Abstract
Banana bunchy top virus (BBTV) is the most damaging virus of monocot bananas and plantains, considered the 4th largest crop plants in the world. Belonging to the family Nanoviridae and the genus Babuvirus, BBTV is a multipartite circular single-stranded DNA (ssDNA) virus composed of six viral DNA components. Transmitted from plant to plant by the aphid vector Pentalonia nigronervosa, this virus can also be associated with one or more alphasatellites. Alphasatellites are circular ssDNA molecules belonging to the family Alphasatellitidae which encode a Rep-like protein that allows alphasatellites to self-replicate. However, alphasatellites cannot trans-replicate helper virus components and require the helper virus for encapsidation, movement and transmission by insect vectors. Very few studies have been conducted on alphasatellites, and their role in viral infection and disease transmission is not defined. In this thesis, we report the emergence of a new alphasatellite associated with BBTV in Democratic Republic of the Congo. This is the first BBTV alphasatellite discovered in Africa. We demonstrate that this alphasatellite affects its helper virus transmission, replication, gene expression and evasion of antiviral defences generating viral small interfering RNAs. Furthermore, this alphasatellite forms a new genus Banaphisatellite and is more phylogenetically related to alphasatellites infecting dicots than those previously found to be associated with monocot-infecting BBTV in South-East Asia. In addition, our surveys of BBTV-infected plants from South-East Asia showed the high prevalence of alphasatellites in this region of the world and among other alphasatellites we discovered two additional novel species representing the genus Banaphisatellite. Finally, we demonstrate that dicot plants can serve as alternative hosts of BBTV and its alphasatellites including the banaphisatellites, thus supporting the hypothesis of a dicot to monocot passage of this alphasatellite genus.