Genetic Structure And Populations Variability Of Chimeras Of Tomato Yellow Leaf Curl China Betasatellite And Tobacco Curly Shoot Betasatellite

It was reported that Tomato yellow leaf curl China virus (TYLCCNV) and Tobacco curly shoot virus (TbCSV) and their associated betasatellites (TYLCCNB and TbCSB) have similar genetic variation with that of a plant RNA virus, the population structure are designated as "quasispecies". Our previous studies have shown that, TYLCCNB and TbCSB have richer population diversity and higher mutation frequency than that of their cognate helper virus. The mutation frequency and percentage of mutated clone of TYLCCNB were higher than that of TbCSB, especially associated with a heterologous betasatellite. βC1 is the only functional gene on betasatellite, βC1 protein is not essential for replication, but plays a key role in symptom induction. TbCSB-10C1 and TYLCCNB-35C1 are chimeras of betasatellites generated between TbCSB isolate Y35 (Y35β) and TYLCCNB isolate Y10 (Y10(3) by exchanging (3C1. In this study, the two chimeras respectively associated with TbCSV isolateY35 (Y35A) were the experimental material. By inoculating Nicotiana benthamiana and Nicotiana glutinosa with infectious clone of Y35A and the two chimeras respectively, four Y35β-10C1 and four Y10β-35C1 populations were obtained from inoculated plants, and the genetic structure and variability of the eight populations were analyzed.Population genetic structure of the satellite chimera analyses showed that all Y10β-35C1 and Y35β-10C1 populations were of rich genetic diversity, in line with the structural features of "quasispecies". The average mutation percentage of Y10β-35C1 and Y35β-10C1 clones were 100% and 37.68%, respectively; average mutation frequencies were 6.31×10-3 and 3.55×10-4 with a difference of an order of magnitude, and the variability of Y35β-10C1 population was lower than that of Y10P-35C1 population in N. benthamiana and N. glutinosa. It revealed that Y10β-35C1 was of richer genetic structure and higher mutation frequency than those of Y35β-10C1.Variability comparison of Y10β-35C1 and Y35β-10C1 in different plants was investigated in this study. Results showed Y35β-10C1 and Y10β-35C1 were highly pathogenic,9-12d after inoculation, N. benthamiana and N. glutinosa demonstrated severe and typical symptoms of geminivirus, the variation levels differ in the two hosts.12.5% and 20.09% Y35β-10C1 clones mutated and with 2.07×10-4 and 5.03×10-4 of mutation frequency in N. benthamiana and N. glutinosa in average. It showed that population variation of Y35β-10C1 in N. glutinosa was higher than that in N. benthamiana; All Y10β-35C1 clones were mutated in both N. benthamiana and N. glutinosa. Mutation frequency of Y10β-35C1 was 6.12×10-4 and 7.71×10-4 in N. benthamiana and N. glutinosa, respectively. It showed that population variation of Y10β-10C1 in N. glutinosa was higher than that in N. benthamiana. The mutation frequencies of Y35β-10C1 were 2.38×10-4 and 4.73×10-4 in 60 and 120 days after inoculation, respectively. The mutation frequencies of Y35β-10C1 were 6.51×10-3 and 7.32×10-3 in 60 and 120 days after inoculation, respectively. These results showed that the variation level of the two chimeras in 120d after inoculation were higher than that in 60d.The mutation distribution analysis of chimeras showed similar mutation distribution in both Y35β-10C1 population and Y10β-35C1 population, Base mutation of Y35β-10C1 population and Y10β-35C1 population occured mainly in the A-rich region, with 3.46×10-4 and 4.79×10-3 of mutation frequencies, respectively; Y10β-35C1 population mutation mainly occurred in 950-964 nt of A-rich region, Y35β-10C1 population has no significant mutations sections. Satellite chimera rarely occurred in SCR region and βCl, mutation frequencies of Y35β-10C1 and Y10β-35C1 population in SCR region were 1.73×10-5 and 1.62×10-5, respectively; those in βCl were 8.66×10-6 and 2.43×10-5, respectively, lower than those in A-rich region.The mutation type analysis of chimeras showed that substitution and insertion mutations were frequently occurred in Y35β-10C1 population, there were no obvious mutation tendencies in N. benthamiana; the base transition from G toT, from G to A occurred frequently in N. glutinosa, with insertion of A. Substitution, insertion and deletion mutations were frequently occurred in Y10β-35Cl population. The base substitutions from G to A, T to G, T to C, C to T and C to A occurred frequently in N. benthamiana and those from T to G、T to C、C to T occurred frequently in N. glutinosa. Insertion and substitution of A occurred most frequently in both N. benthamiana and N. glutinosa, insertion and deletion of A were 26.17% and 18.91% in N. benthamiana, insertion and deletion of A were 41.45% and 20.09% in N. glutinosa. These results revealed that mutations from T to C, from C to T, deletion and insertion of A were predominant in Y10β-35C1 population in N. benthamiana and N. glutinosa.