricultural pests. This knowledge should significantly advance the means of controlling the size and invasive potential of medfly populations. Its close relationship to Drosophila, and other insect species important to agriculture and human health, will further comparative functional and structural studies of insect genomes that should broaden our understanding of gene family evolution.

Electronic supplementary material

The online version of this article (doi:10.1186/s13059-016-1049-2) contains supplementary material, which is available to authorized users.

A draft genome sequence of an invasive mosquito: an Italian Aedes albopictus

The draft genome sequence of Italian specimens of the Asian tiger mosquito Aedes (Stegomyia) albopictus (Diptera: Culicidae) was determined using a standard NGS (next generation sequencing) approach. The size of the assembled genome is comparable to that of Aedes aegypti; the two mosquitoes are also similar as far as the high content of repetitive DNA is concerned, most of which is made up of transposable elements. Although, based on BUSCO (Benchmarking Universal Single-Copy Orthologues) analysis, the genome assembly reported here contains more than 99% of protein-coding genes, several of those are expected to be represented in the assembly in a fragmented state. We also present here the annotation of several families of genes (tRNA genes, miRNA genes, the sialome, genes involved in chromatin condensation, sex determination genes, odorant binding proteins and odorant receptors). These analyses confirm that the assembly can be used for the study of the biology of this invasive vector of disease.

Molecular markers for analyses of intraspecific genetic diversity in the Asian Tiger mosquito, Aedes albopictus

Background

The dramatic worldwide expansion of Aedes albopictus (the Asian tiger mosquito) and its vector competence for numerous arboviruses represent a growing threat to public health security. Molecular markers are crucially needed for tracking the rapid spread of this mosquito and to obtain a deeper knowledge of population structure. This is a fundamental requirement for the development of strict monitoring protocols and for the improvement of sustainable control measures.

Methods

Wild population samples from putative source areas and from newly colonised regions were analysed for variability at the ribosomal DNA internal transcribed spacer 2 (ITS2). Moreover, a new set of 23 microsatellite markers (SSR) was developed. Sixteen of these SSRs were tested in an ancestral (Thailand) and two adventive Italian populations.

Results

Seventy-six ITS2 sequences representing 52 unique haplotypes were identified, and AMOVA indicated that most of their variation occurred within individuals (74.36%), while only about 8% was detected among populations. Spatial analyses of molecular variance revealed that haplotype genetic similarity was not related to the geographic proximity of populations and the haplotype phylogeny clearly indicated that highly related sequences were distributed across populations from different geographical regions. The SSR markers displayed a high level of polymorphism both in the ancestral and in adventive populations, and F_ST estimates suggested the absence of great differentiation. The ancestral nature of the Thai population was corroborated by its higher level of variability.

Conclusions

The two types of genetic markers here implemented revealed the distribution of genetic diversity within and between populations and provide clues on the dispersion dynamics of this species. It appears that the diffusion of this mosquito does not conform to a progressive expansion from the native Asian source area, but to a relatively recent and chaotic propagule distribution mediated by human activities. Under this scenario, multiple introductions and admixture events probably play an important role in maintaining the genetic diversity and in avoiding bottleneck effects. The polymorphic SSR markers here implemented will provide an important tool for reconstructing the routes of invasion followed by this mosquito.

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.

Identification of pheromone components and their binding affinity to the odorant binding protein CcapOBP83a-2 of the Mediterranean fruit fly, Ceratitis capitata

The Mediterranean fruit fly (or medfly), Ceratitis capitata (Wiedemann; Diptera: Tephritidae), is a serious pest of agriculture worldwide, displaying a very wide larval host range with more than 250 different species of fruit and vegetables. Olfaction plays a key role in the invasive potential of this species. Unfortunately, the pheromone communication system of the medfly is complex and still not well established. In this study, we report the isolation of chemicals emitted by sexually mature individuals during the "calling" period and the electrophysiological responses that these compounds elicit on the antennae of male and female flies. Fifteen compounds with electrophysiological activity were isolated and identified in male emissions by gas chromatography coupled to electroantennography (GC-EAG). Within the group of 15 identified compounds, 11 elicited a response in antennae of both sexes, whilst 4 elicited a response only in female antennae. The binding affinity of these compounds, plus 4 additional compounds known to be behaviourally active from other studies, was measured using C. capitata OBP, CcapOBP83a-2. This OBP has a high homology to Drosophila melanogaster OBPs OS-E and OS-F, which are associated with trichoid sensilla and co-expressed with the well-studied Drosophila pheromone binding protein LUSH. The results provide evidence of involvement of CcapOBP83a-2 in the medfly's odorant perception and its wider specificity for (E,E)-α-farnesene, one of the five major compounds in medfly male pheromone emission. This represents the first step in the clarification of the C. capitata and pheromone reception pathway, and a starting point for further studies aimed towards the creation of new powerful attractants or repellents applicable in the actual control strategies.

Presence of extensive Wolbachia symbiont insertions discovered in the genome of its host Glossina morsitans morsitans

Tsetse flies (Glossina spp.) are the cyclical vectors of Trypanosoma spp., which are unicellular parasites responsible for multiple diseases, including nagana in livestock and sleeping sickness in humans in Africa. Glossina species, including Glossina morsitans morsitans (Gmm), for which the Whole Genome Sequence (WGS) is now available, have established symbiotic associations with three endosymbionts: Wigglesworthia glossinidia, Sodalis glossinidius and Wolbachia pipientis (Wolbachia). The presence of Wolbachia in both natural and laboratory populations of Glossina species, including the presence of horizontal gene transfer (HGT) events in a laboratory colony of Gmm, has already been shown. We herein report on the draft genome sequence of the cytoplasmic Wolbachia endosymbiont (cytWol) associated with Gmm. By in silico and molecular and cytogenetic analysis, we discovered and validated the presence of multiple insertions of Wolbachia (chrWol) in the host Gmm genome. We identified at least two large insertions of chrWol, 527,507 and 484,123 bp in size, from Gmm WGS data. Southern hybridizations confirmed the presence of Wolbachia insertions in Gmm genome, and FISH revealed multiple insertions located on the two sex chromosomes (X and Y), as well as on the supernumerary B-chromosomes. We compare the chrWol insertions to the cytWol draft genome in an attempt to clarify the evolutionary history of the HGT events. We discuss our findings in light of the evolution of Wolbachia infections in the tsetse fly and their potential impacts on the control of tsetse populations and trypanosomiasis.

