Prospects for the application of genomics

Whiteflies are classified in the family Aleyrodidae (Sternorrhyncha: Hemiptera [suborder Homoptera]) (Mound 1984; Mound and Halsey 1978). The closest relatives to whiteflies are aphids, mealybugs, psyllids, and scales, which all feed using piercing and sucking mouthparts (Martin 1987, 2003; Martin and Mound 2007). Reproductive modulation is one of the many examples of plasticity in the Aleyrodidae, an ancient insect family. They are unique among most of their close relatives in employing a haplodiploid sex determination system, in which fertilized eggs yield females and males are produced from unfertilized eggs (Blackman and Cahill 1998; Schrader 1920). Thus, all male offspring inherit only the maternal genome, whereas female offspring inherit genes from both parents. The sex ratio is modulated by increased production of females when males are abundant (see Byrne et al. 1990). Haplodiploid sex determination, combined with infection by bacteria such as Wolbachia and Cardinium spp. that cause cytoplasmic incompatibility (CI), could result in the demise of the germline, or at the least, a severely bottlenecked population due to increased inbreeding. CI in B. tabaci is expressed as a reduction in the number of female offspring resulting from crosses between infected males and uninfected females (or females infected with different bacterial strains) (Caballero 2007; Stouthamer et al. 1999). This process could decrease genetic diversity while also reducing fitness and altering other traits which B. tabaci is widely known to employ, presumably for adaptive advantage. How the interactions between the host and its prokaryotic passengers influences genetic diversity and evolution in B. tabaci is not well understood. Whiteflies, like aphids and many other plant-feeding hemipterans, feed on plant sap, which is deficient in certain essential amino acids. A hallmark of this group is that they harbor species-specific mutualistic endosymbionts (referred to as "primary") having a genome reduced in size compared to free-living counterparts that is housed in a specialized structure called a bacteriome. The bacteriome (Fig. 2.1) is tightly associated with whitefly ovaries from which founder bacteria migrate to the immature egg and are passed on to the offspring (Costa et al. 1996). Based on studies of other homopterans (hemipterans), primary symbionts are less complex in comparison to free-living bacteria (Douglas 1998; Thao and Baumann 2004a, b; Zchori-Fein and Brown 2002). These insect host-bacterial complexes have evolved a mutualistic relationship in which each contributes to the survival of the other. Tightly co-evolved associations have been corroborated for certain whitefly species based on evidence for congruent evolution of both host and bacterial genes (Campbell 1993; Clark et al. 1992; Zchori-Fein and Brown 2002). See Rosell et al. (Chapter 5) for further discussion on endosymbionts. The B. tabaci complex is a "cryptic species" in that its members exhibit a range of genetic variation and are collectively considered a sibling species group, although morphological characters in the pupal case (Fig. 2.2) useful for identification to species lack variation sufficient for finer scale taxonomic purposes. This external morphology for the species complex is thought to have remained static since ancient times (Gill 1990; Martin 2003; Rosell et al. 1997). Variants of B. tabaci for which biological (phenotypic) differences are recognized have most recently been referred to as "biotypes", and previously, as races (Bird 1957; Bird and Sanchez 1971; Bird and Maramorosch 1975, 1978). More than fifteen biotypes have been characterized to varying degrees in biological and genetic terms, and a number of additional variants are recognized but are incompletely studied. In fact, the majority of biological variants that occur throughout the world probably remain unstudied. The best studied phenotypic differences among B. tabaci biotypes include hostspecialization, host range of polyphagous haplotypes, dispersal behavior, mating behavior (Fig. 2.3), reproductive compatibility, differential resistance to distinct classes of insecticides, variable efficiency in the transmission of plant viruses, and secondary endosymbiont composition. Aside from certain basic knowledge about whitefly species included in higherlevel taxonomic studies, there have been far fewer studies of whiteflies at the species level compared to homopterans that predominantly inhabit temperate zones (Campbell et al. 1994; 1996; Gill 1990,1992; Neil and Bentz 1999). As a result, the evolutionary origin or basal and derived taxa have not been ascertained, and so the evolutionary history of B. tabaci is not yet understood. Relatively few molecular markers are available for inferring the evolutionary history of B. tabaci. At present only the 16S rRNA, the cytochrome oxidase I genes in the mitochondrial genome, and the nuclear ribosomal intergenic spacer 1 (ITS1), a non-coding sequence, have been explored Microsatellite markers have been developed to study population structure (Hadjistylli et al., Chapter 3), revealing broad geographic affiliations and levels of substructure not yet revealed for the sibling group. A study of the Asian-Pacific region revealed robust geographic structure accompanied by reduced or negligible gene flow, suggesting that as many as 10 major groupings (sibling species?) could occur there (DeBarro et al. 2005). Mound (1993) postulated that B. tabaci could follow one of two scenarios based on the duration that these whiteflies have inhabited earth, i.e. at least 1.2 MY (Schlee 1970). Depending on extent of isolation, B. tabaci would either be limited in its ecological and therefore geographical distribution and would have low intraspecific genetic variability, or conversely, it would have a broad distribution across variable ecological geographic boundaries, and wide genetic variation. As predicted by Mound, the latter scenario seems to best explain the evolutionary history of B.tabaci and is supported by population and molecular phyologenetic studies (see Hadjistylli et al. Chapter 3). Even so, more extensive population studies and new and more informative molecular markers are needed to facilitate deeper phylogenetic predictions. Exciting times lie ahead! This chapter will review the present status of the phylogenetic biology of the B. tabaci sibling species group (see Gill and Brown, Chapter 1), and highlight well known examples of phenotypic variation and genetic diversity revealed by a limited number of molecular markers. Although insufficient in the larger sense, molecular studies have revealed the existence of many B. tabaci haplotypes that group phylogeographically with their extant origin. Furthermore, a bar coding system employing the cytochrome oxidase I gene to identify haplotypes, coupled with definitive biological characteristics, can provide the necessary criteria to define a biotype. Thus, at least some commonly occurring variants are distinguishable for the first time.