Toroviruses: replication, evolution and comparison with other members of the coronavirus-like superfamily.

Based on their morphological and physicochemical characteristics, the Toroviridae were initially proposed to constitute a new family of enveloped RNA viruses (Horzinek & Weiss, 1984; Horzinek et al., 1987). However, recent analysis of the genetic information and replication strategy of the prototype Berne virus (BEV) (Snijder et al., 1988, 1990a, c) has revealed that toroviruses are not unique: they are clearly related to the Coronaviridae and, more distantly, to the arteriviruses (den Boon et al., 1991 b). This information has led to the reclassification of the toroviruses as a new genus in the coronavirus family (Pringle, 1992) and to the introduction of the unofficial term 'coronavirus-like superfamily' to indicate the evolutionary ties between the three virus groups mentioned above. The history of torovirus research not only illustrates the taxonomic consequences present-day molecular analysis may have; the BEV genome has also turned out to be a showcase for the two driving forces in RNA virus evolution: divergence from a common ancestor and RNA recombination (Snijder et al., 1991 a). Comparison of the torovirus genome with those of coronaand arteriviruses has contributed to our understanding of the evolution and replication of all three virus groups. The aim of this review is to present an overview of the data obtained during the molecular characterization of the torovirus prototype BEV. In addition, evolutionary aspects of our results and their relevance for the classification of toroviruses, coronaviruses and arteriviruses will be discussed. For information about the clinical and epidemiological aspects of toroviruses the reader is referred to a recent review by Koopmans & Horzinek (1993). History of torovirology