Diversity of Small, Single-Stranded DNA Viruses of Invertebrates and Their Chaotic Evolutionary Past

•Comparison of genome structure and expression of single-stranded DNA viruses of invertebrates.•Pathogenicity of single-stranded DNA viruses of invertebrates.•Origin and evolution of single-stranded DNA viruses.•Key genes of genomes of single-stranded DNA viruses.•3-D structure of single-stranded DNA viruses.•Frequent horizontal gene transfer and chimerism of single-stranded DNA virus genomes.