Fig. 6.

Origin and evolution of the nidovirus genome plan. A tentative evolutionary scenario leading to the origin of the LCAs of nidoviruses and large nidoviruses from a progenitor with an astrovirus-like genome organization is illustrated. The three shown genomes are fictitious, although they are drawn to a relative common size scale and include major replicative domains found in genomes of respective contemporary viruses, as discussed in the text.