African trypanosomes are transmitted to man and animals by tsetse fly, a blood sucking insect. Tsetse flies include all Glossina species with the genome of Glossina morsitans morsitans (Gmm) being sequenced under the International Glossina Genome Initiative. The endosymbionts Wigglesworthia glossinidia, Sodalis glossinidius and Wolbachia pipientis (Wolbachia) have been found to establish symbiotic associations with Gmm. Wolbachia is known to be present in natural and laboratory populations of Glossina species. In this study we report the genome sequence of the Wolbachia strain that is associated with Gmm. With the aid of in silico and molecular and cytogenetic analyses, multiple insertions of the Wolbachia genome were revealed and confirmed in Gmm chromosome. Comparison of the cytoplasmic Wolbachia draft genome and the chromosomal insertions enabled us to infer the evolutionary history of the Wolbachia horizontal transfer events. These findings are discussed in relation to their impact on the development of Wolbachia-based strategies for the control of tsetse flies and trypanosomiasis.

Sniffing out chemosensory genes from the Mediterranean fruit fly, Ceratitis capitata

The Mediterranean fruit fly, Ceratitis capitata (medfly), is an extremely invasive agricultural pest due to its extremely wide host range and its ability to adapt to a broad range of climatic conditions and habitats. Chemosensory behaviour plays an important role in many crucial stages in the life of this insect, such as the detection of pheromone cues during mate pursuit and odorants during host plant localisation. Thus, the analysis of the chemosensory gene repertoire is an important step for the interpretation of the biology of this species and consequently its invasive potential. Moreover, these genes may represent ideal targets for the development of novel, effective control methods and pest population monitoring systems. Expressed sequence tag libraries from C. capitata adult heads, embryos, male accessory glands and testes were screened for sequences encoding putative odorant binding proteins (OBPs). A total of seventeen putative OBP transcripts were identified, corresponding to 13 Classic, three Minus-C and one Plus-C subfamily OBPs. The tissue distributions of the OBP transcripts were assessed by RT-PCR and a subset of five genes with predicted proteins sharing high sequence similarities and close phylogenetic affinities to Drosophila melanogaster pheromone binding protein related proteins (PBPRPs) were characterised in greater detail. Real Time quantitative PCR was used to assess the effects of maturation, mating and time of day on the transcript abundances of the putative PBPRP genes in the principal olfactory organs, the antennae, in males and females. The results of the present study have facilitated the annotation of OBP genes in the recently released medfly genome sequence and represent a significant contribution to the characterisation of the medfly chemosensory repertoire. The identification of these medfly OBPs/PBPRPs permitted evolutionary and functional comparisons with homologous sequences from other tephritids of the genera Bactrocera and Rhagoletis.

Transcriptional profiles of mating-responsive genes from testes and male accessory glands of the Mediterranean fruit fly, Ceratitis capitata

BACKGROUND

Insect seminal fluid is a complex mixture of proteins, carbohydrates and lipids, produced in the male reproductive tract. This seminal fluid is transferred together with the spermatozoa during mating and induces post-mating changes in the female. Molecular characterization of seminal fluid proteins in the Mediterranean fruit fly, Ceratitis capitata, is limited, although studies suggest that some of these proteins are biologically active.

METHODOLOGY/PRINCIPAL FINDINGS

We report on the functional annotation of 5914 high quality expressed sequence tags (ESTs) from the testes and male accessory glands, to identify transcripts encoding putative secreted peptides that might elicit post-mating responses in females. The ESTs were assembled into 3344 contigs, of which over 33% produced no hits against the nr database, and thus may represent novel or rapidly evolving sequences. Extraction of the coding sequences resulted in a total of 3371 putative peptides. The annotated dataset is available as a hyperlinked spreadsheet. Four hundred peptides were identified with putative secretory activity, including odorant binding proteins, protease inhibitor domain-containing peptides, antigen 5 proteins, mucins, and immunity-related sequences. Quantitative RT-PCR-based analyses of a subset of putative secretory protein-encoding transcripts from accessory glands indicated changes in their abundance after one or more copulations when compared to virgin males of the same age. These changes in abundance, particularly evident after the third mating, may be related to the requirement to replenish proteins to be transferred to the female.

CONCLUSIONS/SIGNIFICANCE

We have developed the first large-scale dataset for novel studies on functions and processes associated with the reproductive biology of Ceratitis capitata. The identified genes may help study genome evolution, in light of the high adaptive potential of the medfly. In addition, studies of male recovery dynamics in terms of accessory gland gene expression profiles and correlated remating inhibition mechanisms may permit the improvement of pest management approaches.

Transcriptome profiling of sexual maturation and mating in the Mediterranean fruit fly, Ceratitis capitata

Sexual maturation and mating in insects are generally accompanied by major physiological and behavioural changes. Many of these changes are related to the need to locate a mate and subsequently, in the case of females, to switch from mate searching to oviposition behaviour. The prodigious reproductive capacity of the Mediterranean fruit fly, Ceratitis capitata, is one of the factors that has led to its success as an invasive pest species. To identify the molecular changes related to maturation and mating status in male and female medfly, a microarray-based gene expression approach was used to compare the head transcriptomes of sexually immature, mature virgin, and mated individuals. Attention was focused on the changes in abundance of transcripts related to reproduction, behaviour, sensory perception of chemical stimulus, and immune system processes. Broad transcriptional changes were recorded during female maturation, while post-mating transcriptional changes in females were, by contrast, modest. In male medfly, transcriptional changes were consistent both during maturation and as a consequence of mating. Of particular note was the lack of the mating-induced immune responses that have been recorded for Drosophila melanogaster, that may be due to the different reproductive strategies of these species. This study, in addition to increasing our understanding of the molecular machinery behind maturation and mating in the medfly, has identified important gene targets that might be useful in the future management of this pest.

Polyandry is a common event in wild populations of the Tsetse fly Glossina fuscipes fuscipes and may impact population reduction measures

BACKGROUND

Glossina fuscipes fuscipes is the main vector of human and animal trypanosomiasis in Africa, particularly in Uganda. Attempts to control/eradicate this species using biological methods require knowledge of its reproductive biology. An important aspect is the number of times a female mates in the wild as this influences the effective population size and may constitute a critical factor in determining the success of control methods. To date, polyandry in G.f. fuscipes has not been investigated in the laboratory or in the wild. Interest in assessing the presence of remating in Ugandan populations is driven by the fact that eradication of this species is at the planning stage in this country.

