Background The group of > 40 cryptic whitefly species called Bemisia tabaci sensu lato are amongst the world’s worst agricultural pests and plant-virus vectors. Outbreaks of B. tabaci s.l. and the associated plant-virus diseases continue to contribute to global food insecurity and social instability, particularly in sub-Saharan Africa and Asia. Published B. tabaci s.l. genomes have limited use for studying African cassava B. tabaci SSA1 species, due to the high genetic divergences between them. Genomic annotations presented here were performed using the ‘Ensembl gene annotation system’, to ensure that comparative analyses and conclusions reflect biological differences, as opposed to arising from different methodologies underpinning transcript model identification. Results We present here six new B. tabaci s.l. genomes from Africa and Asia, and two re-annotated previously published genomes, to provide evolutionary insights into these globally distributed pests. Genome sizes ranged between 616—658 Mb and exhibited some of the highest coverage of transposable elements reported within Arthropoda. Many fewer total protein coding genes (PCG) were recovered compared to the previously published B. tabaci s.l. genomes and structural annotations generated via the uniform methodology strongly supported a repertoire of between 12.8—13.2 × 103 PCG. An integrative systematics approach incorporating phylogenomic analysis of nuclear and mitochondrial markers supported a monophyletic Aleyrodidae and the basal positioning of B. tabaci Uganda-1 to the sub-Saharan group of species. Reciprocal cross-mating data and the co-cladogenesis pattern of the primary obligate endosymbiont ‘Candidatus Portiera aleyrodidarum’ from 11 Bemisia genomes further supported the phylogenetic reconstruction to show that African cassava B. tabaci populations consist of just three biological species. We include comparative analyses of gene families related to detoxification, sugar metabolism, vector competency and evaluate the presence and function of horizontally transferred genes, essential for understanding the evolution and unique biology of constituent B. tabaci. s.l species. Conclusions These genomic resources have provided new and critical insights into the genetics underlying B. tabaci s.l. biology. They also provide a rich foundation for post-genomic research, including the selection of candidate gene-targets for innovative whitefly and virus-control strategies.

Background Whiteflies are agricultural pests that have caused worldwide negative impacts that have led to severe financial losses. The Bemisia tabaci whitefly species complex is the most damaging in terms of their broad crop host range and its ability to serve as vector for over 300 plant viruses. Whitefly genomes of the species complex provide valuable genomic data; however, transposable elements (TEs) within the species complex remain unexplored. This study provides the first accurate exploration of TE content within the B. tabaci species complex.Results This study identified an average of 40.61% of the genomes of three whitefly species (MEAM1, MEDQ, and SSA-ECA) consists of TEs. Majority of the TEs identified were DNA transposons (22.85% average) while SINEs (0.14% average) were the least represented. This study also compared the TE content the three whitefly genomes with three other hemipteran genomes and found a significant difference in the presence of DNA transposons and LINEs. A total of 63 TE superfamilies were identified to be present across the three whitefly species (39 DNA transposons, six LTR, 16 LINE, and two SINE) of which 11 TE superfamilies were identified to not be present in the three other hemipteran genomes (nine DNA transposon, and two LINE). This study is the first to characterize TEs found within different B. tabaci species and has created a standardized annotation workflow that could be used to analyze future whitefly genomes.Conclusion This study is the first to characterize the landscape of TEs within the B. tabaci species complex. The characterization of these elements within the three whitefly genomes shows that TEs occupy a significant portion of the whitefly genome, majority of which are DNA transposons. This study also identified TE superfamilies of note and provides a framework for future TE studies within the species complex.

Next generation DNA sequencing technologies have led to an exponential growth in the number of genomes being sequenced. While generating whole genome shotgun (WGS) assemblies using next generation sequencing (NGS) is relatively fast and inexpensive, the application of this approach to the assembly of highly repetitive and complex genomes such as wheat results in inferior assemblies thus slowing efforts in identifying markers for crop improvement.The wheat genome is large, highly repetitive and polyploid. Several approaches have been used to sequence and assemble the wheat genome to variable success. Published approaches such as whole chromosome shotgun (WCS) and whole genome shotgun (WGS) have resulted in draft assemblies that are incomplete, fragmented or only represent a subset of the targeted genomic region. BAC by BAC approaches offer the most accurate assemblies although BAC by BAC approaches are expensive and labour intensive.This thesis presents the application of a novel BAC sequencing approach which combines indexed pools of BACs, Illumina paired read sequencing, a sequence assembler specifically designed for complex BAC assembly, and a custom bioinformatics pipeline.The approach is demonstrated by sequencing and assembling BAC cloned fragments from bread wheat chromosome arm 7DS. This approach enables the generation of accurate scalable and reproducible assemblies cost effectively compared to traditional BAC by BAC approaches.Rigorous assembly validation prior to gene annotation and onward analysis is critical in genome sequencing projects but often missing. This thesis demonstrates rigorous assembly validation of bread wheat chromosome arm 7DS BAC assemblies using multiple independent platforms. Novel approaches for de novo assembly validation are also presented. The BAC assemblies were successfully validated using BAC end sequences