A. Varsani and others Although there was no significant difference in the range of host genera from which MSV-A and -B isolates were sampled (P50.28), we noted that the 14 MSV-A samples were obtained from grasses in eight genera, whereas the 34 MSV-B samples were obtained from grasses in only six genera. While this may indicate that MSV-A has a broader host range than MSV-B – something that may explain differences in the continent-wide distribution of the strains – more intensive sampling in a greater variety of hosts will be required to conclusively prove this. Nevertheless, our discovery of MSV-A isolates infecting a variety of uncultiv- ated grasses is significant in that these species are probably both the natural hosts from which this strain originally emerged as a maize pathogen, and represent the hosts that currently sustain it between maize growing seasons. Conclusions While our analysis of MSV diversity in uncultivated indigenous African grasses has revealed six new MSV strains, we have also detected for the first time, a degree of recombination amongst mastreviruses paralleling that seen in begomoviruses. Our demonstration that recom- bination patterns are partially conserved across the family Geminiviridae is particularly significant in that it indicates that early ancestral geminivirus genomes had largely the same recombinational predispositions and constraints as those experienced by modern geminiviruses. Of potentially greater immediate relevance, however, is our discovery that the maize-adapted MSV-A strain is possibly the product of an ancestral recombination event between Digitaria- adapted MSV-G/-F and MSV-B viruses. While this recombination event may have produced a virus with increased severity in maize – a host to which MSV-A seems particularly well adapted – our data are also consistent with the possibility that it may have enabled MSV-A to spread more efficiently throughout the continent by allowing it to infect a wider variety of hosts. Importantly, these hypotheses can be directly tested by reconstructing and analysing the virulence and host ranges of the ancestral MSV-A and its parental viruses. Despite the amount of speculation about how recombination may produce new viral species or strains with altered host ranges, cell tropisms or pathogenicities, there are actually very few well supported examples of this having occurred in nature (see Fondong et al., 2000; Pita et al., 2001; Monci et al., 2002; Garcı´a-Andre´s et al., 2007a, b for good exceptions). The possibility that a recombination event is ultimately responsible for the existence of maize streak disease certainly deserves thorough investigation as MSV-A might be an important example of how recombinational acquisition of novel traits can sometimes trigger pathogen emergence. ACKNOWLEDGEMENTS This research was funded by the National Research Foundation (South Africa). A. V. is supported by the Carnegie Corporation of 2072 New York. D. N. S. is supported by PANNAR (Pty) Ltd; D. P. M. is supported by the Wellcome trust; B. O. is supported by the Rockefeller foundation through the USHEPiA programme; A. L. M. is supported by the Canon Collins Trust for Southern Africa and a University of Cape Town International Scholarship. P. L. is supported by the French Ministe`re de la Recherche et de l’Enseignement Supe´rieur. J. M. L. is supported by CIRAD. REFERENCES Awadalla, P. (2003). The evolutionary genomics of pathogen recombination. Nat Rev Genet 4, 50–60. 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