METHODOLOGY/PRINCIPAL FINDINGS

Two well established populations, Kabukanga in the West and Buvuma Island in Lake Victoria, were sampled to assess the presence and frequency of female remating. Six informative microsatellite loci were used to estimate the number of matings per female by genotyping sperm preserved in the female spermathecae. The direct count of the minimum number of males that transferred sperm to the spermathecae was compared to Maximum Likelihood and Bayesian probability estimates. The three estimates provided evidence that remating is common in the populations but the frequency is substantially different: 57% in Kabukanga and 33% in Buvuma.

CONCLUSIONS/SIGNIFICANCE

The presence of remating, with females maintaining sperm from different mates, may constitute a critical factor in cases of re-infestation of cleared areas and/or of residual populations. Remating may enhance the reproductive potential of re-invading propagules in terms of their effective population size. We suggest that population age structure may influence remating frequency. Considering the seasonal demographic changes that this fly undergoes during the dry and wet seasons, control programmes based on SIT should release large numbers of sterile males, even in residual surviving target populations, in the dry season.

Interchromosomal duplications on the Bactrocera oleae Y chromosome imply a distinct evolutionary origin of the sex chromosomes compared to Drosophila

BACKGROUND

Diptera have an extraordinary variety of sex determination mechanisms, and Drosophila melanogaster is the paradigm for this group. However, the Drosophila sex determination pathway is only partially conserved and the family Tephritidae affords an interesting example. The tephritid Y chromosome is postulated to be necessary to determine male development. Characterization of Y sequences, apart from elucidating the nature of the male determining factor, is also important to understand the evolutionary history of sex chromosomes within the Tephritidae. We studied the Y sequences from the olive fly, Bactrocera oleae. Its Y chromosome is minute and highly heterochromatic, and displays high heteromorphism with the X chromosome.

METHODOLOGY/PRINCIPAL FINDINGS

A combined Representational Difference Analysis (RDA) and fluorescence in-situ hybridization (FISH) approach was used to investigate the Y chromosome to derive information on its sequence content. The Y chromosome is strewn with repetitive DNA sequences, the majority of which are also interdispersed in the pericentromeric regions of the autosomes. The Y chromosome appears to have accumulated small and large repetitive interchromosomal duplications. The large interchromosomal duplications harbour an importin-4-like gene fragment. Apart from these importin-4-like sequences, the other Y repetitive sequences are not shared with the X chromosome, suggesting molecular differentiation of these two chromosomes. Moreover, as the identified Y sequences were not detected on the Y chromosomes of closely related tephritids, we can infer divergence in the repetitive nature of their sequence contents.

CONCLUSIONS/SIGNIFICANCE

The identification of Y-linked sequences may tell us much about the repetitive nature, the origin and the evolution of Y chromosomes. We hypothesize how these repetitive sequences accumulated and were maintained on the Y chromosome during its evolutionary history. Our data reinforce the idea that the sex chromosomes of the Tephritidae may have distinct evolutionary origins with respect to those of the Drosophilidae and other Dipteran families.

Identification and expression profiling of Ceratitis capitata genes coding for β-hexosaminidases

The goal of this study was to identify the genes coding for β-N-acetylhexosaminidases in the Mediterranean fruit fly (medfly) Ceratitis capitata, one of the most destructive agricultural pests, belonging to the Tephritidae family, order Diptera. Two dimeric β-N-acetylhexosaminidases, HEXA and HEXB, have been recently identified on Drosophila sperm. These enzymes are involved in egg binding through interactions with complementary carbohydrates on the surface of the egg shell. Three genes, Hexosaminidase 1 (Hexo1), Hexosaminidase 2 (Hexo2) and fused lobes (fdl), encode for HEXA and HEXB subunits. The availability of C. capitata EST libraries derived from embryos and adult heads allowed us to identify three sequences homologous to the D. melanogaster Hexo1, Hexo2 and fdl genes. Here, we report the expression profile analysis of CcHexo1, CcHexo2 and Ccfdld in several tissues, organs and stages. Ccfdl expression was highest in heads of both sexes and in whole adult females. In the testis and ovary the three genes showed distinct spatial and temporal expression patterns. All the mRNAs were detectable in early stages of spermatogenesis; CcHexo2 and Ccfdl were also expressed in early elongating spermatid cysts. All three genes are expressed in the ovarian nurse cells. CcHexo1 and Ccfdl are stage specific, since they have been observed in stages 12 and 13 during oocyte growth, when programmed cell death occurs in nurse cells. The expression pattern of the three genes in medfly gonads suggests that, as their Drosophila counterparts, they may encode for proteins involved in gametogenesis and fertilization.

Sperm storage and use in polyandrous females of the globally invasive fruitfly, Ceratitis capitata

The medfly, Ceratitis capitata, is an invasive species in which polyandry, associated with sperm precedence, is a common behaviour in the wild. In this species, characterized by internal fertilization, we disclose how the sperm from two males are stored in the female storage organs and how they are used in terms of paternity outcome. The experiments were designed to furnish comparable and unbiased estimates of sperm numbers and progeny in twice-mated females. Results are incorporated in a model through which it is possible to relate the amount of stored sperm with the progeny of twice-mated females. The results show that polyandrous medfly females conserve equal amounts of sperm from the two males to fertilize their eggs. However, we observed a clear advantage of the second male's sperm in siring progeny, which interestingly decreases in favor of the first male as ovipositions progress. The results enable us to exclude differential sperm mortality and suggest that it is the mechanics governing the storage organs which causes the initial, but decreasing second male sperm precedence during the female reproductive life. These outcomes allow us to correlate sperm use in polyandrous females with the mating strategies and invasiveness of this fly.

Safe and fit genetically modified insects for pest control: from lab to field applications

Insect transgenesis is continuously being improved to increase the efficacy of population suppression and replacement strategies directed to the control of insect species of economic and sanitary interest. An essential prerequisite for the success of both pest control applications is that the fitness of the transformant individuals is not impaired, so that, once released in the field, they can efficiently compete with or even out-compete their wild-type counterparts for matings in order to reduce the population size, or to spread desirable genes into the target population. Recent research has shown that the production of fit and competitive transformants can now be achieved and that transgenes may not necessarily confer a fitness cost. In this article we review the most recent published results of the fitness assessment of different transgenic insect lines and underline the necessity to fulfill key requirements of ecological safety. Fitness evaluation studies performed in field cages and medium/large-scale rearing will validate the present encouraging laboratory results, giving an indication of the performance of the transgenic insect genotype after release in pest control programmes.

