Genetic diversity in viruses is shaped by high rates of recombination and i
s constrained by host defenses and the requirements of transmission. Recent
studies of insect-transmitted plant viruses demonstrate highly conserved m
olecular motifs in viral genomes that regulate the specificity of insect tr
ansmission. In contrast, advances in our understanding of host plant respon
se to virus infection reveal some generalized patterns of host defense to a
diversity of viruses.