The Population Genetic Structure of the Whitefly, Bemisia tabaci, at Different Spatial Scales

Adam Dinsdale (2011). The Population Genetic Structure of the Whitefly, Bemisia tabaci, at Different Spatial Scales MPhil Thesis, School of Biological Sciences, The University of Queensland.

       
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Author Adam Dinsdale
Thesis Title The Population Genetic Structure of the Whitefly, Bemisia tabaci, at Different Spatial Scales
School, Centre or Institute School of Biological Sciences
Institution The University of Queensland
Publication date 2011-11
Thesis type MPhil Thesis
Supervisor Dr Yvonne Buckley
Dr Cynthia Riginos
Dr Nancy Schellhorn
Dr Paul De Barro
Total pages 110
Total colour pages 21
Total black and white pages 89
Language eng
Subjects 050103 Invasive Species Ecology
060411 Population, Ecological and Evolutionary Genetics
Abstract/Summary Bemisia tabaci (Hemiptera: Sternorrhyncha: Aleyrodoidea: Aleyrodidea) biotype B, is one of the most severe agricultural pests of cropping systems. In the case of B. tabaci there is no clear understanding of the genetic limits of the numerous genetic groups and biotypes so far identified. This has resulted in a lack of consistency in both the application of the terms, the approaches used to apply them and in our understanding of what genetic structure within B. tabaci means. A background of B. tabaci biotype B is outlined in Chapter 1, along with the importance of understanding the genetic makeup and bounds of this pest species at a global scale, and a scale of localised agricultural regions. Globally, an accurate understanding and characterisation of species boundaries within the B. tabaci complex is essential as part of biosecurity. Within a localised agricultural environment, gaining knowledge of population movement and localised genetic structure can provide a better understanding of potential dispersal in an agricultural region, and the population dynamics. Analyses of the genetic bounds and putative species number of B. tabaci globally is provided in chapter 2. Identifying species boundaries within morphologically indistinguishable cryptic species complexes is often contentious. Our response has been to use mitochondrial gene cytochrome oxidase 1 to consider how to clearly and consistently define genetic separation. Using Bayesian phylogenetic analysis and analysis of sequence pairwise divergence we found a considerably higher number of genetic groups than had been previously determined with two breaks in the distribution, one at 11% and another at 3.5%. At greater than 11% divergence 11 distinct groups were resolved whereas at greater than 3.5% divergence 24 groups were identified. Consensus sequences for each of these groups were determined and were shown to be useful in the correct assignment of sequences of unknown origin. The 3.5% divergence bound is consistent with species level separations in other insect taxa and suggests that B. tabaci is a species complex composed of at least 24 distinct species. We further show that the placement of B. atriplex within the B. tabaci in- group adds further weight to the argument for species level separation within B. tabaci. This new analysis, which constructs consensus sequences and uses these as a standard against which unknown sequences can be compared, provides for the first time a consistent means of identifying the genetic bounds of each species with a high degree of certainty. Finally, with this species level revision the old nomenclature for biotype B has been changed to Middle East-Asia Minor 1 (MEAM1), and referred to as such in chapter 3. A spatial and temporal study of population genetic differentiation across an agricultural landscape is presented in chapter 3. Organisms differ greatly in dispersal ability and landscapes differ in amenability to an organism’s movement. Thus, landscape structure and heterogeneity can affect genetic composition of populations. While many agricultural pests are known for their ability to disperse rapidly, it is unclear how fast and over what spatial scale insect pests might respond to temporally dynamic agricultural landscapes they inhabit. We used population genetic analyses of a severe crop pest, a member of the B. tabaci cryptic species complex known as Middle East-Asia Minor 1 (commonly known as biotype B), to estimate spatial and temporal genetic diversity over four months of the summer growing season, based on 559 individuals from 8 sites, scored for 8 microsatellite loci. Temporal genetic structure vastly exceeded spatial structure. There was significant temporal change in local genetic composition from the beginning to the end of the season with heterozygote deficits and inbreeding. This temporal structure suggests entire cohorts of pests can occupy a large and variable agricultural landscape but are rapidly replaced. These rapid genetic fluctuations reinforce the concept that agricultural landscapes are dynamic mosaics in time and space and may contribute to better decisions for pest and insecticide resistance management. The results I present in this thesis have progressed our understanding of the bounds and genetic structure of the B. tabaci complex at different spatial and temporal scales. By increasing our understanding and refining what we know of this serious global agricultural pest, my results have also enabled the identification of key research areas to continue to improve efforts to manage this species.
Keyword Cytochrome Oxidase 1
Phylogenetics
Barcoding
Microsatellites
Gene Flow
Movement
Aleyrodidea
Bemisia
Additional Notes Pages 90-110 in colour Pages 52 & 87 in landscape

 
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