Sex and the single embryo: early deveiopment in the Mediterranean fruit fly, Ceratitis capitata

Background

In embryos the maternal-to-zygotic transition (MTZ) integrates post-transcriptional regulation of maternal transcripts with transcriptional activation of the zygotic genome. Although the molecular mechanisms underlying this event are being clarified in Drosophila melanogaster, little is know about the embryogenic processes in other insect species. The recent publication of expressed sequence tags (ESTs) from embryos of the global pest species Ceratitis capitata (medfly) has enabled the investigation of embryogenesis in this species and has allowed a comparison of the embryogenic processes in these two related dipteran species, C. capitata and D. melanogaster, that shared a common ancestor 80-100 mya.

Results

Using a novel PCR-based sexing method, which takes advantage of a putative LTR retrotransposon MITE insertion on the medfly Y chromosome, the transcriptomes of individual early male and female embryos were analysed using RT-PCR. This study is focused on two crucial aspects of the onset of embryonic development: sex determination and cellular blastoderm formation. Together with the three known medfly genes (Cctransformer, Cctransformer2 and Ccdoublesex), the expression patterns of other medfly genes that are similar to the D. melanogaster sex-determination genes (sisterlessA, groucho, deadpan, Sex-lethal, female lethal d, sans fille and intersex) and four cellular blastoderm formation genes (Rho1, spaghetti squash, slow-as-molasses and serendipity-α) were analyzed, allowing us to sketch a preliminary outline of the embryonic process in the medfly. Furthermore, a putative homologue of the Zelda gene has been considered, which in D. melanogaster encodes a DNA-binding factor responsible for the maternal-to-zygotic transition.

Conclusions

Our novel sexing method facilitates the study of i) when the MTZ transition occurs in males and females of C. capitata, ii) when and how the maternal information of "female-development" is reprogrammed in the embryos and iii) similarities and differences in the regulation of gene expression in C. capitata and D. melanogaster. We suggest a new model for the onset of the sex determination cascade in the medfly: the maternally inherited Cctra transcripts in the female embryos are insufficient to produce enough active protein to inhibit the male mode of Cctra splicing. The slow rate of development and the inefficiency of the splicing mechanism in the pre-cellular blastoderm facilitates the male-determining factor (M) activity, which probably acts by inhibiting CcTRA protein activity.

Uncovering the tracks of a recent and rapid invasion: the case of the fruit fly pest Bactrocera invadens (Diptera: Tephritidae) in Africa

Phytophagous insects of the genus Bactrocera are among the most economically important invasive fruit fly pests. In 2003, an unknown Bactrocera species was found in Kenya. First identified as an 'aberrant form' of the Asian B. dorsalis complex, it was later recognized as a new species, Bactrocera invadens. Within 2 years of its discovery, the species was recorded in several African countries, becoming an important quarantine pest. As this invasive fly was discovered only recently, no data are available on its invasion pattern in Africa. This pilot study attempts to infer from genetic data the dynamic aspects of the African invasion of this pest. Using microsatellite markers, we evaluated the level of genetic diversity and the extent of common ancestry among several African populations collected across the invaded areas. A sample from the Asian Sri Lankan population was analysed to confirm the Asian origin of this pest. Genetic data cast no doubt that Sri Lanka belongs to the native range, but only a small percentage of its genotypes can be found in Africa. African populations display relatively high levels of genetic diversity associated with limited geographical structure and no genetic footprints of bottlenecks. These features are indicative of processes of rapid population growth and expansion with possible multiple introductions. In the span of relatively few years, the African invasion registered the presence of at least two uncorrelated outbreaks, both starting from the East. The results of the analyses support that invasion started in East Africa, where B. invadens was initially isolated.

Nitric oxide synthase-dependent NADPH-diaphorase activity in the optic lobes of male and female Ceratitis capitata mutants

Nitric oxide (NO) is acknowledged as a messenger molecule in the nervous system with a pivotal role in the modulation of the chemosensory information. It has been shown to be present in the optic lobes of several insect species. In the present study, we used males and females from four different strains of the medfly Ceratitis capitata (Diptera, Tephritidae): or; or,wp (both orange eyed); w,M360 and w,Heraklion (both white eyed), as models to further clarify the involvement of NO in the mutants' visual system and differences in its activity and localization in the sexes. Comparison of the localization pattern of NO synthase (NOS), through NADPH-diaphorase (NADPHd) staining, in the optic lobes of the four strains, revealed a stronger reaction intensity in the retina and in the neuropile region lamina than in medulla and lobula. Interestingly, the intensity of NADPHd staining differs, at least in some strains, in the optic lobes of the two sexes; all the areas are generally strongly labelled in the males of the or and w,M360 strains, whereas the w,Heraklion and or,wp mutants do not show evident sex-dependent NADPHd staining. Taken as a whole, our data point to NO as a likely transmitter candidate in the visual information processes in insects, with a possible correlation among NOS distribution, eye pigmentation and visual function in C. capitata males. Moreover, NO could influence behavioural differences linked to vision in the two sexes.

Gene discovery in an invasive tephritid model pest species, the Mediterranean fruit fly, Ceratitis capitata

Background

The medfly, Ceratitis capitata, is a highly invasive agricultural pest that has become a model insect for the development of biological control programs. Despite research into the behavior and classical and population genetics of this organism, the quantity of sequence data available is limited. We have utilized an expressed sequence tag (EST) approach to obtain detailed information on transcriptome signatures that relate to a variety of physiological systems in the medfly; this information emphasizes on reproduction, sex determination, and chemosensory perception, since the study was based on normalized cDNA libraries from embryos and adult heads.

Results

A total of 21,253 high-quality ESTs were obtained from the embryo and head libraries. Clustering analyses performed separately for each library resulted in 5201 embryo and 6684 head transcripts. Considering an estimated 19% overlap in the transcriptomes of the two libraries, they represent about 9614 unique transcripts involved in a wide range of biological processes and molecular functions. Of particular interest are the sequences that share homology with Drosophila genes involved in sex determination, olfaction, and reproductive behavior. The medfly transformer2 (tra2) homolog was identified among the embryonic sequences, and its genomic organization and expression were characterized.

