A CENTURY OF TOBAMOVIRUS EVOLUTION IN AN AUSTRALIAN POPULATION OF NICOTIANA-GLAUCA

The evolution over the past century of two tobamoviruses infecting pop ulations of the immigrant plant Nicotiana glauca in New South Wales (N SW), Australia, has been studied, This plant species probably entered Australia in the 1870s. Isolates of the viruses were obtained from N. glauca specimens deposited in the NSW Herbarium between 1899 and 1972, and others were obtained from living plants in 1985 and 1993. It was found that the NSW N. glauca population was infected with tobacco mosa ic tobamovirus (TMV) and tobacco mild green mosaic tobamovirus (TMGMV) before 1950 but only with TMGMV after that date. Half the pre-1950 in fections were mixtures of the two viruses, and one was a recombinant, Remarkably, sequence analyses showed no increase in the genetic divers ity among the TMGMV isolates over the period, However, for TMV, the ge netic diversity of synonymous (but not of nonsynonymous) differences b etween isolates varied and was correlated with their time of isolation , TMV accumulated to smaller concentrations than TMGMV in N. glauca pl ants, and in mixed experimental infections, the accumulation of TMV, b ut not of TMGMV, was around 1/10 that in single infections, However, n o evidence was found of isolate-specific interaction between the virus es, We conclude that although TMV may have colonized N. glauca in NSW earlier or faster than TMGMV, the latter virus caused a decrease of th e TMV population below a threshold at which deleterious mutations were eliminated, This phenomenon, called Muller's ratchet, or a ''mutation al meltdown,'' probably caused the disappearance of TMV from the niche .