S. Bertin

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.

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.

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.

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 males 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.

Unfaithful Mediterranean Fruit Fly Ceratitis capitata Females: Impact on the SIT?

An understanding of the levels of remating and paternity skew in the field can be important for polyphagous pest species with a high colonization potential such as the Mediterranean fruit fly Ceratitis capitata (Wiedemann). The use of polymorphic simple sequence repeats on flies from two Mediterranean populations in combination with various statistical methods showed not only that Mediterranean fruit fly females remate in the wild, but most importantly, that the level of sperm precedence could influence the effect of remating itself since one male, presumably the last, tends to sire most of the progeny. Levels of remating and paternity skew may have important implications for the evolution of the species in terms of maintenance of genetic variability. Moreover, these features of mating behaviour may locally affect the efficiency of the sterile insect technique (SIT), which is a commonly applied control strategy against the Mediterranean fruit fly.