Recombination Between Cauliflower Mosaic Virus and Transgenic Plants that Contain CaMV Transgenes : Influence of Selection Pressure on Isolation of Recombinants

Abstract

We have shown that some cauliflower mosaic caulimovirus (CaMV) strains are able to recombine with Nicotiana bigelovii plants that contain a CaMV transgene that determines systemic infection of solanaceous species. The transgenic N. bigelovii expressed gene VI of CaMV strain D4, a gene known to determine systemic infection in solanaceous species including N. bigelovii. Recombinants between CaMV and the transgene can be readily isolated under conditions of strong selection pressure, but surprisingly can also be isolated from transgenic plants under moderate selection pressure. To simulate conditions of strong selection pressure, we inoculated the transgenic N. bigelovii with CaMV strain CM1841, a strain that is unable to infect N. bigelovii systemically. Recombinants between CM1841 and the transgene were isolated from 30% of the plants after mechanical inoculation. When CM1841 was agroinoculated, a recombinant virus was isolated from 100% of the transgenic plants. To simulate conditions of moderate selection pressure, we inoculated transgenic plants with CaMV strain W260, a virus that is able to infect nontransformed N. bigelovii systemically. We hypothesized that a recombinant virus formed between W260 and the transgene would have little or no selective advantage over the wild-type W260 virus. However, we found evidence that W260 had recombined with the transgene in three of 23 transgenic N. bigelovii infected with W260. The observation that a recombinant virus could predominate over the wild-type W260 virus was unforeseen, but might reflect differences in the aggressiveness of the recombinant relative to the initial inoculum. To determine whether the W260 recombinant virus, designated W260R, was more competitive than the wild-type W260, the two viruses were inoculated separately and in varying ratios onto non-transformed N. bigelovii. An analysis of the viruses responsible for the infections revealed that W260R could predominate in mixed infections, even when it was only a minor component of the initial virus population.