Mixed Infections and Genetic Exchange Occur in Natural Populations of Cucumber Mosaic Cucumovirus

Abstract

A potential risk of transgenic plants expressing viral sequences derives from the possibility of new viral genotypes being generated by recombination between the genomic RNA of infecting viruses and RNA transcribed from the transgene. It is important to know if the probability of generation of new variants through this mechanism is higher than through recombination between natural variants in mixed infections. To date, no quantitative data have been reported on the frequency of mixed infections or on the frequency of recombination in natural populations of a plant virus. We will report such an estimation derived from the analysis of about 250 field isolates of cucumber mosaic cucumovirus (CMV), representing different epidemic episodes on different host plants in Spain. Isolates were characterized by the pattern of fragments in ribonuclease protection assays of probes representing different regions of the three genomic RNAs. For each probe, only three well defined patterns (haplotypes) were found, this analysis permitted estimation of the frequency of host plants with mixed infections of different haplotypes to be at least 15%. From these mixed infections new types could arise by genetic enchange, either by recombinanten or by reassortment. The frequency of genetic exchange in the analyzed CMV populations has also been estimated.