Viruses and other selfish elements: the replication strategies, genome size distribution, global ecology, and hallmark proteins. For each class of viruses and related elements, the approximate range of genome sizes is indicated (kb, kilobases). '+' denotes positive strand (same polarity as mRNA) and '-' denotes negative strand. Tr, transcription; T, translation; R, replication; E, encapsidation; A, archaea; B, bacteria; F, fungi; Mz, Metazoa; P, plants; UE, unicellular eukaryotes. For each class of viruses (elements), characteristic structures of hallmark proteins and characteristic electron-microscopic images of viruses are shown. RdRp, RNA-dependent RNA polymerase; JRC, jelly-roll capsid protein; RT, reverse transcriptase; RCRE, rolling-circle replication (initiating) endonuclease. The rightmost panel shows the host range, with the size of the respective image and acronym roughly proportionate to the abundance of the given virus class in the respective taxon.