The past three decades have witnessed a dramatic increase in the economic importance of the whitefly Bemisia tabaci (Genn.) (Aleyrodidae; Hemiptera) in subtropical and mild temperate agriculture systems, owing to the damage it causes to plants when it feeds in the phloem, and its ability to transmit plant viruses. B. tabaci occupies tropical and subtropical habitats, producing 11–15 generations per year [1, 2]. The B. tabaci complex consists of diverse biological 'types' with distinct genetic polymorphisms [3, 4], and differences in host range, fecundity, dispersal behaviours, prokaryotic endosymbiont composition, and competency with respect to begomovirus transmission, a group of small circular ssDNA plant viruses (genus Begomovirus, family Geminiviridae) [5, 6]. The highly fecund Old World B biotype can produce ~ 300 eggs/female, colonizes over 500 host species, while the New World A type colonizes about 200 species and has a lower fecundity (~ 100 eggs/female). In contrast, the Jatropha type colonizes only a few species within the genus Jatropha and exhibits low fecundity (~ 30–50 eggs/female) .
B. tabaci adults develop from eggs, after passing through four instars in approximately 2–3 wk and development is temperature dependent. Members of this complex are haplodiploid and thus unfertilized eggs give rise to haploid males; fertilized eggs develop into diploid females (arrhenotoky) [1, 2].
The B type of B. tabaci transmits begomoviruses to a large number of crop, ornamental, and weed species . Begomovirus have either one (monopartite) or two (bipartite) genomic components . Those infecting tomato constitute a large group of begomoviruses. Among them the bipartite Tomato mottle virus (ToMoV) originated in the New World (Florida/Caribbean region), whereas, the monopartite Old World Tomato yellow leaf curl virus (TYLCV) is indigenous to the Old World (Middle East and Africa). TYLCV recently was introduced to the Caribbean Islands and has since spread into the South eastern States of the U.S.A. .
Begomoviruses are transmitted by B. tabaci in a circulative manner [10, 11]. Virus particles ingested through the stylets enter the oesophagus and the filter chamber, are transported through the gut into the hemocoel, reach the salivary glands and are finally 'transmitted' during feeding, about 8–12 h after the beginning of an acquisition access period . VeloCity of translocation is reported to constitute an intrinsic property of the vector, not of the virus [12, 13]. B. tabaci is able to transmit begomoviruses, and in particular TYLCV, for its lifetime, after the latent period has been achieved [14, 15]. The ingestion of TYLCV by the whitefly vector is accompanied by a marked decrease in whitefly longevity and fertility . In contrast whiteflies that have ingested ToMoV displayed higher fecundity when reared on virus-free tomato than whiteflies not exposed to the virus . TYLCV transcripts have been found in B. tabaci harbouring this virus, whereas viral transcripts are not detected in whiteflies that have ingested ToMoV , suggesting a fundamental difference in interactions between these two begomoviruses and their whitefly vector.
At least one whitefly species that colonizes some of the same hosts as B. tabaci (e.g. the greenhouse whitefly, Trialeurodes vaporariorum) is known to be capable of ingesting, but does not transmit begomoviruses , and at least one barrier to transmission has been shown to occur at the gut/hemocoel interface [12, 18]. The receptors that are hypothesized to mediate begomovirus translocation into the salivary glands of B. tabaci, which is a requisite to transmission, and their genes, are presently unidentified.
Surprisingly very little is known about the genetic make up of this insect. The nuclear DNA content of B. tabaci male and female was estimated as 1.04 and 2.06 pg respectively, using flow cytometry, indicating that the haploid genome of B. tabaci contains about one billion bp, which is approximately five times the size of the genome of the fruitfly Drosophila melanogaster . However, it is still not clear if this size estimate will prove to be accurate and so a long-term goal is to determine the complete genome of this whitefly. Ultimately it is of interest to isolate and identify the genes expressed during the life cycle of the whitefly B. tabaci and to understand the genetic makeup of this pest. Of particular interest is the identification of specific genes and their functions, which are expressed during the development of B. tabaci, as well as those involved in circulative virus transmission, the detoxification of insecticides, and the determination of polyphagy or monophagy in different B. tabaci biotypes. Consequently, the construction of cDNA libraries and the analyses of the sequences for the widespread 'B' biotype of B. tabaci constitute a first step in this endeavour.