Asymmetric patterns of reassortment and concerted evolution in Cardamom bushy dwarf virus.

Nanoviruses are single-stranded DNA (ssDNA) plant viruses which have multipartite genomes consisting of discrete, individually encapsidated components. This multipartite strategy may lead to high rates of reassortment, whereby entire genome components are exchanged among different strains. However, few studies have explored the extent to which reassortment shapes the genetic diversity of nanovirus populations. Here we present an extensive analysis of reassortment among 163 Cardamom bushy dwarf virus (CBDV; Nanoviridae family, Babuvirus genus) isolates collected in Northeast India. We also examined evidence of recombination, which is known to play a role in the evolutionary dynamics of nanovirus populations. By sequencing six discrete genome components for each isolate, we demonstrate that over 40% of the isolates display evidence of at least one reassortment event during their evolutionary histories. Nevertheless, a bias in the frequencies at which different genome components reassort was observed, with the DNA-M and DNA-N components being the most predisposed to reassortment. This may reflect variation in the ability of different genome components to function efficiently in a foreign genomic background. Comparisons of the common regions of different genome components revealed signatures of concerted evolution mediated by frequent inter-component homologous recombination. This process, which has previously been reported in nanoviruses and other multipartite ssDNA viruses, may allow proteins which initiate replication to maintain control over distinct genome components. Notably, DNA-N, one of the genome components most prone to reassortment, also exhibited the most frequent inter-component homologous recombination. This supports the idea that inter-component homologous recombination may promote the efficient replication of novel components which are introduced into a genome via reassortment.