Anna R. Malacrida

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.

Polymorphism within and between populations of Ceratitis capitata: comparison between RAPD and multilocus enzyme electrophoresis data.

Random amplified polymorphic DNA (RAPD) analysis and multilocus enzyme electrophoresis (MLEE) were used to assess genetic variability in six wild populations and in five laboratory strains of Ceratitis capitata. The RAPD technique reveals larger amounts of genetic variation than the conventional MLEE, and can improve discrimination within and between populations and strains. In our experimental conditions, RAPD analysis with four different primers produces 174 polymorphic bands out of 176, while MLEE analysis at 26 enzyme loci scores 74 alleles. RAPD fingerprints are peculiar to African flies, while different laboratory strains have similar patterns, independently of their origins. The results obtained by these two methods are significantly correlated, and are in agreement with the general trend of decreasing variability from African populations towards the peripheral and laboratory ones. UPGMA dendrograms derived from MLEE (protein) and RAPD (DNA) data show that a major part of intraspecific variability involves the differentiation of central vs. peripheral populations.

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.

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 capitatawas 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 medflys genome and the genetic relationships among populations of this pest is a prerequisite for any control programme.

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.

On the origins of medfly invasion and expansion in Australia.

As a result of their rapid expansion and large larval host range, true fruit flies are among the worlds most important agricultural pest species. Among them, Ceratitis capitata has become a model organism for studies on colonization and invasion processes. The genetic aspects of the medfly invasion process have already been analysed throughout its range, with the exception of Australia. Bioinvasion into Australia is an old event: medfly were first captured in Australia in 1895, near Perth. After briefly appearing in Tasmania and the eastern states of mainland Australia, medfly had disappeared from these areas by the 1940s. Currently, they are confined to the western coastal region. South Australia seems to be protected from medfly infestations both by the presence of an inhospitable barrier separating it from the west and by the limited number of transport routes. However, numerous medfly outbreaks have occurred since 1946, mainly near Adelaide. Allele frequency data at 10 simple sequence repeat loci were used to study the genetic structure of Australian medflies, to infer the historical pattern of invasion and the origin of the recent outbreaks. The combination of phylogeographical analysis and Bayesian tests showed that colonization of Australia was a secondary colonization event from the Mediterranean basin and that Australian medflies were unlikely to be the source for the initial Hawaiian invasion. Within Australia, the Perth area acted as the core range and was the source for medfly bioinvasion in both Western and South Australia. Incipient differentiation, as a result of habitat fragmentation, was detected in some localized areas at the periphery of the core range.

Fluorescent sperm marking to improve the fight against the pest insect Ceratitis capitata (Wiedemann; Diptera: Tephritidae)

The Sterile Insect Technique (SIT) involving area-wide release of mass-reared and sterilized pest insects has proven successful to reduce, control and eradicate economically important pest species, such as the Mediterranean fruit fly (medfly). For the efficient application, effective monitoring to assess the number and mating success of the released medflies is essential. Here, we report sperm-specific marking systems based on the spermatogenesis-specific Ceratitis capitata beta2-tubulin (Ccbeta2t) promoter. Fluorescent sperm can be isolated from testes or spermathecae. The marking does not cause general disadvantages in preliminary laboratory competitiveness assays. Therefore, transgenic sperm marking could serve as a major improvement for monitoring medfly SIT programs. The use of such harmless transgenic markers will serve as an ideal initial condition to transfer insect transgenesis technology from the laboratory to field applications. Moreover, effective and easily recognizable sperm marking will make novel studies possible on medfly reproductive biology which will help to further improve SIT programs.

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

BackgroundThe 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.ResultsA 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.ConclusionThe 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.

Microsatellite analysis reveals remating by wild Mediterranean fruit fly females, Ceratitis capitata

Accurate estimates of remating in wild female insects are required for an understanding of the causes of variation in remating between individuals, populations and species. Such estimates are also of profound importance for major economic fruit pests such as the Mediterranean fruit fly (Ceratitis capitata). A major method for the suppression of this pest is the sterile insect technique (SIT), which relies on matings between mass‐reared, sterilized males and wild females. Remating by wild females will thus impact negatively on the success of SIT. We used microsatellite markers to determine the level of remating in wild (field‐collected) Mediterranean fruit fly females from the Greek Island of Chios. We compared the four locus microsatellite genotypes of these females and their offspring. Our data showed 7.1% of wild females remated. Skewed paternity among progeny arrays provided further evidence for double matings. Our lowest estimate of remating was 3.8% and the highest was 21%.

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.