The presence of morphologically cryptic lineages with divergent molecular, ecological, and physiological traits within a species is an intriguing evolutionary phenomenon that offers unique opportunities for evolutionary genetics studies. One such system is the whitefly species complex Bemisia tabaci (Hemiptera: Aleyrodidae), which comprises several cryptic lineages, known as “biotypes” with worldwide distribution, including two of the world's worst invasive pests. In this dissertation I take a population genetics approach to examine the global genetic structure of B. tabaci biotypes, with a focus on the origins, historical demography, and invasion pathways of the two invasive biotypes, known as “B” and “Q”.
I begin with a historical overview of multilocus molecular markers used to examine aspects of the biology, ecology, and genetics of the B. tabaci species complex. The first markers employed were allozymes, particularly esterases, which became the basis for the biotype nomenclature, and were substantiated by ecological and biological data. The exploration of various DNA based markers has established that biotypes within B. tabaci are exceptionally diverse genetically, in spite of their identical morphologies.
Global population genetics analyses using microsatellite markers showed that well-characterized B. tabaci biotypes correspond to real genetic entities with strong geographic structure, and limited or no gene flow among them. The resulting genetic clusters from this analysis are in general agreement with the only well-resolved global phylogeny of the species, which is based on a single mitochondrial gene (cytochrome oxidase I). However, some cases of conflict in the two sets of markers do exist, perhaps associated with the different modes of inheritance, thus cautioning against the use of mitochondrial DNA as a single marker for species or subspecies delineation.
Analysis of genetic data with more sophisticated Bayesian coalescent-based approaches offers the opportunity to study both contemporary and ancestral invasion pathways. Using such an approach, I showed that divergence histories of the invasive biotypes B and Q coincided with periods of extensive human movement and trade of agricultural goods in the Mediterranean, the Middle East, and Africa during the Iron and Bronze Ages, and the Roman period. Results also indicate that ancestral populations to the current B and Q biotypes had much larger effective sizes than those of emerging biotypes, a pattern consistent with expectations of diversification in invasive species.
In a contemporary context, I investigated the recent invasion history of biotype Q in the USA. I found that populations introduced into the USA originated from both the Western and Eastern Mediterranean, in at least three independent cryptic invasions, and spread directly from a single initial introduction site, likely through plant trade.
Findings from this dissertation underscore the practical importance of better monitoring invasions of this insect and other invasive pests at points of entry and dispersal through trade of plant material. From a theoretical perspective, this work adds insights into the origins of biotypes, both in the B. tabaci complex and more generally, emphasizing the demographic processes involved in diversification of invasive biotypes. The research highlights the potential to use B. tabaci in studies of broader applied as well as evolutionary significance.