Conclusion

The sequences obtained in this study represent the first major dataset of expressed genes in a tephritid species of agricultural importance. This resource provides essential information to support the investigation of numerous questions regarding the biology of the medfly and other related species and also constitutes an invaluable tool for the annotation of complete genome sequences. Our study has revealed intriguing findings regarding the transcript regulation of tra2 and other sex determination genes, as well as insights into the comparative genomics of genes implicated in chemosensory reception and reproduction.

Highly similar piggyBac transposase-like sequences from various Bactrocera (Diptera, Tephritidae) species

The piggyBac transposable element is currently the vector of choice for transgenesis, enhancer trapping, gene discovery and gene function determination in both insects and mammals. However, the recent discovery of sequences with similarity to piggyBac in a wide diversity of organisms suggests that piggyBac may be horizontally transferred to distantly related species. This has raised concern on the wide-range application of piggyBac-based transformation vectors and their stability. In this paper, the presence of sequences homologous to the piggyBac transposase was investigated in 17 species belonging to six genera within the Tephritidae family, including many pest species for which transformation has already been achieved. piggyBac-like sequences, with a high degree of similarity to the original Trichoplusia ni transposase sequence were identified only in six species of the Bactrocera genus.

Inferences on the population structure and colonization process of the invasive oriental fruit fly,Bactrocera dorsalis(Hendel)

The phytophagous insects of the Tephritidae family offer different case histories of successful invasions. An example is Bactrocera dorsalis sensu stricto, the oriental fruit fly which has been recognized as a key pest of Asia and the Pacific. It is known to have the potential to establish adventive populations in various tropical and subtropical areas. Despite the economic risk associated with a putative stable presence of this fly, the genetic aspects of its invasion process have remained relatively unexplored. Using microsatellite markers we have investigated the population structure and genetic variability in 14 geographical populations across the four areas of the actual species range: Far East Asia, South Asia, Southeast Asia and the Pacific Area. Results of clustering and admixture, associated with phylogenetic and migration analyses, were used to evaluate the changes in population genetic structure that this species underwent during its invasion process and establishment in the different areas. The colonization process of this fly is associated with a relatively stable population demographic structure, especially in an unfragmented habitat, rich in intensive cultivation such as in Southeast Asia. In this area, the results suggest a lively demographic history, characterized by evolutionary recent demographic expansions and no recent bottlenecks. Cases of genetic isolation attributable to geographical factors, fragmented habitats and/or fruit trade restrictions were observed in Bangladesh, Myanmar and Hawaii. Regarding the pattern of invasion, the overall genetic profile of the considered populations suggests a western orientated migration route from China to the West.

Diffusion of the Nearctic leafhopper Scaphoideus titanus Ball in Europe: a consequence of human trading activity

Scaphoideus titanus Ball is a Nearctic leafhopper that was introduced for the first time in Europe probably at the beginning of the 20th century. In Europe, this species is a specialist on cultivated grapevines and is of great economic importance as the vector of Flavescence dorée (FD), a Grapevine Yellows disease caused by Candidatus Phytoplasma vitis. The Random Amplified Polymorphic DNA (RAPD) technique was employed to obtain genetic information about the diffusion and the structure of S. titanus populations. Two American and 14 European populations were analysed. A total of 188 reproducible bands, obtained from three arbitrary primers, were considered to assess the amount and the pattern of genetic variation within and among leafhopper populations. American populations showed high levels of intra-population polymorphism and dissimilarity and appeared to be the most isolated of all the tested samples. The results confirm the historical role of American samples as the sources for the more recently founded European populations. RAPD analyses revealed a weak genetic structure of European samples that could probably be explained invoking the human role in their diffusion. The non-natural spreading of S. titanus across Europe is in fact attributable to the exchange of grapevine canes and grafts carrying eggs that the insect laid under the bark to overwinter.

Globalization and fruitfly invasion and expansion: the medfly paradigm

The phytophagous insects of the Tephritidae family commonly referred to as "true fruit flies" offer different case histories of successful invasions. Mankind has played an important role in altering the distributions of some of the more polyphagous and oligophagous species. However, the question arises why only a few species have become major invaders. The understanding of traits underlying adaptation in different environments is a major topic in invasion biology. Being generalists or specialists, along the K-r gradient of the growth curve, make a difference in term of food resources exploitation and interspecies competition and displacement. The species of the genus Ceratitis are good examples of r-strategists. The genetic and biological data of the most notorious Ceratitis species, the Mediterranean fruit fly Ceratitis capitata (medfly), are reviewed to investigate the traits and behaviours that make the medfly an important invader. It can be learnt from medfly, that invasions in a modern global trade network tend to be due to multiple introductions. This fact allows a maintenance or enhancement of genetic variability in the adventive populations, which in turn increases their potential invasiveness. Our current knowledge of the medfly genome opens the way for future studies on functional genomics.

Is polyandry a common event among wild populations of the pest Ceratitis capitata?

In many insect species, females can mate more than once and store sperm from more than one male. An assessment and understanding of polyandry in the field can be important for pest species with a high colonization potential, such as the Mediterranean fruit fly, Ceratitis capitata (Wiedemann), which is also highly polyphagous and among the most destructive agricultural insects. The use of polymorphic microsatellite markers, combined with different statistical approaches, provides evidence that polyandry occurs in two C. capitata natural populations, one population from the Greek island of Chios and one population from Rehovot, in Israel. The observed different level of polyandry is discussed in relation to the genetic diversity, seasonality, and demography of the two populations. When polyandry is present, paternity analysis also indicates that one male, presumably the last, tends to sire most of the progeny. Polyandry and paternity skew may have important implications for the evolution of the species, in terms of maintenance of the genetic variability. Moreover, these aspects of the mating behavior, i.e., remating frequency and paternity skew, may locally affect the sterile insect technique, the most commonly applied control strategy against C. capitata.

Genomic organization and functional characterization of the alcohol dehydrogenase locus of Ceratitis capitata (Medfly)

Approximately 30 kb of genomic DNA enclosing the Adh locus from the medfly, Ceratitis capitata have been cloned and about 15 kb has been structurally and functionally characterized. The locus consists of two genes, Adh-1 and Adh-2, separated by an intergenic region, which is polymorphic in size ranging from approximately 6.4 kb to 8.1 kb. Both genes consist of three exons and two introns. The introns are below 200 bp in size, except the 1st intron of Adh-1, which is unexpectedly long, variable in size and contains a deleted mariner-like element (postdoc). The two genes are transcribed in different orientations. The Adh-2 gene shows the typical pattern of transcription seen in the homologous genes of Drosophilidae presenting high levels of expression in the fat body, gut and ovaries. The Adh-1 gene is only expressed in the body muscle tissues of embryos, larvae and adult flies, raising the question of what its biological function may be. A DNA fragment containing bases -102 to -1666 relative to the first base of the initiating ATG of Adh-1 is sufficient to drive the expression of a reporter gene in body muscles of Drosophila melanogaster embryos, larvae and adult flies. The study provides further insights into the evolution of the Adh genes of higher diptera.

