Journal of General Virology (2008), 89, 2063–2074 DOI 10.1099/vir.0.2008/003590-0 Correspondence Darren P. Martin darrin.martin@uct.ac.za Received 25 April 2008 Accepted 17 June 2008 Recombination, decreased host specificity and increased mobility may have driven the emergence of maize streak virus as an agricultural pathogen Arvind Varsani,1,2 Dionne N. Shepherd,2 Ad e´rito L. Monjane,2 Betty E. Owor,2 Julia B. Erdmann,2,3 Edward P. Rybicki,2,4 Michel Peterschmitt,5 Rob W. Briddon,6 Peter G. Markham,7 Sunday Oluwafemi,8 Oliver P. Windram,9 Pierre Lefeuvre,10 Jean-Michel Lett10 and Darren P. Martin4 1Electron Microscope Unit, University of Cape Town, Rondebosch, Cape Town 7701, South Africa 2Department of Molecular and Cell Biology, University of Cape Town, Rondebosch, Cape Town 7701, South Africa 3Institute of Biology, Department of Molecular Biology and Plant Virology, University of Stuttgart, Pfaffenwaldring 57, D-70550 Stuttgart, Germany 4Institute of Infectious Disease and Molecular Medicine, University of Cape Town, Anzio Road, Observatory, Cape Town 7925, South Africa 5CIRAD, UMR BGPI, TA A54/K, Campus International de Baillarguet, 34398 Montpellier Cedex 5, France 6National Institute for Biotechnology and Genetic Engineering, Jhang Road, PO Box 577, Faisalabad, Pakistan 7Department of Disease and Stress Biology, John Innes Centre, Norwich NR4 7UH, UK 8Department of Crop, Soil and Environmental Management, Bowen University, PMB 284, Iwo, Osun State, Nigeria 9Warwick HRI Biology Centre, University of Warwick, Wellesbourne CV35 9EF, UK 10CIRAD, UMR 53 PVBMT CIRAD-Universite´ de la Re´ union, Poˆ le de Protection des Plantes, Ligne Paradis, 97410 Saint Pierre, La Re´ union, France Maize streak virus (MSV; family Geminiviridae, genus Mastrevirus), the causal agent of maize streak disease, ranks amongst the most serious biological threats to food security in subSaharan Africa. Although five distinct MSV strains have been currently described, only one of these – MSV-A – causes severe disease in maize. Due primarily to their not being an obvious threat to agriculture, very little is known about the ‘grass-adapted’ MSV strains, MSV-B, -C, -D and -E. Since comparing the genetic diversities, geographical distributions and natural host ranges of MSV-A with the other MSV strains could provide valuable information on the epidemiology, evolution and emergence of MSV-A, we carried out a phylogeographical analysis of MSVs found in uncultivated indigenous African grasses. Amongst the 83 new MSV genomes presented here, we report the discovery of six new MSV strains (MSV-F to -K). The non-random recombination breakpoint distributions detectable with these and other available mastrevirus sequences partially mirror those seen in begomoviruses, implying that the forces shaping these breakpoint patterns have been largely conserved since the earliest geminivirus ancestors. We present evidence that the ancestor of all MSV-A variants was the recombinant progeny of ancestral MSV-B and MSV-G/-F variants. While it remains unknown whether recombination influenced the emergence of MSV-A in maize, our discovery that MSV-A variants may both move between and become established in different regions of Africa with greater ease, and infect more grass species than other MSV strains, goes some way towards explaining why MSV-A is such a successful maize pathogen. Published online ahead of print on 27 June 2008 as DOI 10.1099/vir.0.2008/003590-0. The GenBank/EMBL/DDBJ accession numbers for the 83 MSV genome sequences are EU152254 and EU628564–EU628644. Supplementary material is available with the online version of this paper. 2008/003590 G 2008 SGM Printed in Great Britain 2063