Three distinct mechanisms facilitate genetic isolation of sympatric wheat streak mosaic virus lineages

Cross-protection and vector transmission bottlenecks have been proposed as mechanisms facilitating genetic isolation of sympatric viral lineages. Mole cular markers were used to monitor establishment and resolution of mixed in fections with genetically defined strains of wheat streak mosaic virus (WSM V). Two closely related WSMV strains from the U.S. (Type and Sidney 81) exh ibited reciprocal cross-protection in wheat, confirming this classic phenom enon as a mechanism of genetic isolation. In contrast, cross-protection bet ween either U.S. strain and the divergent El Batan 3 strain from Mexico was unilateral, erratic, and only partially effective. Distribution of WSMV st rains within individual leaves of plants supporting a mixed infection of Ty pe and Sidney 81 was spatially nonuniform. Strain distribution among indivi dual tillers of coinfected plants also was heterogeneous, with some contain ing either Type or Sidney 81 alone and some containing both. Transmission b y wheat curl mites, acquiring virus from source plants simultaneously infec ted with both Type and Sidney 81, often resulted in test plants bearing onl y a single WSMV strain. Spatial subdivision of virus strains within coinfec ted plants likely contributed to vector transmission bottlenecks during acq uisition. Collectively, these three distinct mechanisms enhance genetic iso lation of individual viral lineages, and together with stochastic processes , may explain generation and maintenance of genetic diversity in field popu lations. (C) 2001 Academic Press.