Phylogeny of the subgenus Culicoides and related species in Italy, inferred from internal transcribed spacer 2 ribosomal DNA sequences

Biting midges of the genus Culicoides (Diptera: Ceratopogonidae) include vectors for the economically important animal diseases, bluetongue (BT) and African horse sickness (AHS). In the Mediterranean Basin, these diseases are transmitted by four species of Culicoides: the first three belong in the subgenus Avaritia Fox and are Culicoides imicola Kieffer, Culicoides obsoletus (Meigen) and Culicoides scoticus Downes and Kettle; the fourth is Culicoides pulicaris (Linnaeus) in the subgenus Culicoides Latreille. In the Palaearctic Region, this subgenus (usually referred to as the C. pulicaris group) now includes a loose miscellany of some 50 taxa. The lack of clarity surrounding its taxonomy stimulated the present morphological and molecular study of 11 species collected in Italy. Phylogenetic analysis of nuclear ribosomal DNA internal transcribed spacer 2 (ITS2) sequence variation demonstrated a high degree of divergence. These results, combined with those from a parallel morphological study, disclosed: (1) that some previously described taxa should be resurrected from synonymy; (2) that there are new species to be described; (3) that the subgenus Culicoides (as currently employed) is a polyphyletic assemblage of four lineages - the subgenus Culicoides sensu stricto, the subgenus Silvicola Mirzaeva and Isaev, the subgenus Hoffmania Fox and the hitherto unrecognized Fagineus species complex. Each is discussed briefly (but not defined) and its constituent Palaearctic taxa listed. Strong congruence between morphological and molecular data holds promise for resolving many of the difficult taxonomic issues plaguing the accurate identification of vector Culicoides around the world.

Cchobo, a hobo-related sequence in Ceratitis capitata

A hobo-related sequence, Cchobo, with high similarity to the Drosophila melanogaster HFL1 and hobo108 elements was isolated from the medfly. Thirteen PCR-derived clones, which share 97.9-100% DNA identity, were sequenced, seven of which do not show frame-shift or stop codon mutations in their conceptual translations. The consensus sequence has 99.7% DNA identity with the D. melanogaster hobo element HFLI. In a phylogenetic analysis with other hobo-related elements, Cchobo clusters with the HFL1 and hobo108 elements from D. melanogaster and hobo-related elements from D. simulans, D. mauritiana and Mamestra brassicae. These elements may have undergone horizontal transfer in the recent past. The genomic distribution of Cchobo was studied by FISH to mitotic and polytene chromosomes, which revealed that Cchobo is distributed within both the heterochromatin and euchromatin. Intra- and interstrain polymorphisms were detected both at euchromatic and heterochromatic sites. These findings suggest that active copies of the element may be present in the medfly genome.

The taxonomy of Culicoides vector complexes - unfinished business

The thirty species of Culicoides biting midges that play a greater or lesser role in the transmission of bluetongue (BT) disease in the pantropical regions of the world are listed. Where known, each species is assigned to its correct subgenus and species complex. In the Mediterranean region there are four species of Culicoides involved in the transmission of BT and belong in the subgenera Avaritia Fox, 1955 (three species) and Culicoides Latreille, 1809 (one species). Using both morphological and molecular second internal transcribed spacer (ITS2) sequence data, the authors reappraise the taxonomy of these four species and their congeners. A total of 56 populations of Culicoides collected from across Italy and representing 17 species (18 including the outgroup taxon C. imicola Kieffer, 1913) were analysed. The findings revealed the following: C. imicola is the only species of the Imicola Complex (subgenus Avaritia) to occur in the Mediterranean region. In Europe the subgenera Avaritia and Culicoides (usually, but not quite correctly, equated with the C. obsoletus and C. pulicaris groups, respectively) are both polyphyletic, each comprising three or more species complexes (including a hitherto unknown complex). About half the species studied could not be identified with certainty; furthermore, the results indicate that at least three previously described species of Palaearctic Culicoides should be resurrected from synonymy. Finally, a high level of taxonomic congruence occurred between the morphological and the molecular data. One of the 'new' vector species, C. pulicaris, was described by the father of taxonomy, Carl Linnaeus, in 1758, but today, almost 250 years later, no monograph has appeared that treats the Culicoides fauna of the northern hemisphere as a whole. At a time when such economically important livestock diseases as BT are affecting ever larger areas of Europe, it would seem appropriate to commence the production of such a monograph to aid in the field identification of vector Culicoides. This 'unfinished business' might best be achieved through a collaborative network embracing all ceratopogonid specialists currently active in both the Palaearctic and Nearctic faunal realms.

Molecular Basis of the Size Polymorphism of the First Intron of theAdh-1 Gene of the Mediterranean Fruit Fly, Ceratitis capitata

The first intron of the gene encoding one of the alcohol dehydrogenase isoenzymes (ADH-1) in Ceratitis capitata is highly polymorphic in size. Five size variants of this intron were isolated from different strains and populations and characterized. Restriction map and sequence analysis showed that the intron size polymorphism is due to the presence or absence of (a) a copy of a defective mariner-like element, postdoc; (b) an approximately 550-bp 3' indel which exhibits no similarity to any known sequence; and (c) a central duplication of 704 bp consisting of part of the 3' end of the postdoc element, the region between postdoc and the 3' indel, and the first 20 bp of the 3' indel. The homologous Adh-1 intron was amplified from the congeneric species, Ceratitis rosa, in order to obtain an outgroup for comparative and phylogenetic analyses. The C. rosa introns were polymorphic in size, ranging from about 1100 to 2000 bp, the major difference between them being the presence or absence of a mariner-like element Crmar2, unrelated to the postdoc element. Phylogenetic analysis suggests that the shorter intron variants in C. capitata may represent the ancestral form of the intron, the longest variants apparently being the most recent.

