GENETIC DIVERSITY IN TOBACCO MILD GREEN MOSAIC TOBAMOVIRUS INFECTING THE WILD PLANT NICOTIANA-GLAUCA

The variability and genetic structure of tobacco mild green mosaic tob amovirus (TMGMV) was analyzed in a collection of isolates from Nicotia na glauca plants from Australia, California, Spain, and the east Medit erranean Basin. Ribonuclease protection was assayed on 53 isolates wit h six probes representing 85% of the TMGMV genome. Results showed that conserved domains in the TMGMV genome were different for each geograp hic population. The nucleotide sequence of two regions of the TMGMV ge nome was determined for 33 isolates. Nucleotide diversity values were smaller than those reported for other RNA genomes. For each population , genetic diversity was not related to the size of the area from which the isolates were collected, and was particularly small for Australia and Spain. Diversity values indicated population differentiation, whi ch was largest between the less diverse Spanish and Australian populat ions. Nucleotide diversity values for nonsynonymous vs synonymous posi tions indicate moderate negative selection, except for the Spanish pop ulation for which it gave evidence of positive selection. On the whole , our data indicate that the low genetic diversity for this plant viru s may he due to different factors such as positive and negative select ion or the recent colonization of new areas or host plants. The intens ity of these factors appears to be different for the various TMGMV geo graphical populations. (C) 1996 Academic Press, Inc.