The whitefly Bemisia tabaci (Gennadius) (Hemiptera: Aleyrodidae) is a species complex composed of at least 31 morphologically indistinguishable cryptic species (hereafter referred to as "species") [1–6]. These species differ genetically as well as in host range, fecundity, insecticide resistance, mating behavior and ability to transmit begomoviruses [7–12]. While many species within the B. tabaci complex cause no obvious harms to agricultural production; some members of this species complex are highly invasive and cause extensive damage to agricultural, horticultural, and ornamental crops through direct feeding or the transmission of plant viruses [13, 14]. Two species of the B. tabaci complex, Middle East - Asia Minor 1 (previously known as biotype B; hereafter MEAM1) and Mediterranean (previously known as biotype Q; hereafter MED) have risen to international prominence due to their global invasion during the last 20 years [8, 15]. MEAM1 and MED originated from the Middle East Asia Minor and Mediterranean Basin regions respectively, and have invaded many countries around the world [3, 16]. Extensive evidence has indicated that the invasion of MEAM1 and MED are associated with the displacement of their closely related indigenous whitefly species [8, 14].
The invasion of an alien whitefly species and competition between invasive and indigenous species are mediated by many abiotic and biotic factors. Efforts have been made to understand the factors that contribute to the incursion of the two species into new regions and the displacement of indigenous species. For example, the invasion of MEAM1 is assumed to be associated with its high adaptability under various environmental stresses and host plants [9, 10, 17, 18]. Liu et al.  also revealed that the displacement of indigenous whitefly species by MEAM1 is associated with the behavior of mating interference. On the other hand, the spread of MED is closely related to its ability to maintain high levels of resistance to major classes of insecticides [19–22]. Despite these advances, the molecular mechanisms underlying the extraordinary capacity of MEAM1 and MED to spread and ultimately displace the native species remains largely unknown. Furthermore, previous studies have mainly focused on single gene or individual aspect of the B. tabaci biology, a global picture of the genetic factors associated with the invasion of these two whitefly species is still lacking.
The genomic divergence between invasive and indigenous species is valuable for determining how phenotypes specific to invasive species have been formed . By examining the divergence of large numbers of genes, a overall picture of genetic differences and invasion mechanisms may be attained . Here, we propose that a global analysis of genomic divergence among the B. tabaci species complex will reveal the molecular mechanisms underlying the biological invasions of MEAM1 and MED. First, the B. tabaci species are reproductively isolated, but retain sufficient genetic similarity for comparative analyses [4, 25, 26]. Second, the whitefly species went through an allopatric divergence process and showed significant differences in survival and reproductive performance [16, 27, 28]. This warrants exploring the interspecies evolutionary processes through the comparison of orthologous genes. Third, at least 31 species have been delineated for the B. tabaci complex including 2 invasive species and 29 indigenous species. The rich diversity of invasive and indigenous species allows extensive cross comparisons of orthologous genes among difference members of the complex, which will facilitate the elucidation of invasive mechanisms.
The transcriptomes of two invasive whitefly species MED and MEAM1 have been sequenced using Illumina sequencing technology [29, 30]. In this study, we sequenced the transcriptome of an indigenous B. tabaci species - Asia II 3 (previously known as biotype ZHJ1) and generated 52,535 distinct sequences. These transcriptome sequences provide a rich molecular resource for functional analysis of the native B. tabaci species. In order to gain further insights on how genes have diverged between the indigenous and invasive whiteflies, we compared the global sequence divergence between the transcriptomes of Asia II 3 and the invasive species MEAM1 and MED. The identification and analysis of divergent sequences between the indigenous and invasive whitefly species opens the door for future investigations on the molecular mechanisms of B. tabaci invasion. The approach described in this manuscript will significantly accelerate the identification of genetic variation underlying adaptation in B. tabaci and other invasive species.