Geminivirus taxonomy and nomenclature is growing in complexity with the number of genomic sequences deposited in sequence databases. Taxonomic and nomenclatural updates are published at regular intervals (Fauquet et al. in Arch Virol 145:1743-1761, 2000, Arch Virol 148:405-421, 2003). A system to standardize virus names, and corresponding guidelines, has been proposed (Fauquet et al. in Arch Virol 145:1743-1761, 2000). This system is now followed by a large number of geminivirologists in the world, making geminivirus nomenclature more transparent and useful. In 2003, due to difficulties inherent in species identification, the ICTV Geminiviridae Study Group proposed new species demarcation criteria, the most important of which being an 89% nucleotide (nt) identity threshold between full-length DNA-A component nucleotide sequences for begomovirus species. This threshold has been utilised since with general satisfaction. More recently, an article has been published to clarify the terminology used to describe virus entities below the species level [5]. The present publication is proposing demarcation criteria and guidelines to classify and name geminiviruses below the species level. Using the Clustal V algorithm (DNAStar MegAlign software), the distribution of pairwise sequence comparisons, for pairs of sequences below the species taxonomic level, identified two peaks: one at 85-94% nt identity that is proposed to correspond to "strain" comparisons and one at 92-100% identity that corresponds to "variant" comparisons. Guidelines for descriptors for each of these levels are proposed to standardize nomenclature under the species level. In this publication we review the status of geminivirus species and strain demarcation as well as providing updated isolate descriptors for a total of 672 begomovirus isolates. As a consequence, we have revised the status of some virus isolates to classify them as "strains", whereas several others previously classified as "strains" have been upgraded to "species". In all other respects, the classification system has remained robust, and we therefore propose to continue using it. An updated list of all geminivirus isolates and a phylogenetic tree with one representative isolate per species are provided.

Viruses of the genus Begomovirus (family Geminiviridae) are emergent pathogens of crops throughout the tropical and subtropical regions of the world. By virtue of having a small DNA genome that is easily cloned, and due to the recent innovations in cloning and low-cost sequencing, there has been a dramatic increase in the number of available begomovirus genome sequences. Even so, most of the available sequences have been obtained from cultivated plants and are likely a small and phylogenetically unrepresentative sample of begomovirus diversity, a factor constraining taxonomic decisions such as the establishment of operationally useful species demarcation criteria. In addition, problems in assigning new viruses to established species have highlighted shortcomings in the previously recommended mechanism of species demarcation. Based on the analysis of 3,123 full-length begomovirus genome (or DNA-A component) sequences available in public databases as of December 2012, a set of revised guidelines for the classification and nomenclature of begomoviruses are proposed. The guidelines primarily consider a) genus-level biological characteristics and b) results obtained using a standardized classification tool, Sequence Demarcation Tool, which performs pairwise sequence alignments and identity calculations. These guidelines are consistent with the recently published recommendations for the genera Mastrevirus and Curtovirus of the family Geminiviridae. Genome-wide pairwise identities of 91 % and 94 % are proposed as the demarcation threshold for begomoviruses belonging to different species and strains, respectively. Procedures and guidelines are outlined for resolving conflicts that may arise when assigning species and strains to categories wherever the pairwise identity falls on or very near the demarcation threshold value.