Mastreviruses (family Geminiviridae) are plant-infecting viruses with circular single-stranded (ss) DNA genomes (~2.7kb). The genus Mastrevirus is comprised of thirty-two species which are transmitted by leafhoppers belonging to the genus Cicadulina. Mastreviruses are widely distributed and have been found in the Middle East, Europe, Asia, Australia, Africa and surrounding islands. Only one species, dragonfly-associated mastrevirus has so far been identified in the Americas, isolated from a dragonfly in Puerto Rico. Species can be group based on the host(s) they infect, those which infect monocotyledonous (monocot) plants and those which infect dicotyledonous (dicot) plants. In recent years many new mastrevirus species have been discovered. Several of these new discoveries can largely been attributed to the development of new molecular tools. The current state of sequencing platforms has made it affordable and easier to characterise mastreviruses at a genome level thus allowing scientists to delve deeper into understanding the dynamics of mastreviruses. A few mastrevirus species have been identified as important agricultural pathogens and as a result have been the focus of much of the mastrevirus research. Maize streak virus, strain A (MSV-A) has been the most extensively studied due to the devastating impact it has on maize production in Africa. Studies have shown that MSV-A likely emerged as a pathogen of maize less than 250 years following introduction of maize in Africa by early European settlers. There is compelling evidence to suggest that MSV-A is likely the result of recombination events between wild grass adapted MSV strains. It therefore is equally important to monitor viruses infecting non-cultivated plants in order to gain a greater understanding of the epidemiological dynamics of mastreviruses, which in turn is essential for implementing disease management strategies.

The objective of the research undertaken as part of this PhD thesis was to investigate global mastrevirus dynamics focusing on diversity, host and geographic ranges, mechanisms of evolution, phylogeography and possible origins of these viruses. In addition to this a viral metagenomic approach was used in order to identify novel mastreviruses or mastrevirus-like present in New Zealand.

The dynamics of the monocot-infecting mastreviruses are investigated in Chapter Two and Three. The work described in these two chapters focus mainly on mastreviruses which infect non-cultivated grasses in Africa and Australia, a total of 161 full mastrevirus genomes were recovered collectively in the two studies. Chapter Two reveals a high level of mastrevirus diversity present in Australia with the discovery of four new species and several new strains of previously characterised species. An extensive sampling effort in Africa undertaken in Chapter Three reveals a broader host range and geographic distribution of the African monocot-infecting mastreviruses than previously documented. Mosaic patterns of recombination are evident among both the Australian and African monocot-infecting mastreviruses.

In Chapters Four, Five and Six a comprehensive investigation was undertaken focusing on the dicot-infecting mastreviruses. The study undertaken in Chapter Four entailed the recovery of 49 full mastrevirus genomes from Australia, the Middle East, Africa, Turkey and the Indian Subcontinent to investigate the diversity of dicot-infecting mastreviruses from a global context. Analyses revealed a high degree of CpCDV strain diversity and extended the known geographic range of CpCDV. For the first time phylogeographic analysis was able to investigate the origins of the dicot-infecting mastreviruses. Results revealed the likely origin of the most recent common ancestor (MRCA) of these viruses is likely closer to Australia than anywhere else that dicot-infecting mastreviruses have been sampled and illuminated a supported series of historical movements following the emergence of the MRCA. In Chapter Five two novel mastreviruses Australian-like mastreviruses were isolated from chickpea material from Pakistan. A comprehensive analysis of CpCDV isolates in the major pulse growing regions of Sudan in Chapter Six reveals that this region harbours a high degree of strain diversity. Complex patterns of intra-species recombination indicate these strains are evidently circulating in these regions and infecting the same hosts, driving the emergence of new CpCDV strains. Collectively the results discussed in Chapters Two through Six extended the current knowledge of mastrevirus diversity. The natural host range of many mastreviruses has proven to be more extensive than previously documented, with many species having overlapping host ranges and hence these hosts could be acting as ‘mixing vessels’ enabling inter-species recombination. Patterns of recombination and selection were observed in both the monocot-infecting and the dicot-infecting mastreviruses further elucidating the mechanisms these viruses employ to evolve rapidly. Extensive sampling in a wide range of geographic regions provides insights into the true geographic range of species such as MSV and CpCDV.

Given that mastreviruses have been able to move globally and Australia has been identified as a major mastrevirus diversity hotspot it is conceivable that mastreviruses are also present in New Zealand. In Chapter Seven and Eight this is explored by using a viral metagenomic approach to investigate the ssDNA viral populations associated with wild grasses and sewage material in New Zealand. Although no mastreviruses were recovered, this endeavour resulted in the discovery of more than 50 novel circular Rep-encoding ssDNA (CRESS DNA) viruses associated with non-cultivated grasses and treated sewage material, many of which are similar to mastreviruses and other geminiviruses. These discoveries expand current knowledge on the diversity of ssDNA viruses present in New Zealand and further highlight this viral metagenomic approach as an effective method for ssDNA virus discovery.

Overall the results discussed in this thesis provide insights into mastrevirus diversity and dynamics as well as revealing a wealth of novel CRESS DNA viruses, some of which share similarities to geminiviruses.