The bemisia tabaci complex: Genetic and phenotypic variation and relevance to TYLCV-vector interactions

The purpose of this review is to present an overview of the biotype concept in relation to the whitefly Bemisia tabaci (Gennadius) vector of Tomato yellow leaf curl virus (TYLCV), the plant virus, which is the topic of this volume. It seems an unlikely coincidence that this single species of whitefly, itself widely variable and plastic, is the arthropod vector of a widespread, dynamic suite of closely related viruses species that also diversify rapidly and adapt to human activities. This chapter will contextualize current scientific knowledge, and raise questions where understanding is lacking - or not yet congealed to reach a satisfactory conclusion. This will involve delineating the characteristics, processes, and concepts that unite or set apart the B. tabaci complex from other whitefly species, and other vector-virus complexes. Also discussed will be the characteristics that uniquely delimit variants or biological types of B. tabaci - recognizable both in terms of biological and/or genetic variability, which yield distinct consequences in agroecosystems - that would not prevail if such variability were absent or irrelevant. The review will also address how knowledge of different and shared characters among biotypes and less well-studied haplotypes (phenotypic variants), could assist in predicting whether a variant could become an invasive, or successful vector. And, how greater than expected genetic variation, together with phenotypic plasticity, influence virus-vector competency, virus dispersion, and virus host adaptation or host range shifts, and support diversification or emergence of begomoviral species. The unprecedented invasiveness of this insect pest and plant virus vector has contributed widely to the intrigue that has fostered the recent interest in this ancient whitefly species. As well, so do its fascinating biology, unresolved taxonomy, unprecedented (apparent) interspecific variability, and extent of reproductive isolation. This review will present a historical perspective of the biotype concept, and describe the attributes of the B. tabaci complex relevant to its role as a vector and pest in agriculture. It also will provide examples of the best-studied biological types of B. tabaci and their significance to begomovirus disease outbreaks. A generalized sequence of events outlining the history of the biotype concept and the contributions of many to its legacy is provided in Table 1. It is particularly important to credit our many colleagues whom over the years have generously contributed whitefly and virus collections for molecular analysis.Without them, much of the work described here would not have been possible. It is regrettable that space limitations do not allow inclusion of a comprehensive chronology citing all that have made important contributions to this new field of study. Even so, every effort has been made to highlight key events and the contributions of as many as possible. This chapter is dedicated to Dr. Julio Bird, Emeritus, University of Puerto Rico, a priceless mentor and friend who continues mostly unknowingly through his insights and keen observations, to inspire students of B. tabaci around the world.