Arch Virol (1991) 119:83-93 _Archives Vi rology © Springer-Verlag 1991 Printed in Austria The use of serological differentiation indices for the phylogenetic analysis of plant virus relationships E. P. Rybicki Department of Microbiology, University of Cape Town, Rondebosch, Republic of South Africa Accepted November 29, 1990 Summary. The potential use of serological differentiation indices (SDIs) in the phylogenetic analysis of plant virus coat protein relationships is presented. Relationship dendrograms were constructed from SDI data for viruses in the tymovirus and Subgroup I geminivirus taxonomic groups, using distance or phenetic methods embodied in the computer programmes Fn'cH and NJa'REE. Relationship dendrograms for geminiviruses agreed well with those constructed by others from sequence or other data; the dendrogram for tymoviruses was not in agreement with one calculated from amino acid composition data, or from other properties, but appears consistent with recently-shown sequence similarities between tymoviruses. Implications of these results for the classifi- cation and taxonomy of plant viruses are discussed. Introduction Some of the first quantitative estimates of "difference" between plant viruses were based on serological data, and serology has long been used for basic grouping of plant viruses [8, 27]. However, the concept of quantitating these serological differences is more recent: van Regenmortel and von Wechmar introduced the concept of the serological differentiation index, or SDI, only in 1970. This is the serological cross-reactivity between two viruses expressed as the number of twofold dilution steps separating homologous and heterologous titres. Formerly such values were determined using precipitin techniques; re- cently, however, ELISA techniques have also been used, and proved to be perhaps more accurate for such determinations, and certainly capable of dem- onstrating a wider range of relationships. Jaegle and van Regenmortel [12J demonstrated the general use of the technique, and Dekker et al. [2J and Pinner and Markham [ 18, 19-]have used it for determining relationships among isolates of maize streak virus (MSV) and other related Subgroup I geminiviruses of