Population genetics of the potentially invasive African fruit fly species, Ceratitis rosa and Ceratitis fasciventris (Diptera: Tephritidae)

A set of 10 microsatellite markers was used to survey the levels of genetic variability and to analyse the genetic aspects of the population dynamics of two potentially invasive pest fruit fly species, Ceratitis rosa and C. fasciventris, in Africa. The loci were derived from the closely related species, C. capitata. The degree of microsatellite polymorphism in C. rosa and C. fasciventris was extensive and comparable to that of C. capitata. In C. rosa, the evolution of microsatellite polymorphism in its distribution area reflects the colonization history of this species. The mainland populations are more polymorphic than the island populations. Low levels of differentiation were found within the Africa mainland area, while greater levels of differentiation affect the islands. Ceratitis fasciventris is a central-east African species. The microsatellite data over the Uganda/Kenya spatial scale suggest a recent expansion and possibly continuing gene flow within this area. The microsatellite variability data from C. rosa and C. fasciventris, together with those of C. capitata, support the hypothesis of an east African origin of the Ceratitis spp.

Medfly transposable elements: diversity, evolution, genomic impact and possible applications

The medfly genome has been shown to contain a rich assortment of transposable elements from the mariner, Tc1, hAT and gypsy/Ty3 families. These elements display different levels of diversity, abundance and distribution in the genome. The presence of actively transposing elements in the medfly genome is revealed by hybrid dysgenesis phenomena, insertion site polymorphisms and other genetic instabilities. The medfly has been a target of transformation studies involving the exogenous elements Minos, Hermes and piggyBac from three families. The presence of active endogenous homologous elements can have important implications for the stability of such transgenic lines. The potential applications of endogenous elements for medfly population analysis and control are discussed.

Comparative analysis of microsatellite loci in four fruit fly species of the genus Ceratitis (Diptera: Tephritidae)

The possibility to cross-species amplify microsatellites in fruit flies of the genus Ceratitis was tested with the polymerase chain reaction (PCR) by analysing 23 Ceratitis capitata (Wiedemann) microsatellite markers on the genomic DNA of three other economically important, congeneric species: C. rosa (Karsch), C. fasciventris (Bezzi) and C. cosyra (Walker). Twenty-two primer pairs produced amplification products in at least one of the three species tested. The majority of the products were similar, if not identical in size to those expected in C. capitata. The structures of the repeat motifs and their flanking sequences were examined for a total of 79 alleles from the three species. Sequence analysis revealed the same repeat type as the homologous C. capitata microsatellites in the majority of the loci, suggesting their utility for population analysis across the species range. A total of seven loci were differentially present/absent in C. capitata, C. rosa, C. fasciventris and C. cosyra, suggesting that it may be possible to differentiate these four species using a simple sequence repeat-based PCR assay. It is proposed that medfly-based microsatellite markers could be utilized in the identification and tracing of the geographical origins of colonist pest populations of the four tested species and in the assessment of their risk and invasive potentials; thereby assisting regulatory authorities in implementing quarantine restrictions and other pest control measures.

Genetic differentiation, gene flow and the origin of infestations of the medfly, Ceratitis capitata

The genetic structure of natural populations of the economically important dipteran species Ceratitis capitata was analysed using both biochemical and molecular markers. This revealed considerable genetic variation in populations from different geographic regions. The nature of this variation suggests that the evolutionary history of the species involved the spread of individuals from the ancestral African populations through Europe and, more recently, to Latin America, Hawaii and Australia. The observed variation can be explained by various evolutionary forces acting differentially in the different geographic areas, including genetic drift, bottleneck effects, selection and gene flow. The analysis of the intrinsic variability of the medfly's genome and the genetic relationships among populations of this pest is a prerequisite for any control programme.

A new basal subfamily of mariner elements in Ceratitis rosa and other tephritid flies

Several copies of highly related transposable elements, Crmar2, Almar1, and Asmar1, are described from the genomes of Ceratitis rosa, Anastrepha ludens, and A. suspensa, respectively. One copy from C. rosa, Crmar2.5, contains a full-length, uninterrupted ORF. All the other copies, from the three species contain a long deletion within the putative ORF. The consensus Crmar2 element has features typical of the mariner/Tc1 superfamily of transposable elements. In particular, the Crmar2 consensus encodes a D,D41D motif, a variant of the D,D34D catalytic domain of mariner elements. Phylogenetic analysis of the relationships of these three elements and other members of the mariner/Tc1 superfamily, based on their encoded amino acid sequences, suggests that they form a new basal subfamily of mariner elements, the rosa subfamily. BLAST analyses identified sequences from other diptera, including Drosophila melanogaster, which appear to be members of the rosa subfamily of mariner elements. Analyses of their molecular evolution suggests that Crmar2 entered the genome of C. rosa in the recent past, a consequence of horizontal transfer.

Microsatellite analysis of medfly bioinfestations in California

The Mediterranean fruit fly, Ceratitis capitata, is a destructive agricultural pest with a long history of invasion success. This pest has been affecting different regions of the United States for the past 30 years, but a number of studies of medfly bioinfestations has focused on the situation in California. Although some progress has been made in terms of establishing the origin of infestations, the overall status of this pest in this area remains controversial. Specifically, do flies captured over the years represent independent infestations or the persistence of a resident population? We present an effort to answer this question based on the use of multilocus genotyping. Ten microsatellite loci were used to analyse 109 medflies captured in several infestations within California between 1992 and 1998. Using these same markers, 242 medflies from regions of the world having 'established' populations of this pest including Hawaii, Guatemala, El Salvador, Ecuador, Brazil, Argentina and Peru, were also analysed. Although phylogenetic analysis, amova analysis, the IMMANC assignment test and geneclass exclusion test analysis suggest that some of the medflies captured in California are derived from independent invasion events, analysis of specimens from the Los Angeles basin provides support for the hypothesis that an endemic population, probably derived from Guatemala, has been established.

Molecular differentiation of the Old World Culicoides imicola species complex (Diptera, Ceratopogonidae), inferred using random amplified polymorphic DNA markers

Samples of seven of the 10 morphological species of midges of the Culicoides imicola complex were considered. The importance of this species complex is connected to its vectorial capacity for African horse sickness virus (AHSV) and bluetongue virus (BTV). Consequently, the risk of transmission may vary dramatically, depending upon the particular cryptic species present in a given area. The species complex is confined to the Old World and our samples were collected in Southern Africa, Madagascar and the Ivory Coast. Genomic DNA of 350 randomly sampled individual midges from 19 populations was amplified using four 20-mer primers by the random amplified polymorphic DNA (RAPD) technique. One hundred and ninety-six interpretable polymorphic bands were obtained. Species-specific RAPD profiles were defined and for five species diagnostic RAPD fragments were identified. A high degree of polymorphism was detected in the species complex, most of which was observed within populations (from 64 to 76%). Principal coordinate analysis (PCO) and cluster analysis provided an estimate of the degree of variation between and within populations and species. There was substantial concordance between the taxonomies derived from morphological and molecular data. The amount and the different distributions of genetic (RAPD) variation among the taxa can be associated to their life histories, i.e. the abundance and distribution of the larval breeding sites and their seasonality.

