Résumé
A maize streak virus population from La Reunion, used to assess maize cultivar response to infection at a CIRAD breeding station, was analysed after 2 different modes of selection. SP1 was selected in 1991 from this population through single-insect transmission following short acquisition and inoculation access periods. SP1 was then transmitted to a wild perennial host (Coix lacryma jobi) on which it was maintained for 4 years, resulting in an isolate called SP2. The virulence of the initial population was increased in 1993 by a series of transfers to almost totally resistant inbred lines, resulting in an isolate called N2A. The genetic complexity of each isolate was analysed by RFLP and sequencing on a range of clones; 120 clones for SP1, 36 for SP2 and 40 for N2A. This study showed that the MSV isolates are quasispecies in nature since they are composed of different but related clones. The heterogeneity of the MSV quasispecies was reduced in SP1 by bottleneck selection and subsequently increased through drift during the 4 years of maintenance on the wild host. N2A was composed of sub-populations and was shown to be the most pathogenic isolate compared to African MSV clones and isolates on a range of differential maize cultivars. The quasispecies structure seems to be a source of adaptation to new environments (host change, host resistance). The results are discussed with a view to epidemiology and maize breeding for resistance to MS V. (Résumé d'auteur)