Title page for ETD etd-02272006-092153


Document Type Doctoral Thesis
Author Ndunguru, Joseph
URN etd-02272006-092153
Document Title Molecular characterization of cassava mosaic geminiviruses in Tanzania
Degree PhD
Department Microbiology and Plant Pathology
Supervisor
Advisor Name Title
Prof A S Aveling
Keywords
  • cassava mosaic geminiviruses
  • cassava
  • African cassava mosaic virus
  • cassava mosaic disease
  • Tanzania
  • East AFrican cassava mosaic virus
  • DNA-A
  • DNA-B molecular characterisation
Date 2005-08-06
Availability restricted
Abstract
Cassava (Manihot esculenta Crantz) is a basic staple food crop in Tanzania. Cassava mosaic disease (CMD) caused by cassava mosaic geminiviruses (CMGs) constitutes a major limiting factor to cassava production in the country. This study was undertaken to characterize the CMGs occurring in Tanzania using molecular techniques and to map their geographical distribution to generate information on which the formulation of control measures can be based. Using Polymerase Chain Reaction (PCR) and Restriction Fragment Length Polymorphism (RFLP) for analysis of CMGs DNA-A genomes, different CMGs were found to be associated with CMD. Higher molecular diversity was observed among East African cassava mosaic viruses (EACMVs)

than African cassava mosaic viruses (ACMVs), which was confirmed later by complete

nucleotide sequence analysis. In addition to EACMV and ACMV isolates, two isolates of

EACMV Cameroon virus (EACMCV) were found in Tanzania. These were confirmed to be strains of EACMCV Cameroon, originally described in Cameroon, West Africa and here named EACMCV- [TZ1] and EACMCV-[TZ7]. They had high (92%) overall DNA-A nucleotide sequence identity and EACMCV-[TZ1] was widespread in the southern part of the country. A subgenomic DNA form of CMG that appeared to be truncated was identified in a CMD-infected cassava plant. It was confirmed upon sequence analysis to be a defect of EACMV DNA-A and had a capacity of attenuating symptoms when coinoculated with wild-type EACMV. In addition, this study revealed for the first time the presence of two novel non-geminivirus single-stranded DNA (ssDNA) sub-genomic molecules associated with CMG infection. They were shown to be dependent on CMG for replication and movement within the plants, confirming their status as satellite molecules named here as satDNA-II and satDNA-III. When present in coinfection with CMGs, they enhance symptoms and can break high levels of resistance in a cassava landrace. Finally a simple, inexpensive technique is described of archiving, transporting and recovering plant DNA for downstream geminivirus characterisation.

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