The medfly, Ceratitis capitata
, is a highly invasive agricultural pest species that has expanded from its native range in sub-Saharan Africa to become a cosmopolitan species in less than 200 years. Its success as an invasive species is partially due to its unusually wide host range and its ability to adapt to a wide range of climatic conditions and habitats [1
]. As such, it has become the target of extensive control programs and a model organism for the sterile insect technique (SIT), a method considered to be among the most efficient and environmentally friendly control procedures [2
]. This technique, designed to reduce the size of the target population, is based on the release of sterile males that compete for wild females. Indeed, the medfly was the first non-drosophilid organism to be transformed [4
], with the goal of introducing genes capable of improving genetic sexing systems for the SIT. Although molecular genetics studies of the medfly began in the early 1990s, at present (January 2008) only 182 putative coding sequences are known, almost half of which are fragmentary [5
]. This lack of molecular data is in sharp contrast to the mass of data that has been accrued on the classical and population genetics of this model insect.
The number of published complete genome sequences has grown exponentially since the first two bacterial genomes were reported in 1995, with over 600 available as of 2008 [6
]. These genome sequences include a number of important insect genomes, such as those of Drosophila melanogaster
, the malarial mosquito, Anopheles gambiae
, the silkworm Bombyx mori
, and the honeybee Apis mellifera
]. Numerous other insect genome-sequencing projects are in progress, including those for numerous species of Drosophila
, mosquitoes of the genera Aedes
, the cotton bollworm Helicoverpa armigera
, the tobacco budworm Heliothis virescens
, the human louse Pediculus humanus
, the vector of Chagas disease Rhodnius prolixus
, the tsetse fly Glossina morsitans
, the sandfly Lutzomyia longipalpis
, parasitic wasps of the genus Nasonia
, the flour beetle Tribolium castaneum
, and several aphids and ticks [6
The initial goal of these genome sequence projects is to identify a complete set of genes and subsequently to determine their expression in different life stages and tissues and to characterize their regulation and function. Given that the haploid genome size of the medfly is relatively large (540 Mb), three times larger than that of D. melanogaster, the sequencing of the complete genome would be prohibitively expensive except by a large consortium.
To address the lack of sequence data available for the medfly, we have initiated a functional genomics approach based on expressed sequence tags (ESTs). ESTs represent a relatively quick and inexpensive technology for discovering new genes, for obtaining data on their expression and regulation, and for the construction of genome maps [13
]. They are an ideal means for the rapid exploration of transcriptomes, especially those of species with large genome sizes. ESTs can also form a very solid basis for evolutionary studies.
The genetic information obtained from this EST initiative will be of enormous value for identifying and determining the functions of genes involved in a number of important biological processes, including sex determination, sex differentiation, reproduction, courtship behavior, and olfaction. Such processes represent ideal targets for the development of novel control methods and pest-monitoring systems. To target these biological processes we have utilised cDNA libraries derived from medfly embryos and adult heads as the source of our ESTs. The embryo library permits the identification of genes involved in sex determination and development whereas the head library permits the identification of genes involved in different behaviours, in olfaction etc. The availability of a large number of transcripts also permits the development of oligonucleotide-based microarrays that will facilitate the study of these biological processes by means of mass expression profile analyses.
Apart from its economic importance, the medfly also represents an alternative model dipteran species. Drosophila melanogaster is the model dipteran par excellence, but in many ways it is an atypical species. The availability of mosquito genomes has helped to balance this bias, and hopefully the medfly data presented here will also contribute to that end.
Here we present a comprehensive EST-based gene discovery project that has provided sequences of 11,885 transcripts and yielded novel insights into various biological activities of an important agricultural pest, the medfly.