Comparative genomics and pathogenicity of Xanthomonas vasicola

Abstract

In the mid-2000s an outbreak of bacterial blight and dieback caused by Xanthomonas vasicola pv. vasculorum was observed on a single Eucalyptus grandis clone in KwaZulu-Natal. It was suggested that this outbreak was as a result of a host jump from sugarcane. Therefore, the purpose of this dissertation is to determine the host range of X. vasicola strains and to identify the genetic determinants which may have played a role in this pathogen’s ability to jump to a new host. This dissertation will be presented in three independent chapters. Chapter 1 will review previous literature on the changes that take place in a bacterial genome leading to its adaptation to a changing environment. The chapter will include sections on the modifications of existing genes by mutations, gene duplications and gene rearrangements, acquisition of new genes by horizontal gene transfer, and the loss of genes as pathogens become specialised to their hosts. In chapter 2, the host range of X. vasicola strains isolated from different hosts will be tested. Host’s include the monocotyledonous plant species banana, maize, sorghum, sugarcane and the dicotyledonous plant species Eucalyptus grandis. The significance of the differences observed between the number of infected plants and the severity of the disease symptoms will be determined. In chapter 3, the differences between the genomes of five Xvv isolated from E. grandis and X. vasicola isolated from other hosts will be determined. The genomes of five Xvv strains from Eucalyptus were sequenced on the Illumina HiSeq 2500 platform. The chromosomes and plasmids were assembled and annotated, and the genomes were compared to those of other X. vasicola strains isolated from different hosts. These genome differences may have played a role in their adaptation to a new host.