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Title: Integrative approach to discovering species diversity within the Mediterranean group of the Bemisia tabaci complex
Author: Vyskocilova, Sona
ISNI:       0000 0004 7969 7102
Awarding Body: University of Greenwich
Current Institution: University of Greenwich
Date of Award: 2019
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Bemisia tabaci is a complex of cryptic whitefly species, which includes some of the world's most damaging agricultural pests. The Mediterranean (MED) putative species is globally invasive and its populations are often resistant to insecticides. The intra-species genetic variability identified in partial sequences of the mitochondrial cytochrome c oxidase 1 (mtCOI) gene has led to the recognition of four MED groups: Q1, Q2, Q3 and African silver-leafing (ASL). A lack of hybridisation between Q1 and ASL populations has been reported, but the taxonomic and biological significance of these groups remained unclear. The aim of this study, therefore, was to evaluate the species status of MED groups using an integrative approach, combining (i) molecular analyses of high-throughput sequencing-derived mitogenomes, (ii) reciprocal crossing experiments to investigate reproductive compatibility among Q1, Q2 and ASL populations and developing a molecular marker for hybrid verification, (iii) detection of bacterial endosymbionts and (iv) bioassays to compare their host-plant ranges and performances on 13 plants. Our mitogenome phylogeny showed close relationships among Q1 and Q2 populations, while ASL was placed outside the Q1/Q2 cluster with 100% bootstrap support. Using the mitogenomes as a reference enabled the identification of sequence errors in 155 of 289 published MED mtCOI haplotypes. Crossing experiments revealed that only Q1 from Spain and Q2 from Israel were compatible, confirming that they belong to the same species. In contrast, ASL from Uganda and Q1 from Sudan both failed to interbreed with any other population. Parental origin of the Q1xQ2 hybrids was verified by the novel nuclear marker GC1 and their fertility by backcrossing. The F2 offspring showed asymmetry in numbers and sex ratio. Hypotheses were formulated about the involvement of endosymbionts Rickettsia and/or Hamiltonella in the F2 asymmetry, and Wolbachia and/or Cardinium in the incompatibility between Spain Q1 and Sudan Q1 populations. Lastly, significant differences (P < 0.05) in host use occurred amongst all four populations. Spain Q1 had the widest host range and Israel Q2 the narrowest, but the most distinct pattern of host use was observed for Uganda ASL. In addition, despite its name, we found no evidence of the capacity of ASL to induce silver-leafing symptoms in squash. From this combined evidence, we conclude that the ASL group belongs to a distinct biological species and we highlight the inconsistency between the biological species concept and the currently used species delimitation based on the partial mtCOI sequence and 3.5% nucleotide distance threshold. Accurate knowledge of the number and biology of cryptic species will allow more targeted and efficient design of pest control strategies.
Supervisor: Colvin, John ; Seal, Susan Sponsor: University of Greenwich ; Bill and Melinda Gates Foundation
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: SB Plant culture