However, good phylogenetic evidence of at least some MSV-A diversification along geographical lines indicates that if long-distance human transportation of MSV-A across the continent occurs at all, it is probably infrequent. Host range variation amongst MSV strains In an attempt to directly determine whether differential host preferences (either by virus strains or vector biotypes) might at least partially account for differences in the geographical distributions of different strains, we analysed the strains in the context of the hosts from which they were isolated. Grouping hosts by genus (11 groups excluding cultivated host species) and viruses by strain (10 groups – excluding MSV-D for which no clear host identification could be made) we found very strong statistical support (P,161028; 11610 x2 test) for significant differences between the hosts from which members of different MSV- strains were sampled. However, we noted that we had oversampled certain host genera (e.g. Digitaria species, n539) and undersampled others (e.g. Axonopus species, Rottboellia species and Pennisetum species, n51 each; Supplementary Fig. S3 available in JGV Online). We also realized that on the island of La Re´union we had apparently oversampled Cenchrus species relative to other regions (3/4 of all Cenchrus samples were obtained on this island). Importantly, we found evidence of significant differences between the host types sampled in different regions [P50.027, 10 (host genera)64 (geographical region) x2 test]. This sampling bias was primarily accounted for by the greater numbers of Cenchrus species sampled from La Re´union as it could be resolved by removing the La Re´union sample from the analysis (P50.149). Given that only MSV-B isolates were sampled on La Re´union and that the sampling bias was only marginally significant, we did not anticipate that it would have a substantial effect on our assessment of the prevalence of different MSV strains in different host species. It is important to point out, however, that our sampling of obviously symptomatic MSV diversity in African grasses plants may have unpredictably biased our analysis of natural host range distributions, in that unsampled host species/MSV strain combinations associated with mild or asymptomtic infections might be just as epidemiologically relevant as those associated with severely symptomatic infections. We sought to offset some of these potential sampling biases by investigating the frequencies with which different MSV strains were sampled only in the three host genera for which we obtained seven or more samples: Digitaria species (n539), Urochloa species (n58) and Setaria species (n57; Fig. 5). We found significant differences in the MSV strains isolated from the plants of these three genera [P,161028, 3 (host genera)69 (virus strains2no MSV-D and -J isolates were sampled from the three genera considered) x2 test]. Considering the host genera in pairs we only found a significant difference between the relative frequencies with which different MSV strains were sampled from Digitaria and Setaria plants (P,161028). Whereas Setaria plants tended to be infected with the closely related MSV-K and -C strains, Digitaria plants tended to be infected with the closely related MSV-A, -B and -G strains. We then looked for specific differences between the apparent host ranges of different virus strains. Despite our sampling too few viruses to achieve enough statistical power to differentiate between the host ranges of most of the MSV strains, we observed significant natural host range differences between MSV-B and both MSV-C and -K [P51.061024 and 2.061024, respectively; 2 (virus strains)611 (host genera) x2 test] and between MSV-G and both MSV-C and -K (P52.961022 and 5.061022, respectively). Whereas MSV- B and -G isolates tended to come primarily from Digitaria species, MSV-K and -C isolates tended to come primarily from Setaria species. Should the MSV-C, -D and -K strains (Fig. 1) be separated into another species following the next revision of geminivirus taxonomic criteria, it seems logical, therefore, that the species be named Setaria streak virus and that these strains be renamed as SetSV-A, -B and -C, respectively (Fig. 1). Fig. 5. The frequencies with which different MSV strains were sampled from three common groups of MSV hosts. Different strains are represented by different colours and P values indicate significant differences (values in bold) between the three groups with respect to the MSV strains isolated from their members. http://vir.sgmjournals.org 2071