The whitefly Bemisia tabaci (Hemiptera: Aleyrodidae) (Fig. 1) is a pest and virus vector in all warm-to-hot climate regions. A strong geographical structure of B. tabaci has been detected using various genetic markers, particularly microsatellites (De Barro 2005), ITS1 and mitochondrial cytochrome oxidase I (mtCOI) (De Barro et al. 2005; Boykin et al. 2007). The high genetic differentiation between populations and biological differences with respect to host plant and reproductive incompatibility has triggered an exciting debate as to whether B. tabaci is a complex species consisting of various biotypes or races, or a species complex (Brown et al. 1995; Frohlich et al. 1999; De Barro et al. 2005). Most recently, based on genetic and biological data, Dinsdale et al. (2010) and De Barro et al. (2011) concluded that B. tabaci is a species complex. These authors used mtCOI sequences to provide a rationale to classify previously identified individuals of B. tabaci into 11 groups containing 24 species. The species classification is consistent with crossing experiments in which mostly no reproductive compatibility could be detected (reviewed in Liu et al. 2012). Although this classification needs to be confirmed with other genetic markers to establish a more reliable phylogeny, the terminology derived from this classification is used here and was preferred to the usual biotype terminology because the distinctive biological features that are required to distinguish biotypes were mostly missing.
Species in nature are often incompletely isolated for millions of years after their formation (Mallet 2005). As divergence between some putative species of B. tabaci is estimated to be within 3–4 million years (Delatte et al. 2005; De Barro et al. 2011), hybridization may still be expected. Consistently, hybrids were obtained in seven of the tested combinations of the putative species (reviewed in Liu et al. 2012): Middle East-Asia Minor 1 × Mediterranean (MEAM1 × Med), Asia II-1 × Asia II-3, Asia II-1 × Asia II-7, MEAM1 × Asia II-7, MEAM1 × China 1, Sub-Saharan Africa 1 × Med (S-Afr1 × Med), S-Afr1 × S-Afr2. Hybridization between putative B. tabaci species is detectable by generation of female progeny because whiteflies exhibit haplodiploidy consisting of male production from unfertilized eggs and female production from fertilized eggs. The F1 progenies obtained with these inter-species crosses were dominated by males (Byrne et al. 1995; De Barro and Hart 2000; Maruthi et al. 2004; Omondi et al. 2005; Xu et al. 2010), whereas the sex ratio was nearly 50% with intra-species crosses performed as a control. The fertility of the F1 females was not formally tested except for crosses S-Afr1 × S-Afr2, in which an F2 generation was obtained (Maruthi et al. 2004), and for crosses MEAM1 × Med, in which F1 females were sterile (Sun et al. 2011). Interestingly, whereas crosses between Chinese representatives of MEAM1 and Med species generated 0–2% females (Sun et al. 2011; Li et al. 2012), crosses between Israeli representatives did not generate any females (Elbaz et al. 2010). Different whitefly densities and durations of observation may explain this contrasting result, but potential differences between whitefly populations in the two studies cannot be ruled out (Sun et al. 2011). These crosses were carried out under laboratory conditions, and there are presently insufficient observations of potential crosses in natural conditions to estimate the importance of hybridization in the evolution of nascent species within the B. tabaci complex. The geographical spread of B. tabaci populations belonging to MEAM1 into many agro-ecosystems throughout the world has provided the opportunity to observe the various outcomes of secondary contacts between previously allopatric divergent populations. Besides the displacement scenario reported from the American continent (Brown et al. 1995) with complete reproductive isolation between the putative species MEAM1 and New World (Xu et al. 2010), hybrids were detected following secondary contact of MEAM1 species and indigenous species from Australia (Austr) using isozymes profiles (Gunning et al. 1997) and the Indian Ocean (IndOc) using microsatellite markers (Delatte et al. 2005). Unexpectedly, no hybrids were detected following secondary contact between MEAM1 and the indigenous species from Spain (Med) using 11 RAPD markers (Moya et al. 2001), although the genetic distance between Med and MEAM1 with respect to mtCOI sequences is lower than the distance between MEAM1 and Austr, and MEAM1 and IndOc (Delatte et al. 2005; Dinsdale et al. 2010), the pairs for which natural hybrids were detected. However, signatures that suggested possible hybridization between MEAM1 and Med were detected in North-American natural populations, although at very low frequency (McKenzie et al. 2012). Thus, from a very comprehensive sampling (4647 individuals analyzed from 517 collections), only 16 were suspected to be hybrids, 15 according to esterase zymogram assays, and 1 according to two microsatellite markers which proved relatively diagnostic for the two putative species. Consistent with field results, hybrids were obtained in experimental crosses with all three pairs of species (Ronda et al. 1999; De Barro and Hart 2000; Sun et al. 2011; H. Delatte pers. comm.) indicating some reproductive compatibility with all of them, including the MEAM1 and Med species pair.
Considering the relatively low genetic distance between Med and MEAM1 and the incomplete reproductive isolation, in this study we further investigated the expected gene flow between these two putative species under natural conditions. B. tabaci populations were sampled from the North East of Morocco, where representatives of the two species have been detected previously (Tahiri et al. 2006). A total of 346 female B. tabaci were collected in 2003 and 2005 and assigned to MEAM1 (119), Med (225), and a new putative species (2) based on sequences of the mtCOI gene. MEAM1 and Med individuals were further analyzed using microsatellite markers. A Bayesian clustering analysis of the microsatellite profiles (Structure) distinguished two groups with 100% consistency with the mtCOI groups, except for two individuals identified as hybrids using complementary population genetic analyses. Hybrid detection is discussed in relation to the secondary endosymbiont infection status determined on a sample of MEAM1 and Med populations and the contrasting outcomes of reported crossing experiments between these species (Elbaz et al. 2010; Sun et al. 2011).