Genomic organization and characterization of the white locus of the Mediterranean fruitfly, Ceratitis capitata

An approximately 14-kb region of genomic DNA encoding the wild-type white eye (w+) color gene from the medfly, Ceratitis capitata has been cloned and characterized at the molecular level. Comparison of the intron-exon organization of this locus among several dipteran insects reveals distinct organizational patterns that are consistent with the phylogenetic relationships of these flies and the dendrogram of the predicted primary amino acid sequence of the white loci. An examination of w+ expression during medfly development has been carried out, displaying overall similarity to corresponding studies for white gene homologues in Drosophila melanogaster and other insects. Interestingly, we have detected two phenotypically neutral allelic forms of the locus that have arisen as the result of an apparently novel insertion or deletion event located in the large first intron of the medfly white locus. Cloning and sequencing of two mutant white alleles, w1 and w2, from the we,wp and M245 strains, respectively, indicate that the mutant conditions in these strains are the result of independent events--a frameshift mutation in exon 6 for w1 and a deletion including a large part of exon 2 in the case of w2.

Microsatellite polymorphism in the Mediterranean fruit fly, Ceratitis capitata

A total of forty-three simple sequence repeats (SSRs) were identified in the Mediterranean fruit fly (medfly) Ceratitis capitata. The most common SSR was the dinucleotide (TG)n/(CA)n occurring in thirty of the forty-three microsatellite loci. Polymorphism at ten dinucleotide markers was investigated in 122 flies from six natural populations sampled in the native and colonized areas. A very high level of allelic diversity was detected in the species range. An average of 13.6 alleles was found over all the ten loci indicating the informativeness of SSRs as genetic markers for the medfly. The distribution of microsatellite polymorphism in the species range reflects the medfly colonization history.

Evolution of different subfamilies of mariner elements within the medfly genome inferred from abundance and chromosomal distribution

The abundance and distribution pattern of eight mariner elements from three different subfamilies in the genome of the medfly Ceratitis capitata were determined. The copy numbers, as determined by slot-blot analysis, were very different for these elements. Their abundance did not change significantly within the native, the ancient or the newly derived populations, indicating that the rapid colonization process of the medfly had not affected the copy number of mariner elements. The distribution of the mariner elements was analyzed using fluorescent in situ hybridization (FISH) with charge-coupled device (CCD) camera analysis. The pattern of distribution in euchromatin and heterochromatin varied greatly and was distinctive and specific for each element. The implications of these findings are discussed and it is concluded that they generally support the hypothesis of a transposition/selection model in which the abundance and distribution patterns of these elements are regulated primarily by selection against deleterious effects due to meiotic ectopic recombination, while genetic drift would have played a minor role.

Genetic aspects of the worldwide colonization process of Ceratitis capitata

Multilocus enzyme electrophoresis data from 26 polymorphic loci (124 alleles) were used to analyze the genetic aspects of the worldwide colonization of Ceratitis capitata (medfly). Eighty-two samples of 17 populations were collected from six regions throughout the species range: Africa, extra-Mediterranean islands (Madeira and Gran Canaria), Mediterranean region, Latin America (Guatemala), Pacific (Hawaii), and Australia. The variability parameters (H, P, A) reveal that the geographical dispersal of medfly from its ancestral source area (East Africa) is associated with a great reduction in variability. The pattern of decreasing variability occurs at two regional levels: in the African-Mediterranean region where the differentiation is gradual, and in the Latin American-Pacific region where some ancestral variability is still present as a consequence of recent colonization. The UPGMA phylogenetic tree, derived from Nei's genetic distances, shows the presence of intraspecific differentiative processes affecting mainly the two island populations, Réunion and Hawaii. The population genetic changes observed in the species range are consistent with both the chronology and the historical circuitous course of the medfly colonization process.

Characterization and evolution of mariner elements from closely related species of fruit flies (Diptera: Tephritidae)

Mariner elements were amplified using the polymerase chain reaction from two species of tephritid flies, Ceratitis rosa and Trirhithrum coffeae. The sequences were approximately 1.3 kb in length. None of these elements appeared to be functional, as in every case the open reading frame (ORF) was disrupted by the presence of frameshifts or stop codons. These elements, Crmar1 and Tcmar1, are very similar to the Ccmar1 element previously amplified from the closely related tephritid species C. capitata and are members of the mellifera subfamily of mariner elements. The phylogeny and pattern of divergence of these elements were examined in relation to the phylogeny of the host species. It is highly probable that the elements were present in the ancestral lineage prior to the divergence of the three species. The copy numbers of the elements within each species are very different, ranging from about 10 in T. coffeae to 5,000 in C. rosa. The possible mechanisms which determine the copy number of an element in the host genome are discussed.

Genetic and molecular investigations on the endogenous mobile elements of non-drosophilid fruitflies

A syndrome of abnormal genetic effects, resembling Drosophila hybrid dysgenesis, occurs in Ceratitis capitata when strains of different origin are mated. The pattern of abnormal traits observed appears to be the phenotypic expression of a complex interacting dysgenic system of inducer and suppressor effects; probably more than one system is activated in the crosses. This suggests that different systems of mobile elements occur in different strains and populations of C. capitata. Using a PCR primer specific to the ITR sequence of a deleted element, full length mariner elements were isolated from C. capitata, Ceratitis rosa, and Trirhithrum coffeae. Very high similarities were found in inter- and intraspecific comparisons of the elements. The majority of these elements contained deletions and frame-shifts. However, one clone Ccmar1.18, from C. capitata, was found to possess an uninterrupted ORF coding for 338 amino acids with approximately 60% similarity to the Mos1 element of Drosophila mauritiana. Database searches and phylogenetic analyses showed that the mariner elements isolated in the present study are representatives of Robertson's mellifera mariner subfamily. The copy numbers of the elements within each species are very different, ranging from about 10 in T. coffeae to 5000 in C. rosa.