Abstract:

Cassava (Manihot esculenta Crantz), as a food security crop in Africa is devastated by many biotic factors which include pests and diseases. Of these, diseases caused by cassava brown streak viruses and cassava mosaic viruses, transmitted by whiteflies, Bemisia tabaci are causing the highest yield losses in Sub-Saharan Africa. The aim of this study was to understand the molecular basis of key processes that underpin the capacity of whiteflies to utilize phloem-sap and thereby identify gene targets for novel whitefly control strategies. This study was founded on a principle that phloem sap feeders require osmoregulation genes to maintain fluid homeostasis and metabolic interaction with endosymbionts for the provisioning of essential amino acids, that are deficient in the phloemsap. Guts and bacteriocytes from a cassava B. tabaci population (SSA1-SG1) were dissected, their RNA extracted and sequenced to compare transcripts with enriched expression in the gut or bacteriocytes relative to the wholebody. RNAseq analysis revealed a total of 15 genes encoding alpha-glucosidase, family 13 with sugar processing domain (Aamy, IPR006047) and enriched expression in the gut. Of these, only one gene (ENSSSA1UGT002057) encoded for a protein that had a specific molecular signature that influence sucrose preference. This gene was selected as it encodes the hydrolysis of sucrose to fructose and glucose. It is, therefore, a suitable gene target against SSA1-SG1. In addition to sugar hydrolysis, B. tabaci maintains fluid homeostasis within the gut through water recycling. A water-specific aquaporin encoded by ENSSSA1UGT022145 (Ssa03484), an AQP1 gene in SSA1-SG1 was identified. This was differentially expressed in the whitefly gut where it is considered to mediate water recycling from the distal to the proximal end of the gut, thereby maintaining fluid homeostasis within the gut. RNAseq analysis of bacteriocytes dissected from SSA1-SG1, revealed three sets of genes. These were, (i) aminotransferases, (ii) amino acid transporters and (iii) horizontally transferred genes of either bacterial or fungal origin that influence amino acid provisioning in Bemisia tabaci SSA1- SG1. In addition to RNAseq analysis, genome-scale metabolic reconstruction unraveled the shared-metabolic interaction between SSA1-SG1 and its primary endosymbiont, Portiera aleyrodidarum. Application of constraint based metabolic modeling to this two-compartment model of cassava whitefly (SSA1-SG1) revealed 270 indispensable reactions, which, upon knockdown are predicted to cause devastating effects on both growth and survival of cassava whitefly. A total of four essential symbiosis genes (AA7\ BCAT, argil and lysA), both of intrinsic origin or horizontally transferred genes mediating terminal reactions in the phenylalanine, isoleucine, leucine, valine, arginine and lysine biosynthesis pathways were selected and validated. These arc proposed as candidate gene targets that can be used for future development of cassava management control options such as RNA interference.